blocks|key|3646145|text|因为上面我可以简单地写int？nVal+=+val
|type|blockquote|depth|inlineStyleRanges|entityRanges|data|3646146|unstyled|3646147|实际上，你也不能这么做。不存在从object到Nullable<int>的隐式转换。但是有一个从int到Nullable<int>的隐式转换，所以您可以这样写：|offset|length|style|CODE|3646148|int?+unVal+=+(int)val;|code-block|syntax|javascript|3646149|您可以使用Nullable.GetUnderlyingType方法。|3646150|返回指定可空类型的基础类型参数。
泛型类型定义是包含类型参数列表的类型声明(如Nullable+)，类型参数列表声明一个或多个类型参数。封闭泛型类型是为类型参数指定特定类型的类型声明。
|BOLD|3646151|3646152|Type+t+=+typeof(int?);+//will+get+this+dynamically
Type+u+=+Nullable.GetUnderlyingType(t);
object+val+=+5;+//will+get+this+dynamically
object+nVal+=+Convert.ChangeType(val,+u);//+nVal+will+be+5|3646153|这是一个DEMO。|3646154|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.nullable.getunderlyingtype.aspx|1|http://ideone.com/9t9ljU^0|0|0|G|6|N|D|1C|3|1G|D|0|0|5|Q|5|Q|0|0|9|6|0|0|0|4|4|4|4|1|0^^$0|@$1|2|3|4|5|6|7|18|8|@]|9|@]|A|$]]|$1|B|3|-4|5|C|7|19|8|@]|9|@]|A|$]]|$1|D|3|E|5|C|7|1A|8|@$F|1B|G|1C|H|I]|$F|1D|G|1E|H|I]|$F|1F|G|1G|H|I]|$F|1H|G|1I|H|I]]|9|@]|A|$]]|$1|J|3|K|5|L|7|1J|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|C|7|1K|8|@$F|1L|G|1M|H|I]]|9|@$F|1N|G|1O|1|1P]]|A|$]]|$1|Q|3|R|5|6|7|1Q|8|@$F|1R|G|1S|H|S]]|9|@]|A|$]]|$1|T|3|-4|5|C|7|1T|8|@]|9|@]|A|$]]|$1|U|3|V|5|L|7|1U|8|@]|9|@]|A|$M|N]]|$1|W|3|X|5|C|7|1V|8|@$F|1W|G|1X|H|I]]|9|@$F|1Y|G|1Z|1|20]]|A|$]]|$1|Y|3|-4|5|C|7|21|8|@]|9|@]|A|$]]]|Z|$10|$5|11|12|13|A|$14|15]]|16|$5|11|12|13|A|$14|17]]]]

<blockquote>
 For above I could simply write int? nVal = val
</blockquote>

Actually, you can't do that either. There is no implicit conversion from <code>object</code> to <code>Nullable&lt;int&gt;</code>. But there is an implicit conversion from <code>int</code> to <code>Nullable&lt;int&gt;</code> so you can write this:

<pre><code>int? unVal = (int)val;
</code></pre>

You can use <a href="http://msdn.microsoft.com/en-us/library/system.nullable.getunderlyingtype.aspx" rel="nofollow noreferrer"><code>Nullable.GetUnderlyingType</code></a> method.

<blockquote>
 Returns the underlying type argument of the specified nullable type.
 
 A generic type definition is a type declaration, such as Nullable,
 that contains a type parameter list, and the type parameter list
 declares one or more type parameters. A closed generic type is a type
 declaration where a particular type is specified for a type parameter.
</blockquote>

<pre><code>Type t = typeof(int?); //will get this dynamically
Type u = Nullable.GetUnderlyingType(t);
object val = 5; //will get this dynamically
object nVal = Convert.ChangeType(val, u);// nVal will be 5
</code></pre>

Here's a <a href="http://ideone.com/9t9ljU" rel="nofollow noreferrer"><code>DEMO</code></a>.

blocks|key|3646180|text|您必须使用Nullable.GetUnderlyingType获取底层类型的Nullable。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3646181|这是我用来克服ChangeType对Nullable的限制的方法。|3646182|public+static+T+ChangeType<T>(object+value)+
{
+++var+t+=+typeof(T);

+++if+(t.IsGenericType+&&+t.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))+
+++{
+++++++if+(value+==+null)+
+++++++{+
+++++++++++return+default(T);+
+++++++}

+++++++t+=+Nullable.GetUnderlyingType(t);
+++}

+++return+(T)Convert.ChangeType(value,+t);
}|code-block|syntax|javascript|3646183|非一般方法：|3646184|public+static+object+ChangeType(object+value,+Type+conversion)+
{
+++var+t+=+conversion;

+++if+(t.IsGenericType+&&+t.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))+
+++{
+++++++if+(value+==+null)+
+++++++{+
+++++++++++return+null;+
+++++++}

+++++++t+=+Nullable.GetUnderlyingType(t);
+++}

+++return+Convert.ChangeType(value,+t);
}|3646185|entityMap^0|5|Q|12|8|0|7|A|I|8|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

You have to use <code>Nullable.GetUnderlyingType</code> to get underlying type of <code>Nullable</code>. 

