blocks|key|1728465|text|它被称为空合并(??)运算符：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1728466|myNewValue+=+myValue+??+new+MyValue();|code-block|syntax|javascript|1728467|entityMap^0|8|2|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

It's called the null coalescing (<code>??</code>) operator:

<pre><code>myNewValue = myValue ?? new MyValue();
</code></pre>

blocks|key|1513376|text|遗憾的是，没有与使用DBNull的空合并运算符等效的运算符；为此，需要使用三元运算符：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1513377|newValue+=+(oldValue+is+DBNull)+?+null+:+oldValue;|code-block|syntax|javascript|1513378|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|$]]

Sadly, there's no equivalent to the null coalescing operator that works with DBNull; for that, you need to use the ternary operator:

<pre><code>newValue = (oldValue is DBNull) ? null : oldValue;
</code></pre>

blocks|key|1513576|text|public+static+T+isNull<T>(this+T+v1,+T+defaultValue)
{
++++return+v1+==+null+?+defaultValue+:+v1;
}

myValue.isNull(new+MyValue())|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1513577|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>public static T isNull&lt;T&gt;(this T v1, T defaultValue)
{
 return v1 == null ? defaultValue : v1;
}

myValue.isNull(new MyValue())
</code></pre>

blocks|key|1513562|text|使用Equals方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1513563|object+value2+=+null;
Console.WriteLine(object.Equals(value2,null));|code-block|syntax|javascript|1513564|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|$]]

Use the Equals method:

<pre><code>object value2 = null;
Console.WriteLine(object.Equals(value2,null));
</code></pre>

blocks|key|1728565|text|为了使用DB+Nulls，我为我的VB应用程序创建了一堆。我称它们为Cxxx2，因为它们类似于VB内置的Cxxx函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1728566|您可以在我的CLR扩展项目中看到它们|1728567|http://www.codeplex.com/ClrExtensions/SourceControl/FileView.aspx?itemId=363867&changeSetId=17967|offset|length|1728568|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|2P|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@$F|R|G|S|1|T]]|A|$]]|$1|H|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|E]]]]

For working with DB Nulls, I created a bunch for my VB applications. I call them Cxxx2 as they are similar to VB's built-in Cxxx functions.

You can see them in my CLR Extensions project

<a href="http://www.codeplex.com/ClrExtensions/SourceControl/FileView.aspx?itemId=363867&amp;changeSetId=17967" rel="nofollow noreferrer">http://www.codeplex.com/ClrExtensions/SourceControl/FileView.aspx?itemId=363867&amp;changeSetId=17967</a>

blocks|key|1513605|text|你写了两个函数|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1513606|++++//When+Expression+is+Number
++++public+static+double?+isNull(double?+Expression,+double?+Value)
++++{
++++++++if+(Expression+==null)
++++++++{
++++++++++++return+Value;
++++++++}
++++++++else
++++++++{
++++++++++++return+Expression;
++++++++}
++++}


++++//When+Expression+is+string+(Can+not+send+Null+value+in+string+Expression
++++public+static+string+isEmpty(string+Expression,+string+Value)
++++{
++++++++if+(Expression+==+"")
++++++++{
++++++++++++return+Value;
++++++++}
++++++++else
++++++++{
++++++++++++return+Expression;
++++++++}
++++}|code-block|syntax|javascript|1513607|它们工作得很好。|1513608|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|$]]

You Write Two Function

<pre><code> //When Expression is Number
 public static double? isNull(double? Expression, double? Value)
 {
 if (Expression ==null)
 {
 return Value;
 }
 else
 {
 return Expression;
 }
 }


 //When Expression is string (Can not send Null value in string Expression
 public static string isEmpty(string Expression, string Value)
 {
 if (Expression == "")
 {
 return Value;
 }
 else
 {
 return Expression;
 }
 }
</code></pre>

They Work Very Well

blocks|key|1728814|text|我在我的DataRow类型上使用了以下扩展方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1728815|++++public+static+string+ColumnIsNull(this+System.Data.DataRow+row,+string+colName,+string+defaultValue+=+"")
++++{
++++++++string+val+=+defaultValue;
++++++++if+(row.Table.Columns.Contains(colName))
++++++++{
++++++++++++if+(row[colName]+!=+DBNull.Value)
++++++++++++{
++++++++++++++++val+=+row[colName]?.ToString();
++++++++++++}
++++++++}
++++++++return+val;
++++}|code-block|syntax|javascript|1728816|用法：|1728817|MyControl.Text+=+MyDataTable.Rows[0].ColumnIsNull("MyColumn");
MyOtherControl.Text+=+MyDataTable.Rows[0].ColumnIsNull("AnotherCol",+"Doh!+I'm+null");|1728818|我首先检查该列是否存在，因为如果没有查询结果包含该列的非null值，则DataTable对象甚至不会提供该列。|1728819|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I've been using the following extension method on my DataRow types:

<pre><code> public static string ColumnIsNull(this System.Data.DataRow row, string colName, string defaultValue = "")
 {
 string val = defaultValue;
 if (row.Table.Columns.Contains(colName))
 {
 if (row[colName] != DBNull.Value)
 {
 val = row[colName]?.ToString();
 }
 }
 return val;
 }
</code></pre>

usage:

