blocks|key|1832836|text|您需要检查IsDBNull|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1832837|if(!SqlReader.IsDBNull(indexFirstName))
{
++employee.FirstName+=+sqlreader.GetString(indexFirstName);
}|code-block|syntax|javascript|1832838|这是检测和处理这种情况的唯一可靠方法。|1832839|我将这些内容包装到扩展方法中，如果列确实为null，则倾向于返回默认值|1832840|public+static+string+SafeGetString(this+SqlDataReader+reader,+int+colIndex)
{
+++if(!reader.IsDBNull(colIndex))
+++++++return+reader.GetString(colIndex);
+++return+string.Empty;
}|1832841|现在你可以这样叫它：|1832842|employee.FirstName+=+SqlReader.SafeGetString(indexFirstName);|1832843|而且您再也不用担心异常或null值了。|1832844|entityMap^0|5|8|0|0|0|L|4|0|0|0|0|C|4|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|17|8|@]|D|@]|E|$]]|$1|S|3|T|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|U|3|V|5|6|7|19|8|@$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|X|$]]

You need to check for <code>IsDBNull</code>:

<pre><code>if(!SqlReader.IsDBNull(indexFirstName))
{
 employee.FirstName = sqlreader.GetString(indexFirstName);
}
</code></pre>

That's your only reliable way to detect and handle this situation.

I wrapped those things into extension methods and tend to return a default value if the column is indeed <code>null</code>:

<pre><code>public static string SafeGetString(this SqlDataReader reader, int colIndex)
{
 if(!reader.IsDBNull(colIndex))
 return reader.GetString(colIndex);
 return string.Empty;
}
</code></pre>

Now you can call it like this:

<pre><code>employee.FirstName = SqlReader.SafeGetString(indexFirstName);
</code></pre>

and you'll never have to worry about an exception or a <code>null</code> value again.

blocks|key|1936504|text|对于默认值，应该结合使用as运算符和??运算符。值类型将需要被读取为可空的，并给出默认值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1936505|employee.FirstName+=+sqlreader[indexFirstName]+as+string;
employee.Age+=+sqlreader[indexAge]+as+int?+??+default(int);|code-block|syntax|javascript|1936506|as操作符处理类型转换，包括对DBNull的检查。|1936507|entityMap^0|C|2|I|2|0|0|0|2|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

You should use the <code>as</code> operator combined with the <code>??</code> operator for default values. Value types will need to be read as nullable and given a default.

<pre><code>employee.FirstName = sqlreader[indexFirstName] as string;
employee.Age = sqlreader[indexAge] as int? ?? default(int);
</code></pre>

The <code>as</code> operator handles the casting including the check for DBNull.

blocks|key|1936666|text|employee.FirstName+=+sqlreader[indexFirstName]+as+string;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1936667|对于整数，如果强制转换为可以为空的整数，则可以使用GetValueOrDefault()|unstyled|1936668|employee.Age+=+(sqlreader[indexAge]+as+int?).GetValueOrDefault();|1936669|或空合并运算符(??)。|offset|length|style|CODE|1936670|employee.Age+=+(sqlreader[indexAge]+as+int?)+??+0;|1936671|entityMap^0|0|0|0|8|2|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|V|8|@$K|W|L|X|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$B|C]]|$1|Q|3|-4|5|F|7|Z|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>employee.FirstName = sqlreader[indexFirstName] as string;
</code></pre>
For integers, if you cast to a nullable int, you can use GetValueOrDefault()
<pre><code>employee.Age = (sqlreader[indexAge] as int?).GetValueOrDefault();
</code></pre>
or the null-coalescing operator (<code>??</code>).
<pre><code>employee.Age = (sqlreader[indexAge] as int?) ?? 0;
</code></pre>

blocks|key|528826|text|IsDbNull(int)通常比使用GetSqlDateTime这样的方法慢得多，然后再与DBNull.Value进行比较。尝试这些用于SqlDataReader的扩展方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|528827|public+static+T+Def<T>(this+SqlDataReader+r,+int+ord)
{
++++var+t+=+r.GetSqlValue(ord);
++++if+(t+==+DBNull.Value)+return+default(T);
++++return+((INullable)t).IsNull+?+default(T)+:+(T)t;
}

public+static+T?+Val<T>(this+SqlDataReader+r,+int+ord)+where+T:struct
{
++++var+t+=+r.GetSqlValue(ord);
++++if+(t+==+DBNull.Value)+return+null;
++++return+((INullable)t).IsNull+?+(T?)null+:+(T)t;
}

public+static+T+Ref<T>(this+SqlDataReader+r,+int+ord)+where+T+:+class
{
++++var+t+=+r.GetSqlValue(ord);
++++if+(t+==+DBNull.Value)+return+null;
++++return+((INullable)t).IsNull+?+null+:+(T)t;
}|code-block|syntax|javascript|528828|像这样使用它们：|528829|var+dd+=+r.Val<DateTime>(ords[4]);
var+ii+=+r.Def<int>(ords[0]);
int+nn+=+r.Def<int>(ords[0]);|528830|entityMap^0|0|D|I|E|19|C|1W|D|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

<code>IsDbNull(int)</code> is usually much slower than using methods like <code>GetSqlDateTime</code> and then comparing to <code>DBNull.Value</code>. Try these extension methods for <code>SqlDataReader</code>.

