Type.IsAssignableFrom Method (Type)

 

Determines whether an instance of a specified type can be assigned to an instance of the current type.

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

Public Overridable Function IsAssignableFrom (
	c As Type
) As Boolean

Parameters

c
Type: System.Type

The type to compare with the current type.

Return Value

Type: System.Boolean

true if any of the following conditions is true:

  • c and the current instance represent the same type.

  • c is derived either directly or indirectly from the current instance. c is derived directly from the current instance if it inherits from the current instance; c is derived indirectly from the current instance if it inherits from a succession of one or more classes that inherit from the current instance.

  • The current instance is an interface that c implements.

  • c is a generic type parameter, and the current instance represents one of the constraints of c.

    In the following example, the current instance is a Type object that represents the Stream class. GenericWithConstraint is a generic type whose generic type parameter must be of type Stream. Passing its generic type parameter to the IsAssignableFrom indicates that an instance of the generic type parameter can be assigned to an Stream object.

    Imports System.IO
    
    Module Example
       Public Sub Main()
          Dim t As Type = GetType(Stream)
          Dim genericT As Type = GetType(GenericWithConstraint(Of ))
          Dim genericParam As Type = genericT.GetGenericArguments()(0)
          Console.WriteLine(t.IsAssignableFrom(genericParam))  
          ' Displays True.
       End Sub
    End Module
    
    Public Class GenericWithConstraint(Of T As Stream)
    End Class
    
  • c represents a value type, and the current instance represents Nullable<c> (Nullable(Of c) in Visual Basic).

false if none of these conditions are true, or if c is null.

The IsAssignableFrom method can be used to determine whether an instance of c can be assigned to an instance of the current type, The method is most useful when you are handling objects whose types are not known at design time and allows for conditional assignment, as the following example shows.

Imports System.Collections

Module Example
   Public Sub Main()
      Dim t As Type = GetType(IEnumerable)
      Dim c As Type = GetType(Array)

      Dim instanceOfT As IEnumerable
      Dim instanceOfC As Integer() = { 1, 2, 3, 4 }
      If t.IsAssignableFrom(c) Then
         instanceOfT = instanceOfC
      End If  
   End Sub
End Module

This method thus ensures that a line of code like the following will execute at runtime without throwing an InvalidCastException exception or a similar exception:

instanceOfT = instanceOfC

This method can be overridden by a derived class.

System_CAPS_noteNote

A generic type definition is not assignable from a closed constructed type. That is, you cannot assign the closed constructed type MyGenericList<int> (MyGenericList(Of Integer) in Visual Basic) to a variable of type MyGenericList<T>.

If the c parameter is of type TypeBuilder, the result is based on the type that is to be built. The following code example demonstrates this using a built type named B.

Imports System.Reflection
Imports System.Reflection.Emit

Public Class A
End Class

Module Example
   Public Sub Main()
      Dim domain As AppDomain = AppDomain.CurrentDomain
      Dim assemName As New AssemblyName()
      assemName.Name = "TempAssembly"

      ' Define a dynamic assembly in the current application domain.
      Dim assemBuilder As AssemblyBuilder = domain.DefineDynamicAssembly(assemName,
                                                   AssemblyBuilderAccess.Run)

      ' Define a dynamic module in this assembly.
      Dim moduleBuilder As ModuleBuilder = assemBuilder.DefineDynamicModule("TempModule")

      Dim b1 As TypeBuilder = moduleBuilder.DefineType("B", TypeAttributes.Public, GetType(A))
      Console.WriteLine(GetType(A).IsAssignableFrom(b1))
   End Sub
End Module
' The example displays the following output:
'       True

The following example demonstrates the IsAssignableFrom method using defined classes, integer arrays, and generics.

Imports System.Collections.Generic

Module Example
    Public Sub Main()
        Console.WriteLine("Defined Classes:")
        Dim room1 As New Room()
        Dim kitchen1 As New Kitchen()
        Dim bedroom1 As New Bedroom()
        Dim guestroom1 As New Guestroom()
        Dim masterbedroom1 As New MasterBedroom()

        Dim room1Type As Type = room1.GetType()
        Dim kitchen1Type As Type = kitchen1.GetType()
        Dim bedroom1Type As Type = bedroom1.GetType()
        Dim guestroom1Type As Type = guestroom1.GetType()
        Dim masterbedroom1Type As Type = masterbedroom1.GetType()

        Console.WriteLine("room assignable from kitchen: {0}", room1Type.IsAssignableFrom(kitchen1Type))
        Console.WriteLine("bedroom assignable from guestroom: {0}", bedroom1Type.IsAssignableFrom(guestroom1Type))
        Console.WriteLine("kitchen assignable from masterbedroom: {0}", kitchen1Type.IsAssignableFrom(masterbedroom1Type))

        ' Demonstrate arrays:
        Console.WriteLine()
        Console.WriteLine("Integer arrays:")

        Dim array10(10) As Integer
        Dim array2(2) As Integer
        Dim array22(2, 2) As Integer
        Dim array24(2, 4) As Integer

        Dim array10Type As Type = array10.GetType
        Dim array2Type As Type = array2.GetType
        Dim array22Type As Type = array22.GetType
        Dim array24Type As Type = array24.GetType

        Console.WriteLine("Integer(2) assignable from Integer(10): {0}", array2Type.IsAssignableFrom(array10Type))
        Console.WriteLine("Integer(2) assignable from Integer(2,4): {0}", array2Type.IsAssignableFrom(array24Type))
        Console.WriteLine("Integer(2,4) assignable from Integer(2,2): {0}", array24Type.IsAssignableFrom(array22Type))

        ' Demonstrate generics:
        Console.WriteLine()
        Console.WriteLine("Generics:")

        Dim arrayNull(10) As Nullable(Of Integer)
        Dim genIntList As New List(Of Integer)
        Dim genTList As New List(Of Type)

        Dim arrayNullType As Type = arrayNull.GetType
        Dim genIntListType As Type = genIntList.GetType
        Dim genTListType As Type = genTList.GetType

        Console.WriteLine("Integer(10) assignable from Nullable(Of Integer)(10): {0}", array10Type.IsAssignableFrom(arrayNullType))
        Console.WriteLine("List(Of Integer) assignable from List(Of Type): {0}", genIntListType.IsAssignableFrom(genTListType))
        Console.WriteLine("List(Of Type) assignable from List(Of Integer): {0}", genTListType.IsAssignableFrom(genIntListType))
        Console.ReadLine()
    End Sub
End Module

Class Room
End Class

Class Kitchen : Inherits Room
End Class

Class Bedroom : Inherits Room
End Class

Class Guestroom : Inherits Bedroom
End Class

Class MasterBedroom : Inherits Bedroom
End Class
' The example displays the following output:
'    Defined Classes:
'    room assignable from kitchen: True
'    bedroom assignable from guestroom: True
'    kitchen assignable from masterbedroom: False
'
'    Integer arrays:
'    Integer(2) assignable from Integer(10): True
'    Integer(2) assignable from Integer(2,4): False
'    Integer(2,4) assignable from Integer(2,2): True
'
'    Generics:
'    Integer(10) assignable from Nullable(Of Integer)(10): False
'    List(Of Integer) assignable from List(Of Type): False
'    List(Of Type) assignable from List(Of Integer): False

.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Return to top
Show: