Question

I have a normal method

public List<string> FindNearByCity(string targetCity)
{
// ... some implementation
}

I want to add async support for this method, so i wrote this:

public IAsyncResult BeginFindNearByCity(string targetCity, AsyncCallback callback, object obj)
{
    Func<string, List<string>> method = FindNearByCity;
    return method.BeginInvoke(targetCity, callback, obj);
}

public List<string> EndFindNearByCity(IAsyncResult  result)
{
    Func<string, List<string>> method = FindNearByCity;
    return method.EndInvoke(result);
}

The BeginFindNearByCity works fine, however when it comes to EndFindNearByCity, exception will throw when it hit EndInvoke.

I look into the auto gen Async web service method, seems i need to implement something call “ChannelBase”

can anyone point me to something that more simple like tutorial or sample that i can have a look at?

Thanks

Answer 1

The delegate you are creating in your EndXXX method is a separate instance to the delegate you create in your BeginXXX method, therefore it has no knowledge of the IAsyncResult which you are passing to its EndInvoke() method.

You have to use the same delegate in the EndXXX method as the BeginXXX method, e.g.

public class Foo
{
    private readonly Func<string, List<string>> method;

    public Foo()
    {
        this.method = this.FindNearByCity;
    }

    public IAsyncResult BeginFindNearByCity(string targetCity, AsyncCallback callback, object obj)
    {
        return this.method.BeginInvoke(targetCity, callback, obj);
    }

    public List<string> EndFindNearByCity(IAsyncResult  result)
    {
        return this.method.EndInvoke(result);
    }

    public List<string> FindNearByCity(string targetCity)
    {
        // ... some implementation
    }
}