blocks|key|260095|text|因为T可以是引用类型，也可以是值类型，所以如果T是值类型，返回null就不能满足。你应该返回：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|260096|return+default(T);|code-block|syntax|javascript|260097|从链接默认关键字|260098|给定参数化类型T的变量T，只有当T是引用类型时，t=+null语句才有效，并且t=0只适用于数值类型，而不适用于结构。解决方案是使用默认关键字，对于引用类型返回null，对于数值类型返回零。对于结构，它将返回初始化为零或空的结构的每个成员，这取决于它们是值类型还是引用类型。对于可空值类型，default返回一个System.Nullable，它与任何结构一样被初始化。|blockquote|260099|更新(2021-09-27)|260100|使用C#+7.1中的新语法，您可以返回：|260101|return+default;|260102|entityMap|0|LINK|mutability|MUTABLE|url|https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/default^0|2|1|V|4|0|0|3|5|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|19|8|@]|D|@$9|1A|A|1B|1|1C]]|E|$]]|$1|M|3|N|5|O|7|1D|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|1E|8|@]|D|@]|E|$]]|$1|R|3|S|5|6|7|1F|8|@]|D|@]|E|$]]|$1|T|3|U|5|H|7|1G|8|@]|D|@]|E|$I|J]]|$1|V|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|W|$X|$5|Y|Z|10|E|$11|12]]]]

Since <code>T</code> can be either reference type or value type, so returning <code>null</code> will not satisfy if T is value type. You should return:
<pre><code>return default(T);
</code></pre>
From the link <a href="https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/default" rel="noreferrer">default keyword</a>:
<blockquote>
Given a variable t of a parameterized type T, the statement t = null is only valid if T is a reference type and t = 0 will only work for numeric value types but not for structs. The solution is to use the default keyword, which will return null for reference types and zero for numeric value types. For structs, it will return each member of the struct initialized to zero or null depending on whether they are value or reference types. For nullable value types, default returns a System.Nullable, which is initialized like any struct.
</blockquote>
Update (2021-09-27)
With the new syntax from C# 7.1, you can be able to return:
<pre><code>return default;
</code></pre>

blocks|key|260183|text|对于C#，您必须使用默认(T)，如果只返回T而不返回T，则它将返回0而不是零或空。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|260184|现在，在VB.net的情况下，如您的示例所示，您说不返回任何内容，这是正确的，但是您是否试图检查该函数的返回值。|260185|喜欢|260186|Dim+c+=+<<your+class+object>>.FindItem("test",Nothing);++//+you+have+to+pass+valid+values.|code-block|syntax|javascript|260187|如果您检查C的值，那么它是0，而不是零。|260188|现在到了MSIL级别所发生的事情。|260189|当您编译VB.net代码并检查其MSIL时。您将找到initobj+!T，这是default(T)指令。这是自动完成的。因此，如果您从VB.net调用C#库，那么它也会工作。|offset|length|style|BOLD|260190|entityMap^0|0|0|0|0|0|0|Q|A|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|Z|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|12|8|@$Q|13|R|14|S|T]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

In case of C# you have to use default(T) and it will return 0 not Nothing or Null if you just return T and not T?.

Now In case of VB.net as per your example you say that you return Nothing which is correct but did you try to check return value of that function.

Like 

<pre><code>Dim c = &lt;&lt;your class object&gt;&gt;.FindItem("test",Nothing); // you have to pass valid values.
</code></pre>

If you check value of C then it is 0 and not nothing.

Now comes to what happen at MSIL level. 

