Making Extension method Generic

Question

In this post there's a very interesting way of updating UI threads using a static extension method.

public static void InvokeIfRequired(this Control c, Action<Control> action)
{
    if(c.InvokeRequired)
    {
        c.Invoke(() => action(c));
    }
    else
    {
        action(c);
    }
}

What I want to do, is to make a generic version, so I'm not constrained by a control. This would allow me to do the following for example (because I'm no longer constrained to just being a Control)

this.progressBar1.InvokeIfRequired(pb => pb.Value = e.Progress);

I've tried the following:

  public static void InvokeIfRequired<T>(this T c, Action<T> action) where T : Control
    {
        if (c.InvokeRequired)
        {
            c.Invoke(() => action(c));
        }
        else
        {
            action(c);
        }
    }

But I get the following error that I'm not sure how to fix. Anyone any suggestions?

Error 5 Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type

In fact... looking a bit closer, I seem to get the error on the first code example too... although I'm actually compiling for .NET 4.0 at the moment. — Ian, Jun 12 '10 at 14:38

score 4 · Accepted Answer · answered Jun 12 '10 at 17:07

4

replace :

c.Invoke(() => action(c));

with :

c.Invoke(action, c);

answered Jun 12 '10 at 17:07

Thomas Levesque

286,951
70
623
758

score 0 · Answer 2 · edited May 23 '17 at 12:01

0

This is a well known error with lambdas and anonymous methods:

Convert this delegate to an anonymous method or lambda

Your code just needs a cast to compile:

public static void InvokeIfRequired<T>(this T c, Action<T> action) where T : Control
{
    if (c.InvokeRequired)
    {
        c.Invoke((Action<T>)((control) => action(control)));
    }
    else
    {
        action(c);
    }
}

edited May 23 '17 at 12:01

Community

1
1

answered Jun 12 '10 at 14:54

bbudge

1,127
6
7

won't work either. You need to provide the action's argument to the `Invoke` method – Thomas Levesque Jun 12 '10 at 17:09

ckramer · Answer 3 · 2010-06-12T17:27:30.193

0

Try this slight varient:

public static void InvokeIfRequired<T>(this T c, Action<T> action) where T : Control
{
    if (c.InvokeRequired)
    {
        c.Invoke((Action<T>)(() => action(c)));
    }
    else
    {
        action(c);
    }
}

You need to cast it as a Delegate type. Kinda stupid I know. I can't really give you a good reason why a lambda expression isn't implicitly assignable as a delegate.

edited Jun 12 '10 at 17:27

answered Jun 12 '10 at 14:54

ckramer

9,419
1
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It won't work, because `action` takes a parameter, and you're not passing it to `Invoke` – Thomas Levesque Jun 12 '10 at 17:08
good point. edited to account for that. for some reason I was thinking Invoke passed in the current control, which is totally not the case. – ckramer Jun 12 '10 at 17:28

Making Extension method Generic

3 Answers3