Reflection and Nullable

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Joe Bloggs - 13 Jul 2006 17:00 GMT

Hi,

Can someone please kindly show me how to determine if a type (read
value
type) is Nullable.

MSDN has this KB:
How to: Identify a Nullable Type (C# Programming Guide)
http://msdn2.microsoft.com/en-us/library/ms366789.aspx

however, using their code snippet, I couldn't get it to work:

public class MyClass
{
public static void Main()
{
Nullable<int> j = new Nullable<int>(20);
Type type = j.GetType();
if (type.IsGenericType && type.GetGenericTypeDefinition() ==
typeof(Nullable<>))
{
Console.WriteLine("j is Nullable");
}
else
Console.WriteLine("j is NOT Nullable");

Console.WriteLine("Type of j is {0}", j.GetType());
}
}

The Output I got is:

j is NOT Nullable
Type of j is System.Int32

thus, is there no way to determine a nullable or an non-nullable value

types?

Thanks for your help in advanced.

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Barry Kelly - 13 Jul 2006 19:13 GMT

> Can someone please kindly show me how to determine if a type (read
> value
> type) is Nullable.

I answered in your multipost in dotnet.general.

-- Barry

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http://barrkel.blogspot.com/

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Jon Skeet [C# MVP] - 13 Jul 2006 19:23 GMT

> Can someone please kindly show me how to determine if a type (read
> value type) is Nullable.

Finding out if a type is nullable is fine, but you can't find out from
a value in the way you're doing. This is because calling GetType()
involves boxing the nullable value. Now, from the C# spec:

<quote>
A boxing conversion from a nullable type T? is processed as follows:

* If the source value is null (HasValue property is false), the
result is a null reference of the target type.
* Otherwise, the result is a reference to a boxed T produced by
unwrapping and boxing the source value.
</quote>

In other words, by the time you call GetType(), you're either calling
it on null (resulting in an exception) or on the boxed non-nullable
value type.

Now, do you actually need it from a Type object? If so, the test you
showed works fine. (Try it with typeof(Nullable<int>).)

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Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
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Reflection and Nullable

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