<pre><code>public static T Def&lt;T&gt;(this SqlDataReader r, int ord)
{
 var t = r.GetSqlValue(ord);
 if (t == DBNull.Value) return default(T);
 return ((INullable)t).IsNull ? default(T) : (T)t;
}

public static T? Val&lt;T&gt;(this SqlDataReader r, int ord) where T:struct
{
 var t = r.GetSqlValue(ord);
 if (t == DBNull.Value) return null;
 return ((INullable)t).IsNull ? (T?)null : (T)t;
}

public static T Ref&lt;T&gt;(this SqlDataReader r, int ord) where T : class
{
 var t = r.GetSqlValue(ord);
 if (t == DBNull.Value) return null;
 return ((INullable)t).IsNull ? null : (T)t;
}
</code></pre>

Use them like this:

<pre><code>var dd = r.Val&lt;DateTime&gt;(ords[4]);
var ii = r.Def&lt;int&gt;(ords[0]);
int nn = r.Def&lt;int&gt;(ords[0]);
</code></pre>

blocks|key|1833033|text|reader.IsDbNull(ColumnIndex)的工作就像许多答案所说的那样。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1833034|我想提一下，如果您使用列名，那么只比较类型可能会更舒服。|1833035|if(reader["TeacherImage"].GetType()+==+typeof(DBNull))+{+//logic+}|code-block|syntax|javascript|1833036|entityMap^0|0|S|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|R|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|S|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

<code>reader.IsDbNull(ColumnIndex)</code> works as many answers says.

And I want to mention if you working with column names, just comparing types may be more comfortable.

<pre><code>if(reader["TeacherImage"].GetType() == typeof(DBNull)) { //logic }
</code></pre>

blocks|key|531279|text|当在数据读取器中使用列名返回行时，我不认为有NULL列值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|531280|如果你做datareader["columnName"].ToString();，它总是会给你一个可以是空字符串的值(如果你需要比较，就是String.Empty)。|CODE|531281|我会使用下面的代码，不会太担心：|531282|employee.FirstName+=+sqlreader["columnNameForFirstName"].ToString();|code-block|syntax|javascript|531283|entityMap^0|M|4|0|4|10|1X|C|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|H]|$9|X|A|Y|B|H]]|D|@]|E|$]]|$1|I|3|J|5|6|7|Z|8|@]|D|@]|E|$]]|$1|K|3|L|5|M|7|10|8|@]|D|@]|E|$N|O]]|$1|P|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|Q|$]]

I don't think there's a NULL column value, when rows are returned within a datareader using the column name.

If you do <code>datareader["columnName"].ToString();</code> it will always give you a value that can be a empty string (<code>String.Empty</code> if you need to compare).

I would use the following and wouldn't worry too much:

<pre><code>employee.FirstName = sqlreader["columnNameForFirstName"].ToString();
</code></pre>

blocks|key|1936345|text|一种方法是检查数据库是否为空：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1936346|employee.FirstName+=+(sqlreader.IsDBNull(indexFirstName)+
++++?+""
++++:+sqlreader.GetString(indexFirstName));|code-block|syntax|javascript|1936347|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

One way to do it is to check for db nulls:

<pre><code>employee.FirstName = (sqlreader.IsDBNull(indexFirstName) 
 ? ""
 : sqlreader.GetString(indexFirstName));
</code></pre>

blocks|key|1936608|text|此解决方案对供应商的依赖性较低，可与SQL、OleDB和MySQL阅读器配合使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1936609|public+static+string+GetStringSafe(this+IDataReader+reader,+int+colIndex)
{
++++return+GetStringSafe(reader,+colIndex,+string.Empty);
}

public+static+string+GetStringSafe(this+IDataReader+reader,+int+colIndex,+string+defaultValue)
{
++++if+(!reader.IsDBNull(colIndex))
++++++++return+reader.GetString(colIndex);
++++else
++++++++return+defaultValue;
}

public+static+string+GetStringSafe(this+IDataReader+reader,+string+indexName)
{
++++return+GetStringSafe(reader,+reader.GetOrdinal(indexName));
}

public+static+string+GetStringSafe(this+IDataReader+reader,+string+indexName,+string+defaultValue)
{
++++return+GetStringSafe(reader,+reader.GetOrdinal(indexName),+defaultValue);
}|code-block|syntax|javascript|1936610|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This Solution is less vendor-dependent and works with an SQL, OleDB, and MySQL Reader:

<pre><code>public static string GetStringSafe(this IDataReader reader, int colIndex)
{
 return GetStringSafe(reader, colIndex, string.Empty);
}

public static string GetStringSafe(this IDataReader reader, int colIndex, string defaultValue)
{
 if (!reader.IsDBNull(colIndex))
 return reader.GetString(colIndex);
 else
 return defaultValue;
}

public static string GetStringSafe(this IDataReader reader, string indexName)
{
 return GetStringSafe(reader, reader.GetOrdinal(indexName));
}

public static string GetStringSafe(this IDataReader reader, string indexName, string defaultValue)
{
 return GetStringSafe(reader, reader.GetOrdinal(indexName), defaultValue);
}
</code></pre>