This is the method I use to overcome limitation of <code>ChangeType</code> for <code>Nullable</code>

<pre><code>public static T ChangeType&lt;T&gt;(object value) 
{
 var t = typeof(T);

 if (t.IsGenericType &amp;&amp; t.GetGenericTypeDefinition().Equals(typeof(Nullable&lt;&gt;))) 
 {
 if (value == null) 
 { 
 return default(T); 
 }

 t = Nullable.GetUnderlyingType(t);
 }

 return (T)Convert.ChangeType(value, t);
}
</code></pre>

non generic method:

<pre><code>public static object ChangeType(object value, Type conversion) 
{
 var t = conversion;

 if (t.IsGenericType &amp;&amp; t.GetGenericTypeDefinition().Equals(typeof(Nullable&lt;&gt;))) 
 {
 if (value == null) 
 { 
 return null; 
 }

 t = Nullable.GetUnderlyingType(t);
 }

 return Convert.ChangeType(value, t);
}
</code></pre>

blocks|key|3646224|text|我想我应该解释一下为什么这个功能不起作用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3646225|1-抛出异常的行如下：|3646226|throw+new+InvalidCastException(Environment.GetResourceString("InvalidCast_FromTo",+new+object[]
++{
++++value.GetType().FullName,+
++++targetType.FullName
++++}));|code-block|syntax|javascript|3646227|实际上，函数在数组Convert.ConvertTypes中搜索，然后它会查看该函数是否是Enum，当没有发现时，它会抛出上面的异常。|3646228|2-+Convert.ConvertTypes初始化为：|3646229|Convert.ConvertTypes+=+new+RuntimeType[]
+++{
++++++(RuntimeType)typeof(Empty),+
++++++(RuntimeType)typeof(object),+
++++++(RuntimeType)typeof(DBNull),+
++++++(RuntimeType)typeof(bool),+
++++++(RuntimeType)typeof(char),+
++++++(RuntimeType)typeof(sbyte),+
++++++(RuntimeType)typeof(byte),+
++++++(RuntimeType)typeof(short),+
++++++(RuntimeType)typeof(ushort),+
++++++(RuntimeType)typeof(int),+
++++++(RuntimeType)typeof(uint),+
++++++(RuntimeType)typeof(long),+
++++++(RuntimeType)typeof(ulong),+
++++++(RuntimeType)typeof(float),+
++++++(RuntimeType)typeof(double),+
++++++(RuntimeType)typeof(decimal),+
++++++(RuntimeType)typeof(DateTime),+
++++++(RuntimeType)typeof(object),+
++++++(RuntimeType)typeof(string)
+++};|3646230|因此，由于int?不在ConvertTypes数组中，而不是Enum中，因此抛出异常。|offset|length|style|CODE|3646231|因此，若要继续工作，Convert.ChnageType函数必须具备以下功能：|3646232|要转换的对象是IConvertible。|ordered-list-item|3646233|目标类型在ConvertTypes中，而不是Empty或DBNull中(除了抛出异常，这两种类型都有明确的测试)|3646234|3646235|3646236|这种行为是因为int+(以及所有其他默认类型)使用Convert.DefaultToType作为使用ILSpy的IConvertibale.ToType+implementation.+and+here+is+the+code+of+theDefaultToTypeextracted。|3646237|internal+static+object+DefaultToType(IConvertible+value,+Type+targetType,+IFormatProvider+provider)
{
++++if+(targetType+==+null)
++++{
++++++++throw+new+ArgumentNullException("targetType");
++++}
++++RuntimeType+left+=+targetType+as+RuntimeType;
++++if+(left+!=+null)
++++{
++++++++if+(value.GetType()+==+targetType)
++++++++{
++++++++++++return+value;
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[3])
++++++++{
++++++++++++return+value.ToBoolean(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[4])
++++++++{
++++++++++++return+value.ToChar(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[5])
++++++++{
++++++++++++return+value.ToSByte(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[6])
++++++++{
++++++++++++return+value.ToByte(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[7])
++++++++{
++++++++++++return+value.ToInt16(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[8])
++++++++{
++++++++++++return+value.ToUInt16(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[9])
++++++++{
++++++++++++return+value.ToInt32(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[10])
++++++++{
++++++++++++return+value.ToUInt32(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[11])
++++++++{
++++++++++++return+value.ToInt64(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[12])
++++++++{