<pre><code>MyControl.Text = MyDataTable.Rows[0].ColumnIsNull("MyColumn");
MyOtherControl.Text = MyDataTable.Rows[0].ColumnIsNull("AnotherCol", "Doh! I'm null");
</code></pre>

I'm checking for the existence of the column first because if none of query results has a non-null value for that column, the DataTable object won't even provide that column.

blocks|key|1513684|text|使用下面的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1513685|++++///+<summary>
++++///+Returns+replacement+value+if+expression+is+null
++++///+</summary>
++++///+<param+name="expression"></param>
++++///+<param+name="replacement"></param>
++++///+<returns></returns>
++++public+static+long?+IsNull(long?+expression,+long?+replacement)
++++{
++++++++if+(expression.HasValue)
++++++++++++return+expression;
++++++++else
++++++++++++return+replacement;
++++}

++++///+<summary>
++++///+Returns+replacement+value+if+expression+is+null
++++///+</summary>
++++///+<param+name="expression"></param>
++++///+<param+name="replacement"></param>
++++///+<returns></returns>
++++public+static+string+IsNull(string+expression,+string+replacement)
++++{
++++++++if+(string.IsNullOrWhiteSpace(expression))
++++++++++++return+replacement;
++++++++else
++++++++++++return+expression;
++++}|code-block|syntax|javascript|1513686|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|$]]

Use below methods.

<pre><code> /// &lt;summary&gt;
 /// Returns replacement value if expression is null
 /// &lt;/summary&gt;
 /// &lt;param name="expression"&gt;&lt;/param&gt;
 /// &lt;param name="replacement"&gt;&lt;/param&gt;
 /// &lt;returns&gt;&lt;/returns&gt;
 public static long? IsNull(long? expression, long? replacement)
 {
 if (expression.HasValue)
 return expression;
 else
 return replacement;
 }

 /// &lt;summary&gt;
 /// Returns replacement value if expression is null
 /// &lt;/summary&gt;
 /// &lt;param name="expression"&gt;&lt;/param&gt;
 /// &lt;param name="replacement"&gt;&lt;/param&gt;
 /// &lt;returns&gt;&lt;/returns&gt;
 public static string IsNull(string expression, string replacement)
 {
 if (string.IsNullOrWhiteSpace(expression))
 return replacement;
 else
 return expression;
 }
</code></pre>

blocks|key|1728854|text|++++public+static+T+IsNull<T>(this+T+DefaultValue,+T+InsteadValue)
++++{

++++++++object+obj="kk";

++++++++if((object)+DefaultValue+==+DBNull.Value)
++++++++{
++++++++++++obj+=+null;
++++++++}

++++++++if+(obj==null+%7C%7C+DefaultValue==null+%7C%7C+DefaultValue.ToString()=="")
++++++++{
++++++++++++return+InsteadValue;
++++++++}
++++++++else
++++++++{
++++++++++++return+DefaultValue;
++++++++}

++++}

//This+method+can+work+with+DBNull+and+null+value.+This+method+is+question's+answer|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1728855|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> public static T IsNull&lt;T&gt;(this T DefaultValue, T InsteadValue)
 {

 object obj="kk";

 if((object) DefaultValue == DBNull.Value)
 {
 obj = null;
 }

 if (obj==null || DefaultValue==null || DefaultValue.ToString()=="")
 {
 return InsteadValue;
 }
 else
 {
 return DefaultValue;
 }

 }

//This method can work with DBNull and null value. This method is question's answer
</code></pre>

blocks|key|1728662|text|这是半个笑话，因为这个问题有点傻。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1728663|public+static+bool+IsNull+(this+System.Object+o)
{
+++return+(o+==+null);
}|code-block|syntax|javascript|1728664|这是一个扩展方法，但是它扩展了System.Object，所以现在使用的每个对象都有一个IsNull()方法。|1728665|然后你可以通过这样做来节省大量的代码：|1728666|if+(foo.IsNull())|1728667|而不是超级差劲的：|1728668|if+(foo+==+null)|1728669|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

This is meant half as a joke, since the question is kinda silly.

<pre><code>public static bool IsNull (this System.Object o)
{
 return (o == null);
}
</code></pre>

This is an extension method, however it extends System.Object, so every object you use now has an IsNull() method.

Then you can save tons of code by doing:

<pre><code>if (foo.IsNull())
</code></pre>

instead of the super lame:

<pre><code>if (foo == null)
</code></pre>

In SQL Server you can use the <code>IsNull()</code> function to check if a value is null, and if it is, return another value. Now I am wondering if there is anything similar in C#.

For example, I want to do something like:

<pre><code>myNewValue = IsNull(myValue, new MyValue());
</code></pre>

instead of:

<pre><code>if (myValue == null)
 myValue = new MyValue();
myNewValue = myValue;
</code></pre>

Thanks.

C# equivalent of the IsNull() function in SQL Server

SQL Server

在SQL Server中，可以使用IsNull()函数检查某个值是否为null，如果是，则返回另一个值。现在我想知道在C#中是否有类似的东西。例如，我想要这样做：myNewValue = IsNull(myValue, new MyValue());而不是：if (myValue == null)  myValue = new MyValue();myNewValue = myValue;谢谢。

问SQL Server中IsNull()函数的C#等效项
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回答 10

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