Calling the BeginInvoke method of a delegate will allocate some resources that are only freed-up when EndInvoke is called. This is why you should always pair BeginInvoke with an EndInvoke to complete your asynchronous call.

This rule raises an issue when:

• 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 doesn’t contain a call to EndInvoke.

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

See

Calling Synchronous Methods Asynchronously