++++++++++++return+value.ToUInt64(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[13])
++++++++{
++++++++++++return+value.ToSingle(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[14])
++++++++{
++++++++++++return+value.ToDouble(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[15])
++++++++{
++++++++++++return+value.ToDecimal(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[16])
++++++++{
++++++++++++return+value.ToDateTime(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[18])
++++++++{
++++++++++++return+value.ToString(provider);
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[1])
++++++++{
++++++++++++return+value;
++++++++}
++++++++if+(left+==+Convert.EnumType)
++++++++{
++++++++++++return+(Enum)value;
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[2])
++++++++{
++++++++++++throw+new+InvalidCastException(Environment.GetResourceString("InvalidCast_DBNull"));
++++++++}
++++++++if+(left+==+Convert.ConvertTypes[0])
++++++++{
++++++++++++throw+new+InvalidCastException(Environment.GetResourceString("InvalidCast_Empty"));
++++++++}
++++}
++++throw+new+InvalidCastException(Environment.GetResourceString("InvalidCast_FromTo",+new+object[]
++++{
++++++++value.GetType().FullName,+
++++++++targetType.FullName
++++}));
}|3646238|另一方面，强制转换是由Nullable类本身实现的，定义是：|3646239|public+static+implicit+operator+T?(T+value)
{
++++return+new+T?(value);
}
public+static+explicit+operator+T(T?+value)
{
++++return+value.Value;
}|3646240|entityMap^0|0|0|0|0|0|0|5|4|0|0|0|M|5|S|6|0|0|0|7|3|P|L|1E|5|25|17|3P|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1D|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1E|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1F|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1G|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|1H|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|1I|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|1J|8|@$Q|1K|R|1L|S|T]]|9|@]|A|$]]|$1|U|3|V|5|6|7|1M|8|@]|9|@]|A|$]]|$1|W|3|X|5|Y|7|1N|8|@]|9|@]|A|$]]|$1|Z|3|10|5|Y|7|1O|8|@$Q|1P|R|1Q|S|T]|$Q|1R|R|1S|S|T]]|9|@]|A|$]]|$1|11|3|-4|5|6|7|1T|8|@]|9|@]|A|$]]|$1|12|3|-4|5|6|7|1U|8|@]|9|@]|A|$]]|$1|13|3|14|5|6|7|1V|8|@$Q|1W|R|1X|S|T]|$Q|1Y|R|1Z|S|T]|$Q|20|R|21|S|T]|$Q|22|R|23|S|T]|$Q|24|R|25|S|T]]|9|@]|A|$]]|$1|15|3|16|5|F|7|26|8|@]|9|@]|A|$G|H]]|$1|17|3|18|5|6|7|27|8|@]|9|@]|A|$]]|$1|19|3|1A|5|F|7|28|8|@]|9|@]|A|$G|H]]|$1|1B|3|-4|5|6|7|29|8|@]|9|@]|A|$]]]|1C|$]]

I think I should explain why the function does not work:

1- The line that throw the exception is as follows:

<pre><code>throw new InvalidCastException(Environment.GetResourceString("InvalidCast_FromTo", new object[]
 {
 value.GetType().FullName, 
 targetType.FullName
 }));
</code></pre>

in fact the function search in the array Convert.ConvertTypes after that it see if the targer is an Enum and when nothing is found it throw the exception above.

2- the Convert.ConvertTypes is initialized as:

<pre><code>Convert.ConvertTypes = new RuntimeType[]
 {
 (RuntimeType)typeof(Empty), 
 (RuntimeType)typeof(object), 
 (RuntimeType)typeof(DBNull), 
 (RuntimeType)typeof(bool), 
 (RuntimeType)typeof(char), 
 (RuntimeType)typeof(sbyte), 
 (RuntimeType)typeof(byte), 
 (RuntimeType)typeof(short), 
 (RuntimeType)typeof(ushort), 
 (RuntimeType)typeof(int), 
 (RuntimeType)typeof(uint), 
 (RuntimeType)typeof(long), 
 (RuntimeType)typeof(ulong), 
 (RuntimeType)typeof(float), 
 (RuntimeType)typeof(double), 
 (RuntimeType)typeof(decimal), 
 (RuntimeType)typeof(DateTime), 
 (RuntimeType)typeof(object), 
 (RuntimeType)typeof(string)
 };
</code></pre>

So since the <code>int?</code> is not in the ConvertTypes array and not an Enum the exception is thrown.