blocks|key|529058|text|您可以编写一个泛型函数来检查NULL，并在默认值为Null时包含默认值。在读取Datareader时调用此函数|type|unstyled|depth|inlineStyleRanges|entityRanges|data|529059|public+T+CheckNull<T>(object+obj)
++++++++{
++++++++++++return+(obj+==+DBNull.Value+?+default(T)+:+(T)obj);
++++++++}|code-block|syntax|javascript|529060|在读取Datareader时使用|529061|++++++++++++++++++++++++while+(dr.Read())
++++++++++++++++++++++++{
++++++++++++++++++++++++++++tblBPN_InTrRecon+Bpn+=+new+tblBPN_InTrRecon();
++++++++++++++++++++++++++++Bpn.BPN_Date+=+CheckNull<DateTime?>(dr["BPN_Date"]);
++++++++++++++++++++++++++++Bpn.Cust_Backorder_Qty+=+CheckNull<int?>(dr["Cust_Backorder_Qty"]);
++++++++++++++++++++++++++++Bpn.Cust_Min+=+CheckNull<int?>(dr["Cust_Min"]);
+++++++++++++++++++++++++}|529062|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You can write a Generic function to check Null and include default value when it is NULL. Call this when reading Datareader

<pre><code>public T CheckNull&lt;T&gt;(object obj)
 {
 return (obj == DBNull.Value ? default(T) : (T)obj);
 }
</code></pre>

When reading the Datareader use

<pre><code> while (dr.Read())
 {
 tblBPN_InTrRecon Bpn = new tblBPN_InTrRecon();
 Bpn.BPN_Date = CheckNull&lt;DateTime?&gt;(dr["BPN_Date"]);
 Bpn.Cust_Backorder_Qty = CheckNull&lt;int?&gt;(dr["Cust_Backorder_Qty"]);
 Bpn.Cust_Min = CheckNull&lt;int?&gt;(dr["Cust_Min"]);
 }
</code></pre>

blocks|key|528771|text|我倾向于做的是将SELECT语句中的空值替换为适当的值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|528772|SELECT+ISNULL(firstname,+'')+FROM+people|code-block|syntax|javascript|528773|在这里，我用一个空字符串替换每个null。在这种情况下，您的代码不会抛出错误。|528774|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

What I tend to do is replace the null values in the SELECT statement with something appropriate.

<pre><code>SELECT ISNULL(firstname, '') FROM people
</code></pre>

Here I replace every null with a blank string. Your code won't throw in error in that case.

blocks|key|1830859|text|在你尝试阅读之前，请检查sqlreader.IsDBNull(indexFirstName)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1830860|entityMap^0|C|Y|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Check <code>sqlreader.IsDBNull(indexFirstName)</code> before you try to read it.

blocks|key|1831051|text|受getpsyched's+answer的影响，我创建了一个泛型方法，它通过列名检查列值|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1831052|public+static+T+SafeGet<T>(this+System.Data.SqlClient.SqlDataReader+reader,+string+nameOfColumn)
{
++var+indexOfColumn+=+reader.GetOrdinal(nameOfColumn);
++return+reader.IsDBNull(indexOfColumn)+?+default(T)+:+reader.GetFieldValue<T>(indexOfColumn);
}|code-block|syntax|javascript|1831053|用法：|1831054|var+myVariable+=+SafeGet<string>(reader,+"NameOfColumn")|1831055|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/52204396/1262673^0|1|J|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

By influencing from <a href="https://stackoverflow.com/a/52204396/1262673">getpsyched's answer</a>, I created a generic method which checks column value by its name

<pre><code>public static T SafeGet&lt;T&gt;(this System.Data.SqlClient.SqlDataReader reader, string nameOfColumn)
{
 var indexOfColumn = reader.GetOrdinal(nameOfColumn);
 return reader.IsDBNull(indexOfColumn) ? default(T) : reader.GetFieldValue&lt;T&gt;(indexOfColumn);
}
</code></pre>

Usage: 

<pre><code>var myVariable = SafeGet&lt;string&gt;(reader, "NameOfColumn")
</code></pre>

blocks|key|529198|text|作为对marc_s回答的补充，您可以使用更通用的扩展方法从SqlDataReader获取值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|529199|public+static+T+SafeGet<T>(this+SqlDataReader+reader,+int+col)
++++{
++++++++return+reader.IsDBNull(col)+?+default(T)+:+reader.GetFieldValue<T>(col);
++++}|code-block|syntax|javascript|529200|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

As an addition to the answer by marc_s, you can use a more generic extension method to get values from the SqlDataReader:

<pre><code>public static T SafeGet&lt;T&gt;(this SqlDataReader reader, int col)
 {
 return reader.IsDBNull(col) ? default(T) : reader.GetFieldValue&lt;T&gt;(col);
 }
</code></pre>

blocks|key|531381|text|如何创建帮助器方法|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531382|For字符串|531383|private+static+string+MyStringConverter(object+o)
++++{
++++++++if+(o+==+DBNull.Value+%7C%7C+o+==+null)
++++++++++++return+"";

++++++++return+o.ToString();
++++}|code-block|syntax|javascript|531384|用法|531385|MyStringConverter(read["indexStringValue"])|531386|对于Int|531387|+private+static+int+MyIntonverter(object+o)
++++{
++++++++if+(o+==+DBNull.Value+%7C%7C+o+==+null)
++++++++++++return+0;

