GetUnderlyingType Method
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Nullable.GetUnderlyingType Method

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Returns the underlying type argument of the specified nullable type.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public static Type GetUnderlyingType(
	Type nullableType


Type: System.Type
A Type object that describes a closed generic nullable type.

Return Value

Type: System.Type
The type argument of the nullableType parameter, if the nullableType parameter is a closed generic nullable type; otherwise, null.


nullableType is null.

A generic type definition is a type declaration, such as Nullable<T>, 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.

For example, if the nullableType parameter is the type of Nullable<Int32> in C# (Nullable(Of Int32) in Visual Basic), the return value is the type of Int32 (that is, the type argument of the closed generic type).

The following code example defines a method whose return value is of type Nullable<T> of Int32. The code example uses the GetUnderlyingType method to display the type argument of the return value.

// This code example demonstrates the
// Nullable.GetUnderlyingType() method.

using System;
using System.Reflection;

class Example
      Use reflection to obtain a Type object for the Sample type.
      Use the Type object to obtain a MethodInfo object for the MyMethod method.
      Use the MethodInfo object to obtain the type of the return value of
        MyMethod, which is Nullable of Int32.
      Use the GetUnderlyingType method to obtain the type argument of the
        return value type, which is Int32.
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
      Type t = typeof(Sample);
      MethodInfo mi = t.GetMethod("MyMethod");
      Type retval = mi.ReturnType;
      outputBlock.Text += String.Format("Return value type ... {0}", retval) + "\n";
      Type answer = Nullable.GetUnderlyingType(retval);
      outputBlock.Text += String.Format("Underlying type ..... {0}", answer) + "\n";

// Declare a type named Sample.
// The MyMethod member of Sample returns a Nullable of Int32.public class Sample
class Sample
    public int? MyMethod()
        return 0;
This code example produces the following results:

Return value type ... System.Nullable`1[System.Int32]
Underlying type ..... System.Int32


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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