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 string AsyncMethodCaller();

public static void Main()
{
    AsyncExample asyncExample = new AsyncExample();
    AsyncMethodCaller caller = new AsyncMethodCaller(asyncExample.MyMethod);

    // Initiate the asynchronous call.
    IAsyncResult result = caller.BeginInvoke(null, null); // Noncompliant - not paired with EndInvoke
}

BeginInvoke with callback

public delegate string AsyncMethodCaller();

public static void Main()
{
    AsyncExample asyncExample = new AsyncExample();
    AsyncMethodCaller caller = new AsyncMethodCaller(asyncExample.MyMethod);

    IAsyncResult result = caller.BeginInvoke(
        new AsyncCallback((IAsyncResult ar) => {}),
        null); // Noncompliant - not paired with EndInvoke
}

Compliant Solution

BeginInvoke without callback

public delegate string AsyncMethodCaller();

public static void Main()
{
    AsyncExample asyncExample = new AsyncExample();
    AsyncMethodCaller caller = new AsyncMethodCaller(asyncExample.MyMethod);

    IAsyncResult result = caller.BeginInvoke(null, null);

    string returnValue = caller.EndInvoke(result);
}

BeginInvoke with callback

public delegate string AsyncMethodCaller();

public static void Main()
{
    AsyncExample asyncExample = new AsyncExample();
    AsyncMethodCaller caller = new AsyncMethodCaller(asyncExample.MyMethod);

    IAsyncResult result = caller.BeginInvoke(
        new AsyncCallback((IAsyncResult ar) =>
            {
                // Call EndInvoke to retrieve the results.
                string returnValue = caller.EndInvoke(ar);
            }), null);
}

See

Calling Synchronous Methods Asynchronously