++++++++return+Convert.ToInt32(o);
++++}|531388|531389|MyIntonverter(read["indexIntValue"])|531390|对于日期|531391|private+static+DateTime?+MyDateConverter(object+o)
++++{
++++++++return+(o+==+DBNull.Value+%7C%7C+o+==+null)+?+(DateTime?)null+:+Convert.ToDateTime(o);
++++}|531392|531393|MyDateConverter(read["indexDateValue"])|531394|注意:对于DateTime，将变量声明为|531395|DateTime?+variable;|531396|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|18|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|19|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|1A|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|1B|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|1C|8|@]|9|@]|A|$G|H]]|$1|Q|3|J|5|6|7|1D|8|@]|9|@]|A|$]]|$1|R|3|S|5|F|7|1E|8|@]|9|@]|A|$G|H]]|$1|T|3|U|5|6|7|1F|8|@]|9|@]|A|$]]|$1|V|3|W|5|F|7|1G|8|@]|9|@]|A|$G|H]]|$1|X|3|J|5|6|7|1H|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|1I|8|@]|9|@]|A|$G|H]]|$1|10|3|11|5|6|7|1J|8|@]|9|@]|A|$]]|$1|12|3|13|5|F|7|1K|8|@]|9|@]|A|$G|H]]|$1|14|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|15|$]]

how to about creating helper methods

For String

<pre><code>private static string MyStringConverter(object o)
 {
 if (o == DBNull.Value || o == null)
 return "";

 return o.ToString();
 }
</code></pre>

Usage

<pre><code>MyStringConverter(read["indexStringValue"])
</code></pre>

For Int

<pre><code> private static int MyIntonverter(object o)
 {
 if (o == DBNull.Value || o == null)
 return 0;

 return Convert.ToInt32(o);
 }
</code></pre>

Usage

<pre><code>MyIntonverter(read["indexIntValue"])
</code></pre>

For Date

<pre><code>private static DateTime? MyDateConverter(object o)
 {
 return (o == DBNull.Value || o == null) ? (DateTime?)null : Convert.ToDateTime(o);
 }
</code></pre>

Usage

<pre><code>MyDateConverter(read["indexDateValue"])
</code></pre>

Note: for DateTime declare varialbe as

<pre><code>DateTime? variable;
</code></pre>

blocks|key|529399|text|这些都不是我想要的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|529400|+public+static+T+GetFieldValueOrDefault<T>(this+SqlDataReader+reader,+string+name)
+{
+++++int+index+=+reader.GetOrdinal(name);
+++++T+value+=+reader.IsDBNull(index)+?+default(T)+:+reader.GetFieldValue<T>(index);
+++++return+value;
+}|code-block|syntax|javascript|529401|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

None of these was quite what i wanted:
<pre><code> public static T GetFieldValueOrDefault&lt;T&gt;(this SqlDataReader reader, string name)
 {
 int index = reader.GetOrdinal(name);
 T value = reader.IsDBNull(index) ? default(T) : reader.GetFieldValue&lt;T&gt;(index);
 return value;
 }
</code></pre>

blocks|key|1936470|text|我认为你会想要使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1936471|SqlReader.IsDBNull(indexFirstName)|code-block|syntax|javascript|1936472|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I think you would want to use:

<pre><code>SqlReader.IsDBNull(indexFirstName)
</code></pre>

blocks|key|528741|text|我们使用一系列静态方法从数据读取器中提取所有值。因此，在这种情况下，我们将调用DBUtils.GetString(sqlreader(indexFirstName))创建静态/共享方法的好处是，您不必一遍又一遍地执行相同的检查……|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|528742|静态方法将包含检查空值的代码(请参阅本页上的其他答案)。|528743|entityMap^0|13|18|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

We use a series of static methods to pull all of the values out of our data readers. So in this case we'd be calling <code>DBUtils.GetString(sqlreader(indexFirstName))</code> The benefit of creating static/shared methods is that you don't have to do the same checks over and over and over...

The static method(s) would contain code to check for nulls (see other answers on this page).

blocks|key|531371|text|您可以使用条件运算符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531372|employee.FirstName+=+sqlreader["indexFirstName"]+!=+DBNull.Value+?+sqlreader[indexFirstName].ToString()+:+"";|code-block|syntax|javascript|531373|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You may use the conditional operator:

<pre><code>employee.FirstName = sqlreader["indexFirstName"] != DBNull.Value ? sqlreader[indexFirstName].ToString() : "";
</code></pre>

blocks|key|1937016|text|这里有很多有用的信息(和一些错误的信息)的答案，我想把它们放在一起。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1937017|这个问题的简短答案是检查DBNull+-几乎每个人都同意这一点:)|1937018|泛型方法使我们可以用更少的代码来解决这个问题，而不是使用帮助器方法来读取每个SQL数据类型的可空值。但是，不能同时为可空值类型和引用类型使用单个泛型方法，这在Nullable+type+as+a+generic+parameter+possible?和C#+generic+type+constraint+for+everything+nullable中有详细讨论。|offset|length|1937019|因此，根据@ZXX和@getpsyched的回答，我们最终得到了这两个方法，用于获取可空的值，我为非空值添加了第三个方法(它完成了基于方法命名的集合)。|1937020|public+static+T?+GetNullableValueType<T>(this+SqlDataReader+sqlDataReader,+string+columnName)+where+T+:+struct
{
++++int+columnOrdinal+=+sqlDataReader.GetOrdinal(columnName);
++++return+sqlDataReader.IsDBNull(columnOrdinal)+?+(T?)null+:+sqlDataReader.GetFieldValue<T>(columnOrdinal);
}