When you compile you VB.net code and check its MSIL. you will find initobj !T and this is instruction for default(T). It is doing this automatically. So if you call your VB.net library from C# then also it will work.

blocks|key|2074650|text|你可以用这个方法|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2074651|public+static+IList<int>+ToIntList(this+object+values)
++++++++{
++++++++++++//try+to+convert+using+modified+ChangeType+which+handles+nullables
++++++++++++try
++++++++++++{
++++++++++++++++if+(values+==+null)+return+new+List<int>();

++++++++++++++++var+charValues+=+values.ToType<string>();
++++++++++++++++return+charValues.Split(',').Select(n+=>+Convert.ToInt32(n)).ToList();
++++++++++++}
++++++++++++//if+there+is+an+exception+then+get+the+default+value+of+parameterized+type+T
++++++++++++catch+(Exception)
++++++++++++{
++++++++++++++++return+new+List<int>();
++++++++++++}
++++++++}

internal+static+object+ChangeType(object+value,+Type+conversionType)
++++++++{
++++++++++++if+(conversionType+==+null)
++++++++++++{
++++++++++++++++throw+new+ArgumentNullException("conversionType");
++++++++++++}+

++++++++++++if+(conversionType.IsGenericType+&&+conversionType.GetGenericTypeDefinition()+==+typeof+(Nullable<>))
++++++++++++{
++++++++++++++++if+(value+==+null)
++++++++++++++++{
++++++++++++++++++++return+null;
++++++++++++++++}+
++++++++++++++++var+nullableConverter+=+new+NullableConverter(conversionType);
++++++++++++++++conversionType+=+nullableConverter.UnderlyingType;
++++++++++++}
++++++++++++return+Convert.ChangeType(value,+conversionType);
++++++++}|code-block|syntax|javascript|2074652|像这样用|2074653|userVar.ToType<int>()
userVar.ToType<string>()|2074654|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 used this method

<pre><code>public static IList&lt;int&gt; ToIntList(this object values)
 {
 //try to convert using modified ChangeType which handles nullables
 try
 {
 if (values == null) return new List&lt;int&gt;();

 var charValues = values.ToType&lt;string&gt;();
 return charValues.Split(',').Select(n =&gt; Convert.ToInt32(n)).ToList();
 }
 //if there is an exception then get the default value of parameterized type T
 catch (Exception)
 {
 return new List&lt;int&gt;();
 }
 }

internal static object ChangeType(object value, Type conversionType)
 {
 if (conversionType == null)
 {
 throw new ArgumentNullException("conversionType");
 } 

 if (conversionType.IsGenericType &amp;&amp; conversionType.GetGenericTypeDefinition() == typeof (Nullable&lt;&gt;))
 {
 if (value == null)
 {
 return null;
 } 
 var nullableConverter = new NullableConverter(conversionType);
 conversionType = nullableConverter.UnderlyingType;
 }
 return Convert.ChangeType(value, conversionType);
 }
</code></pre>

Used like this

<pre><code>userVar.ToType&lt;int&gt;()
userVar.ToType&lt;string&gt;()
</code></pre>

I'm converting a bunch of code from VB to C# and I'm running in to an issue with a method. This VB method works great:

<pre><code>Public Function FindItem(ByVal p_propertyName As String, ByVal p_value As Object) As T

 Dim index As Int32

 index = FindIndex(p_propertyName, p_value)

 If index &gt;= 0 Then
 Return Me(index)
 End If

 Return Nothing

End Function
</code></pre>

It allow the return of Nothing(null) for T.

The C# equivalent does not work:

<pre><code>public T FindItem(string p_propertyName, object p_value)
{
 Int32 index = FindIndex(p_propertyName, p_value);

 if (index &gt;= 0) {
 return this[index];
 }
 return null;
}
</code></pre>

It won't compile with this error: 

<blockquote>
 The type 'T' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method <code>'System.Nullable&lt;T&gt;'</code>
</blockquote>

I need to be able to have the same functionality or it will break a lot of code. What am I missing?

Why cannot convert null to type parameter T in c#?

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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我正在将一堆代码从VB转换到C#，我正在处理一个方法的问题。这个VB方法工作得很好：Public Function FindItem(ByVal p_propertyName As String, ByVal p_value As Object) As T    Dim index As Int32    index ...

问为什么不能在c#中将null转换为类型参数T？
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问为什么不能在c#中将null转换为类型参数T？EN