IteratorStateMachineAttribute Class

.NET Framework 4.6 and 4.5

Indicates whether a method in Visual Basic is marked with the Iterator modifier.

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


[AttributeUsageAttribute(AttributeTargets.Method, Inherited = false, 
	AllowMultiple = false)]
public sealed class IteratorStateMachineAttribute : StateMachineAttribute


Initializes a new instance of the IteratorStateMachineAttribute class.


Returns the type object for the underlying state machine type that was generated by the compiler to implement the state machine method.(Inherited from StateMachineAttribute.)


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from Attribute.)

You shouldn't apply the IteratorStateMachine attribute to methods in your code. For methods in Visual Basic that have the Iterator (Visual Basic) modifier, the compiler will apply the IteratorStateMachine attribute in the IL that it emits.

When a method (MethodName) in Visual Basic has the Iterator modifier, the compiler emits IL that includes a state machine structure. That structure contains the code in the method. That IL also contains a stub method (MethodName) that calls into the state machine. The compiler adds the IteratorStateMachine attribute to the stub method so that tools can identify the corresponding state machine. Details of the emitted IL might change in future releases of the compilers.

An iterator method performs a custom iteration over a collection by using the Yield (Visual Basic) statement to return each element one at a time. For more information, see Iterators (C# and Visual Basic).


You can't use IteratorStateMachineAttribute to test whether a method is an iterator method in C#.

The following example shows how to determine whether a method is an iterator method. In the example, IsIteratorMethod performs the following steps:

  • Obtains a MethodInfo object for the method name by using Type.GetMethod.

  • Obtains a Type object for the attribute by using GetType Operator (Visual Basic).

  • Obtains an attribute object for the method and attribute type by using MethodInfo.GetCustomAttribute. If GetCustomAttribute returns Nothing (Visual Basic), the method doesn't contain the attribute.

Imports System
Imports System.Collections.Generic
Imports System.Reflection
Imports System.Threading.Tasks
Imports System.Runtime.CompilerServices

Module Module1

    ' This class is used by the code below to discover method attributes.
    Public Class TheClass
        Public Async Function AsyncMethod() As Task(Of Integer)
            Await Task.Delay(5)
            Return 1
        End Function

        Public Iterator Function IteratorMethod() As IEnumerable(Of Integer)
            Yield 1
            Yield 2
        End Function

        Public Function RegularMethod() As Integer
            Return 0
        End Function
    End Class

    Private Function IsAsyncMethod(classType As Type, methodName As String)
        ' Obtain the method with the specified name.
        Dim method As MethodInfo = classType.GetMethod(methodName)

        Dim attType As Type = GetType(AsyncStateMachineAttribute)

        Dim attrib = CType(method.GetCustomAttribute(attType), AsyncStateMachineAttribute)
        ' The above variable contains the StateMachineType property.

        Return (attrib IsNot Nothing)
    End Function

    Private Function IsIteratorMethod(classType As Type, methodName As String)
        ' Obtain the method with the specified name.
        Dim method As MethodInfo = classType.GetMethod(methodName)

        Dim attType As Type = GetType(IteratorStateMachineAttribute)

        ' Obtain the custom attribute for the method.
        ' The value returned contains the StateMachineType property.
        ' Nothing is returned if the attribute isn't present for the method.
        Dim attrib = CType(method.GetCustomAttribute(attType), IteratorStateMachineAttribute)

        Return (attrib IsNot Nothing)
    End Function

    Private Sub ShowResult(classType As Type, methodName As String)
        Console.Write((methodName & ": ").PadRight(16))

        If IsAsyncMethod(classType, methodName) Then
            Console.WriteLine("Async method")
        ElseIf IsIteratorMethod(classType, methodName) Then
            Console.WriteLine("Iterator method")
            Console.WriteLine("Regular method")
        End If

        ' Note: The IteratorStateMachineAttribute applies to Visual Basic methods
        ' but not C# methods.
    End Sub

    Sub Main()
        ShowResult(GetType(TheClass), "AsyncMethod")
        ShowResult(GetType(TheClass), "IteratorMethod")
        ShowResult(GetType(TheClass), "RegularMethod")

    End Sub

    '   AsyncMethod:    Async method
    '   IteratorMethod: Iterator method
    '   RegularMethod:  Regular method

End Module

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 4.5
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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