public+static+T+GetNullableReferenceType<T>(this+SqlDataReader+sqlDataReader,+string+columnName)+where+T+:+class
{
++++int+columnOrdinal+=+sqlDataReader.GetOrdinal(columnName);
++++return+sqlDataReader.IsDBNull(columnOrdinal)+?+null+:+sqlDataReader.GetFieldValue<T>(columnOrdinal);
}

public+static+T+GetNonNullValue<T>(this+SqlDataReader+sqlDataReader,+string+columnName)
{
++++int+columnOrdinal+=+sqlDataReader.GetOrdinal(columnName);
++++return+sqlDataReader.GetFieldValue<T>(columnOrdinal);
}|code-block|syntax|javascript|1937021|我通常使用列名，如果您使用列索引，请更改列名。根据这些方法名称，我可以判断我是否期望数据为空，这在查看很久以前编写的代码时非常有用。|1937022|提示；|1937023|数据库中没有可为空的列的|1937024|可避免此问题。如果您可以控制数据库，那么默认情况下，列应该是非空的，并且只有在必要时才可以为空。|unordered-list-item|1937025|不要使用C#+'as‘操作符转换数据库值，因为如果转换错误，它将自动返回null。对于int、datetime、bit等值类型，使用default+value+expression的|1937026|会将数据库空值更改为非空值。|1937027|1937028|最后，当我在所有SQL+Server数据类型上测试上述方法时，我发现不能直接从SqlDataReader获取char[]，如果需要char[]，则必须获取字符串并使用ToCharArray()。|1937029|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/209160/nullable-type-as-a-generic-parameter-possible?rq=1|1|https://stackoverflow.com/questions/19831157/c-sharp-generic-type-constraint-for-everything-nullable|2|https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/default-value-expressions^0|0|0|27|1A|0|3I|1E|1|0|0|0|0|0|0|0|1U|O|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1H|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1I|8|@]|9|@$F|1J|G|1K|1|1L]|$F|1M|G|1N|1|1O]]|A|$]]|$1|H|3|I|5|6|7|1P|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|1Q|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1R|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1S|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1T|8|@]|9|@]|A|$]]|$1|U|3|V|5|W|7|1U|8|@]|9|@]|A|$]]|$1|X|3|Y|5|W|7|1V|8|@]|9|@$F|1W|G|1X|1|1Y]]|A|$]]|$1|Z|3|10|5|W|7|1Z|8|@]|9|@]|A|$]]|$1|11|3|-4|5|6|7|20|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|21|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|22|8|@]|9|@]|A|$]]]|15|$16|$5|17|18|19|A|$1A|1B]]|1C|$5|17|18|19|A|$1A|1D]]|1E|$5|17|18|19|A|$1A|1F]]]]

There are a lot of answers here with useful info (and some wrong info) spread about, I'd like to bring it all together.

The short answer to the question is to check for DBNull - almost everyone agrees on this bit :)

Rather than using a helper method to read nullable values per SQL data type a generic method allows us to address this with a lot less code. However, you can't have a single generic method for both nullable value types and reference types, this is discussed at length in 
<a href="https://stackoverflow.com/questions/209160/nullable-type-as-a-generic-parameter-possible?rq=1">Nullable type as a generic parameter possible?</a> and <a href="https://stackoverflow.com/questions/19831157/c-sharp-generic-type-constraint-for-everything-nullable">C# generic type constraint for everything nullable</a>. 

So, following on from the answers from @ZXX and @getpsyched we end up with this, 2 methods for getting nullable values and I've added a 3rd for non-null values (it completes the set based on method naming).

<pre><code>public static T? GetNullableValueType&lt;T&gt;(this SqlDataReader sqlDataReader, string columnName) where T : struct
{
 int columnOrdinal = sqlDataReader.GetOrdinal(columnName);
 return sqlDataReader.IsDBNull(columnOrdinal) ? (T?)null : sqlDataReader.GetFieldValue&lt;T&gt;(columnOrdinal);
}

public static T GetNullableReferenceType&lt;T&gt;(this SqlDataReader sqlDataReader, string columnName) where T : class
{
 int columnOrdinal = sqlDataReader.GetOrdinal(columnName);
 return sqlDataReader.IsDBNull(columnOrdinal) ? null : sqlDataReader.GetFieldValue&lt;T&gt;(columnOrdinal);
}

public static T GetNonNullValue&lt;T&gt;(this SqlDataReader sqlDataReader, string columnName)
{
 int columnOrdinal = sqlDataReader.GetOrdinal(columnName);
 return sqlDataReader.GetFieldValue&lt;T&gt;(columnOrdinal);
}
</code></pre>

I usually use column names, alter these if you use column indexes. Based on these method names I can tell whether I'm expecting the data to be nullable or not, quite useful when looking at code written a long time ago.

Tips;

<ul>
<li>Not having nullable columns in the database avoids this issue. If you
have control over the database then columns should be non-null by
default and only nullable where necessary. </li>
<li>Don't cast database values
with the C# 'as' operator because if the cast is wrong it will
silently return null. </li>
<li>Using a <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/default-value-expressions" rel="nofollow noreferrer">default value expression</a> will change
database nulls to non-null values for value types like int, datetime,
bit etc.</li>
</ul>

Lastly, whilst testing the above methods across all SQL Server data types I discovered you can't directly get a char[] from a SqlDataReader, if you want a char[] you will have to get a string and use ToCharArray().

blocks|key|531022|text|和/或使用带赋值的三元运算符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531023|employee.FirstName+=+rdr.IsDBNull(indexFirstName))?+
+++++++++++++++++++++String.Empty:+rdr.GetString(indexFirstName);|code-block|syntax|javascript|531024|根据需要替换每个属性类型的默认值(当为null时)...|531025|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

and / or use ternary operator with assignment:

<pre><code>employee.FirstName = rdr.IsDBNull(indexFirstName))? 
 String.Empty: rdr.GetString(indexFirstName);
</code></pre>

replace the default (when null) value as appropriate for each property type...

blocks|key|531107|text|我使用下面列出的代码来处理读取到datatable中的Excel工作表中的空单元格。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531108|if+(!reader.IsDBNull(2))
{
+++row["Oracle"]+=+(string)reader[2];
}|code-block|syntax|javascript|531109|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I am using the code listed below to handle null cells in an Excel sheet that is read in to a datatable.

<pre><code>if (!reader.IsDBNull(2))
{
 row["Oracle"] = (string)reader[2];
}
</code></pre>

blocks|key|1830936|text|private+static+void+Render(IList<ListData>+list,+IDataReader+reader)
++++++++{
++++++++++++while+(reader.Read())
++++++++++++{

++++++++++++++++listData.DownUrl+=+(reader.GetSchemaTable().Columns["DownUrl"]+!=+null)+?+Convert.ToString(reader["DownUrl"])+:+null;
++++++++++++++++//没有这一列时，让其等于null
++++++++++++++++list.Add(listData);
++++++++++++}
++++++++++++reader.Close();
++++++++}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1830937|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>private static void Render(IList&lt;ListData&gt; list, IDataReader reader)
 {
 while (reader.Read())
 {

 listData.DownUrl = (reader.GetSchemaTable().Columns["DownUrl"] != null) ? Convert.ToString(reader["DownUrl"]) : null;
 //没有这一列时，让其等于null
 list.Add(listData);
 }
 reader.Close();
 }
</code></pre>

blocks|key|1830961|text|这个方法依赖于indexFirstName，它应该是从零开始的列序号。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1830962|if(!sqlReader.IsDBNull(indexFirstName))
{
++employee.FirstName+=+sqlreader.GetString(indexFirstName);
}|code-block|syntax|javascript|1830963|如果您不知道列索引，但又不想检查名称，则可以使用以下扩展方法：|1830964|public+static+class+DataRecordExtensions
{
++++public+static+bool+HasColumn(this+IDataRecord+dr,+string+columnName)
++++{
++++++++for+(int+i=0;+i+<+dr.FieldCount;+i%2B%2B)
++++++++{
++++++++++++if+(dr.GetName(i).Equals(columnName,+StringComparison.InvariantCultureIgnoreCase))
++++++++++++++++return+true;
++++++++}
++++++++return+false;
++++}
}|1830965|并像这样使用方法：|1830966|if(sqlReader.HasColumn("FirstName"))
{
++employee.FirstName+=+sqlreader["FirstName"];
}|1830967|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

This method is dependent on indexFirstName which should be the zero-based column ordinal.

<pre><code>if(!sqlReader.IsDBNull(indexFirstName))
{
 employee.FirstName = sqlreader.GetString(indexFirstName);
}
</code></pre>

If you don't know the column index but wan't to check a name you can use this extension method instead:

<pre><code>public static class DataRecordExtensions
{
 public static bool HasColumn(this IDataRecord dr, string columnName)
 {
 for (int i=0; i &lt; dr.FieldCount; i++)
 {
 if (dr.GetName(i).Equals(columnName, StringComparison.InvariantCultureIgnoreCase))
 return true;
 }
 return false;
 }
}
</code></pre>

And use the method like this:

<pre><code>if(sqlReader.HasColumn("FirstName"))
{
 employee.FirstName = sqlreader["FirstName"];
}
</code></pre>

blocks|key|531416|text|这是个老问题，但也许有人仍然需要一个答案|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531417|实际上，我是这样处理这个问题的。|531418|对于int：|531419|public+static+object+GatDataInt(string+Query,+string+Column)
++++{
++++++++SqlConnection+DBConn+=+new+SqlConnection(ConnectionString);
++++++++if+(DBConn.State+==+ConnectionState.Closed)
++++++++++++DBConn.Open();
++++++++SqlCommand+CMD+=+new+SqlCommand(Query,+DBConn);
++++++++SqlDataReader+RDR+=+CMD.ExecuteReader();
++++++++if+(RDR.Read())
++++++++{
++++++++++++var+Result+=+RDR[Column];
++++++++++++RDR.Close();
++++++++++++DBConn.Close();
++++++++++++return+Result;
++++++++}
++++++++return+0;
++++}|code-block|syntax|javascript|531420|相同的for字符串只是返回"“而不是0，因为"”是空字符串|531421|所以你可以像这样使用它|531422|int+TotalPoints+=+GatDataInt(QueryToGetTotalPoints,+TotalPointColumn)+as+int?;|531423|和|531424|string+Email+=+GatDatastring(QueryToGetEmail,+EmailColumn)+as+string;|531425|非常灵活，因此您可以插入任何查询来读取任何列，并且它永远不会返回错误|531426|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|P|5|H|7|14|8|@]|9|@]|A|$I|J]]|$1|Q|3|R|5|6|7|15|8|@]|9|@]|A|$]]|$1|S|3|T|5|H|7|16|8|@]|9|@]|A|$I|J]]|$1|U|3|V|5|6|7|17|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|X|$]]

