Dispatcher.BeginInvoke: Cannot convert lambda to System.Delegate

Question

I'm trying to call System.Windows.Threading.Dispatcher.BeginInvoke. The signature of the method is this:

BeginInvoke(Delegate method, params object[] args)

I'm trying to pass it a Lambda instead of having to create a Delegate.

_dispatcher.BeginInvoke((sender) => { DoSomething(); }, new object[] { this } );

It's giving me a compiler error saying that I

can't convert the lambda to a System.Delegate.

The signature of the delegate takes an object as a parameter and returns void. My lambda matches this, yet it's not working. What am I missing?

Does this answer your question? [Cannot convert lambda expression to type 'System.Delegate'](https://stackoverflow.com/questions/9549358/cannot-convert-lambda-expression-to-type-system-delegate) — StayOnTarget, Jan 22 '21 at 12:51

score 76 · Answer 1 · edited Jun 11 '14 at 23:03

76

Shorter:

_dispatcher.BeginInvoke((Action)(() => DoSomething()));

edited Jun 11 '14 at 23:03

abatishchev

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answered Oct 06 '11 at 01:50

Erwin Mayer

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8

Even shorter: I don't think you need the braces {} and semicolon around the expression. – sp3ctum May 16 '13 at 09:10
3

You don't even need (), so it can be `_dispatcher.BeginInvoke((Action)(DoSomething));` – mycroes May 19 '15 at 11:55
Or even _dispatcher.BeginInvoke(DoSomething); – Athanviel Apr 09 '21 at 14:51

score 73 · Accepted Answer · edited May 23 '17 at 12:34

73

Since the method takes a System.Delegate, you need to give it a specific type of delegate, declared as such. This can be done via a cast or a creation of the specified delegate via new DelegateType as follows:

_dispatcher.BeginInvoke(
     new Action<MyClass>((sender) => { DoSomething(); }),
     new object[] { this } 
  );

Also, as SLaks points out, Dispatcher.BeginInvoke takes a params array, so you can just write:

_dispatcher.BeginInvoke(
     new Action<MyClass>((sender) => { DoSomething(); }),
     this
  );

Or, if DoSomething is a method on this object itself:

_dispatcher.BeginInvoke(new Action(this.DoSomething));

edited May 23 '17 at 12:34

Community

1
1

answered Feb 08 '11 at 17:50

Reed Copsey

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But doesn't (x) => { DoSomething(); } match the signature of the delegate? I thought that's all I should have to specify. – Micah Feb 08 '11 at 17:53
@Micah: System.Delegate doesn't have a signature - it's just "any delegate" at all. You need to provide it a delegate type with a sig. that matches your usage. – Reed Copsey Feb 08 '11 at 17:54
@Reed But if in place of MyMethod(Action action) (and Action is a delegate) I can call MyMethod(() => { DoSomething(); }); Why can't I do the same thing for BeginInvoke? – Micah Feb 08 '11 at 17:56
16

@Micah: There actually *isn't* a signature for the delegate, which is what causes the issue. `Invoke` and `BeginInvoke` take a generic `Delegate` object, which can represent a method of any signature. Under normal circumstances (where a delegate is strongly typed to a particular signature), the compiler can infer the specific delegate type. This is why you're able to get away with omitting the delegate type in other scenarios. However, since there *is* no actual delegate type here, the compiler does not have a reasonable basis (or, really, even a *means*) to use to select a delegate type. – Adam Robinson Feb 08 '11 at 17:56
2

@Micah: Because BeginInvoke isn't declared as BeginInvoke(Action ..), but rather BeginInvoke(System.Delegate, ..) This allows it to use ANY delegate type, but you must specify it explicitly. – Reed Copsey Feb 08 '11 at 18:00
I'd love to know why they did this! Invoke works in a sane way: Invoke(()=>{}); Why does BeginInvoke force you to do all this? I'll give all DispatcherObjects a comfortable extension method anyways, but still, WHY? – ASA Jul 15 '15 at 09:04

score 10 · Answer 3 · answered Jan 07 '16 at 16:10

10

Using Inline Lambda...

Dispatcher.BeginInvoke((Action)(()=>{
  //Write Code Here
}));

answered Jan 07 '16 at 16:10

JWP

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logicnet.dk · Answer 4 · 2017-02-22T21:06:39.617

7

If you reference System.Windows.Presentation.dll from your project and add using System.Windows.Threading then you can access an extension method that allows you to use the lambda syntax.

using System.Windows.Threading;

...

Dispatcher.BeginInvoke(() =>
{
});

edited Feb 22 '17 at 21:06

answered Nov 24 '14 at 11:16

logicnet.dk

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I cannot get this to work. Could you go a little more into detail? – Tim Pohlmann Feb 20 '17 at 13:21
I've added a simple example. Remember to reference System.Windows.Presentation.dll – logicnet.dk Feb 22 '17 at 21:07
That's exactly what I but now it works...weird. Maybe I did something wrong last time. – Tim Pohlmann Feb 23 '17 at 07:40

score 5 · Answer 5 · answered May 17 '18 at 17:14

5

We create extension methods for this. E.g.

public static void BeginInvoke(this Control control, Action action)
    => control.BeginInvoke(action);

Now we can call it from within a form: this.BeginInvoke(() => { ... }).

answered May 17 '18 at 17:14

Shaun Luttin

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Dispatcher.BeginInvoke: Cannot convert lambda to System.Delegate

5 Answers5

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