Why is this an issue?

When calling the BeginInvoke method of a delegate, resources are allocated that are only freed up when EndInvoke is called. Failing to pair BeginInvoke with EndInvoke can lead to resource leaks and incomplete asynchronous calls.

This rule raises an issue in the following scenarios:

• The BeginInvoke method is called without any callback, and it is not paired with a call to EndInvoke in the same block.
• A callback with a single parameter of type IAsyncResult does not contain a call to EndInvoke in the same block.

How to fix it

Code examples

Noncompliant code example

BeginInvoke without callback:

Public Delegate Function AsyncMethodCaller() As String

Public Class Sample

    Public Sub DoSomething()
        Dim Example As New AsyncExample()
        Dim Caller As New AsyncMethodCaller(Example.SomeMethod)
        ' Initiate the asynchronous call.
        Dim Result As IAsyncResult = Caller.BeginInvoke(Nothing, Nothing) ' Noncompliant: Not paired With EndInvoke
    End Sub

End Class

BeginInvoke with callback:

Public Delegate Function AsyncMethodCaller() As String

Public Class Sample

    Public Sub DoSomething()
        Dim Example As New AsyncExample()
        Dim Caller As New AsyncMethodCaller(Example.SomeMethod)
        ' Initiate the asynchronous call.
        Dim Result As IAsyncResult = Caller.BeginInvoke(New AsyncCallback(Sub(ar)
                                                                          End Sub), Nothing) ' Noncompliant: Not paired With EndInvoke
    End Sub

End Class

Compliant solution

BeginInvoke without callback:

Public Delegate Function AsyncMethodCaller() As String

Public Class Sample

    Public Function DoSomething() As String
        Dim Example As New AsyncExample()
        Dim Caller As New AsyncMethodCaller(Example.SomeMethod)
        ' Initiate the asynchronous call.
        Dim Result As IAsyncResult = Caller.BeginInvoke(Nothing, Nothing)
        ' ...
        Return Caller.EndInvoke(Result)
    End Function

End Class

BeginInvoke with callback:

Public Delegate Function AsyncMethodCaller() As String

Public Class Sample

    Public Sub DoSomething()
        Dim Example As New AsyncExample()
        Dim Caller As New AsyncMethodCaller(Example.SomeMethod)
        ' Initiate the asynchronous call.
        Dim Result As IAsyncResult = Caller.BeginInvoke(New AsyncCallback(Sub(ar)
                                                                              ' Call EndInvoke to retrieve the results.
                                                                              Dim Ret As String = Caller.EndInvoke(ar)
                                                                              ' ...
                                                                          End Sub), Nothing)
    End Sub

End Class

Resources

Documentation

• Calling Synchronous Methods Asynchronously
• Asynchronous Programming Using Delegates
• BeginInvoke()
• EndInvoke()
• AsyncCallback Delegate