Old question but maybe someone still need an answer 

in real i worked around this issue like that

For int :

<pre><code>public static object GatDataInt(string Query, string Column)
 {
 SqlConnection DBConn = new SqlConnection(ConnectionString);
 if (DBConn.State == ConnectionState.Closed)
 DBConn.Open();
 SqlCommand CMD = new SqlCommand(Query, DBConn);
 SqlDataReader RDR = CMD.ExecuteReader();
 if (RDR.Read())
 {
 var Result = RDR[Column];
 RDR.Close();
 DBConn.Close();
 return Result;
 }
 return 0;
 }
</code></pre>

the same for string just return "" instead of 0 as "" is empty string

so you can use it like 

<pre><code>int TotalPoints = GatDataInt(QueryToGetTotalPoints, TotalPointColumn) as int?;
</code></pre>

and

<pre><code>string Email = GatDatastring(QueryToGetEmail, EmailColumn) as string;
</code></pre>

very flexible so you can insert any query to read any column and it'll never return with error

blocks|key|529245|text|以下是帮助器类，其他人可以根据@marc_s答案在需要时使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|529246|public+static+class+SQLDataReaderExtensions
++++{
++++++++public+static+int+SafeGetInt(this+SqlDataReader+dataReader,+string+fieldName)
++++++++{
++++++++++++int+fieldIndex+=+dataReader.GetOrdinal(fieldName);
++++++++++++return+dataReader.IsDBNull(fieldIndex)+?+0+:+dataReader.GetInt32(fieldIndex);
++++++++}

++++++++public+static+int?+SafeGetNullableInt(this+SqlDataReader+dataReader,+string+fieldName)
++++++++{
++++++++++++int+fieldIndex+=+dataReader.GetOrdinal(fieldName);
++++++++++++return+dataReader.GetValue(fieldIndex)+as+int?;
++++++++}

++++++++public+static+string+SafeGetString(this+SqlDataReader+dataReader,+string+fieldName)
++++++++{
++++++++++++int+fieldIndex+=+dataReader.GetOrdinal(fieldName);
++++++++++++return+dataReader.IsDBNull(fieldIndex)+?+string.Empty+:+dataReader.GetString(fieldIndex);
++++++++}

++++++++public+static+DateTime?+SafeGetNullableDateTime(this+SqlDataReader+dataReader,+string+fieldName)
++++++++{
++++++++++++int+fieldIndex+=+dataReader.GetOrdinal(fieldName);
++++++++++++return+dataReader.GetValue(fieldIndex)+as+DateTime?;
++++++++}

++++++++public+static+bool+SafeGetBoolean(this+SqlDataReader+dataReader,+string+fieldName)
++++++++{
++++++++++++return+SafeGetBoolean(dataReader,+fieldName,+false);
++++++++}

++++++++public+static+bool+SafeGetBoolean(this+SqlDataReader+dataReader,+string+fieldName,+bool+defaultValue)
++++++++{
++++++++++++int+fieldIndex+=+dataReader.GetOrdinal(fieldName);
++++++++++++return+dataReader.IsDBNull(fieldIndex)+?+defaultValue+:+dataReader.GetBoolean(fieldIndex);
++++++++}
++++}|code-block|syntax|javascript|529247|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here is helper class which others can use if they need based on @marc_s answer:

<pre><code>public static class SQLDataReaderExtensions
 {
 public static int SafeGetInt(this SqlDataReader dataReader, string fieldName)
 {
 int fieldIndex = dataReader.GetOrdinal(fieldName);
 return dataReader.IsDBNull(fieldIndex) ? 0 : dataReader.GetInt32(fieldIndex);
 }

 public static int? SafeGetNullableInt(this SqlDataReader dataReader, string fieldName)
 {
 int fieldIndex = dataReader.GetOrdinal(fieldName);
 return dataReader.GetValue(fieldIndex) as int?;
 }

 public static string SafeGetString(this SqlDataReader dataReader, string fieldName)
 {
 int fieldIndex = dataReader.GetOrdinal(fieldName);
 return dataReader.IsDBNull(fieldIndex) ? string.Empty : dataReader.GetString(fieldIndex);
 }

 public static DateTime? SafeGetNullableDateTime(this SqlDataReader dataReader, string fieldName)
 {
 int fieldIndex = dataReader.GetOrdinal(fieldName);
 return dataReader.GetValue(fieldIndex) as DateTime?;
 }

 public static bool SafeGetBoolean(this SqlDataReader dataReader, string fieldName)
 {
 return SafeGetBoolean(dataReader, fieldName, false);
 }

 public static bool SafeGetBoolean(this SqlDataReader dataReader, string fieldName, bool defaultValue)
 {
 int fieldIndex = dataReader.GetOrdinal(fieldName);
 return dataReader.IsDBNull(fieldIndex) ? defaultValue : dataReader.GetBoolean(fieldIndex);
 }
 }
</code></pre>

blocks|key|531356|text|在c#+7.0中，我们可以执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531357|var+a+=+reader["ERateCode"]+as+string;
var+b+=+reader["ERateLift"]+as+int?;
var+c+=+reader["Id"]+as+int?;|code-block|syntax|javascript|531358|因此，如果是，它将保留空值。|531359|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

in c# 7.0 we can do :
<pre><code>var a = reader[&quot;ERateCode&quot;] as string;
var b = reader[&quot;ERateLift&quot;] as int?;
var c = reader[&quot;Id&quot;] as int?;
</code></pre>
so it will keep null value if it is.

blocks|key|529284|text|Convert明智地处理DbNull。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|529285|employee.FirstName+=+Convert.ToString(sqlreader.GetValue(indexFirstName));|code-block|syntax|javascript|529286|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Convert handles DbNull sensibly.