So to resume, for the function Convert.ChnageType to work you have:

<ol>
<li>The object to be converted is IConvertible</li>
<li>The target type is within the ConvertTypes and not <code>Empty</code> nor <code>DBNull</code> (There is an explict test on those two with throw exception)</li>
</ol>

This behaviour is because <code>int</code> (and all other default types) uses <code>Convert.DefaultToType</code> as IConvertibale.ToType <code>implementation. and here is the code of the</code>DefaultToType<code>extracted</code> using <code>ILSpy</code>

<pre><code>internal static object DefaultToType(IConvertible value, Type targetType, IFormatProvider provider)
{
 if (targetType == null)
 {
 throw new ArgumentNullException("targetType");
 }
 RuntimeType left = targetType as RuntimeType;
 if (left != null)
 {
 if (value.GetType() == targetType)
 {
 return value;
 }
 if (left == Convert.ConvertTypes[3])
 {
 return value.ToBoolean(provider);
 }
 if (left == Convert.ConvertTypes[4])
 {
 return value.ToChar(provider);
 }
 if (left == Convert.ConvertTypes[5])
 {
 return value.ToSByte(provider);
 }
 if (left == Convert.ConvertTypes[6])
 {
 return value.ToByte(provider);
 }
 if (left == Convert.ConvertTypes[7])
 {
 return value.ToInt16(provider);
 }
 if (left == Convert.ConvertTypes[8])
 {
 return value.ToUInt16(provider);
 }
 if (left == Convert.ConvertTypes[9])
 {
 return value.ToInt32(provider);
 }
 if (left == Convert.ConvertTypes[10])
 {
 return value.ToUInt32(provider);
 }
 if (left == Convert.ConvertTypes[11])
 {
 return value.ToInt64(provider);
 }
 if (left == Convert.ConvertTypes[12])
 {
 return value.ToUInt64(provider);
 }
 if (left == Convert.ConvertTypes[13])
 {
 return value.ToSingle(provider);
 }
 if (left == Convert.ConvertTypes[14])
 {
 return value.ToDouble(provider);
 }
 if (left == Convert.ConvertTypes[15])
 {
 return value.ToDecimal(provider);
 }
 if (left == Convert.ConvertTypes[16])
 {
 return value.ToDateTime(provider);
 }
 if (left == Convert.ConvertTypes[18])
 {
 return value.ToString(provider);
 }
 if (left == Convert.ConvertTypes[1])
 {
 return value;
 }
 if (left == Convert.EnumType)
 {
 return (Enum)value;
 }
 if (left == Convert.ConvertTypes[2])
 {
 throw new InvalidCastException(Environment.GetResourceString("InvalidCast_DBNull"));
 }
 if (left == Convert.ConvertTypes[0])
 {
 throw new InvalidCastException(Environment.GetResourceString("InvalidCast_Empty"));
 }
 }
 throw new InvalidCastException(Environment.GetResourceString("InvalidCast_FromTo", new object[]
 {
 value.GetType().FullName, 
 targetType.FullName
 }));
}
</code></pre>

in other hand the cast is implemented by Nullable class itself and the definition is:

<pre><code>public static implicit operator T?(T value)
{
 return new T?(value);
}
public static explicit operator T(T? value)
{
 return value.Value;
}
</code></pre>

<pre><code>Type t = typeof(int?); //will get this dynamically
object val = 5; //will get this dynamically
object nVal = Convert.ChangeType(val, t);//getting exception here
</code></pre>

I am getting InvalidCastException in above code. For above I could simply write <code>int? nVal = val</code>, but above code is executing dynamically. 

I am getting a value(of non nullable type like int, float, etc) wrapped up in an object (here val), and I have to save it to another object by casting it to another type(which can or cannot be nullable version of it). When 

<blockquote>
 Invalid cast from 'System.Int32' to 'System.Nullable`1[[System.Int32,
 mscorlib, Version=4.0.0.0, Culture=neutral,
 PublicKeyToken=b77a5c561934e089]]'.
</blockquote>

An <code>int</code>, should be convertible/type-castable to <code>nullable int</code>, what is the issue here ?

Invalid cast from 'System.Int32' to 'System.Nullable`1[[System.Int32, mscorlib]]

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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Type t = typeof(int?); //will get this dynamicallyobject val = 5; //will get this dynamicallyobject nVal = Convert.ChangeType(val, t);//getting exception here我在...

问无效的从“System.Int32”到“System.Nullable`1[[System.Int32，mscorlib]]的强制转换
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问无效的从“System.Int32”到“System.Nullable`1[[System.Int32，mscorlib]]的强制转换EN