<pre><code>employee.FirstName = Convert.ToString(sqlreader.GetValue(indexFirstName));
</code></pre>

blocks|key|531521|text|整洁的一行代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531522|++++while+(dataReader.Read())
{
++++employee.FirstName+=+(!dataReader.IsDBNull(dataReader.GetOrdinal("FirstName")))+?+dataReader["FirstName"].ToString()+:+"";
}|code-block|syntax|javascript|531523|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

neat one-liner:
<pre><code> while (dataReader.Read())
{
 employee.FirstName = (!dataReader.IsDBNull(dataReader.GetOrdinal(&quot;FirstName&quot;))) ? dataReader[&quot;FirstName&quot;].ToString() : &quot;&quot;;
}
</code></pre>

blocks|key|529465|text|我尽了最大努力从DataTable重新实现Field方法。https://docs.microsoft.com/en-us/dotnet/api/system.data.datarowextensions.field|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|529466|如果您尝试将DBNull.Value转换为不可为空的类型，则会抛出此错误。否则，它会将DBNull.Value转换为null。|529467|我还没有完全测试过它。|529468|public+static+T+Field<T>(this+SqlDataReader+sqlDataReader,+string+columnName)
{
++++int+columnIndex+=+sqlDataReader.GetOrdinal(columnName);
++++if+(sqlDataReader.IsDBNull(columnIndex))
++++{
++++++++if+(default(T)+!=+null)
++++++++{
++++++++++++throw+new+InvalidCastException("Cannot+convert+DBNULL+value+to+type+"+%2B+typeof(T).Name);
++++++++}
++++++++else
++++++++{
++++++++++++return+default(T);
++++++++}
++++}
++++else
++++{
++++++++return+sqlDataReader.GetFieldValue<T>(columnIndex);
++++}
}|code-block|syntax|javascript|529469|用法：|529470|string+fname+=+sqlDataReader.Field<string>("FirstName");
int?+age+=+sqlDataReader.Field<int?>("Age");
int+yearsOfExperience+=+sqlDataReader.Field<int?>("YearsEx")+??+0;|529471|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/api/system.data.datarowextensions.field^0|8|9|L|5|T|27|0|0|6|C|17|C|1M|4|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]]|E|$]]|$1|F|3|G|5|6|7|18|8|@$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1F|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1G|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1H|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1I|8|@]|D|@]|E|$M|N]]|$1|S|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

I did my best to reimplement the <code>Field</code> method from <code>DataTable</code>. <a href="https://docs.microsoft.com/en-us/dotnet/api/system.data.datarowextensions.field" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/dotnet/api/system.data.datarowextensions.field</a>
It will throw if you try to convert a <code>DBNull.Value</code> to a non-nullable type. Otherwise it will convert <code>DBNull.Value</code> to <code>null</code>.
I haven't fully tested it.
<pre><code>public static T Field&lt;T&gt;(this SqlDataReader sqlDataReader, string columnName)
{
 int columnIndex = sqlDataReader.GetOrdinal(columnName);
 if (sqlDataReader.IsDBNull(columnIndex))
 {
 if (default(T) != null)
 {
 throw new InvalidCastException(&quot;Cannot convert DBNULL value to type &quot; + typeof(T).Name);
 }
 else
 {
 return default(T);
 }
 }
 else
 {
 return sqlDataReader.GetFieldValue&lt;T&gt;(columnIndex);
 }
}
</code></pre>
Usage:
<pre><code>string fname = sqlDataReader.Field&lt;string&gt;(&quot;FirstName&quot;);
int? age = sqlDataReader.Field&lt;int?&gt;(&quot;Age&quot;);
int yearsOfExperience = sqlDataReader.Field&lt;int?&gt;(&quot;YearsEx&quot;) ?? 0;
</code></pre>

blocks|key|531247|text|你也可以检查一下这个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|531248|if(null+!=x+&&+x.HasRows)
{+....}|code-block|syntax|javascript|531249|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

you can ever check for this as well

<pre><code>if(null !=x &amp;&amp; x.HasRows)
{ ....}
</code></pre>

I'm using a SQLdatareader to build POCOs from a database. The code works except when it encounters a null value in the database. For example, if the FirstName column in the database contains a null value, an exception is thrown. 

<pre><code>employee.FirstName = sqlreader.GetString(indexFirstName);
</code></pre>

What is the best way to handle null values in this situation?

SQL Data Reader - handling Null column values

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在使用SQLdatareader从数据库构建POCOs。代码可以正常工作，除非它在数据库中遇到空值。例如，如果数据库中的FirstName列包含空值，则会引发异常。employee.FirstName = sqlreader.GetString(indexFirstName);在这种情况下，处理空值的最佳方法是什么？

问SQL数据读取器-处理空列值
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