anonymous delegates in C#

问题:

I can't be the only one getting tired of defining and naming a delegate for just a single call to something that requires a delegate. For example, I wanted to call .Refresh() in a form from possibly other threads, so I wrote this code:

private void RefreshForm()
{
    if (InvokeRequired)
        Invoke(new InvokeDelegate(Refresh));
    else
        Refresh();
}

I'm not even sure I have to, I just read enough to be scared that it won't work at some later stage.
InvokeDelegate is actually declared in another file, but do I really need an entire delegate dedicated just for this? aren't there any generic delegates at all?
I mean, for example, there's a Pen class, but there's also Pens.pen-of-choice so you don't have to remake the whole thing. It's not the same, but I hope you understand what I mean.

回答1:

Yes. In .NET 3.5 you can use Func and Action delegates. The Func delegates return a value, while Action delegates return void. Here is what the type names would look like:

System.Func<TReturn> // (no arg, with return value)
System.Func<T, TReturn> // (1 arg, with return value)
System.Func<T1, T2, TReturn> // (2 arg, with return value)
System.Func<T1, T2, T3, TReturn> // (3 arg, with return value)
System.Func<T1, T2, T3, T4, TReturn> // (4 arg, with return value)

System.Action // (no arg, no return value)
System.Action<T> // (1 arg, no return value)
System.Action<T1, T2> // (2 arg, no return value)
System.Action<T1, T2, T3> // (3 arg, no return value)
System.Action<T1, T2, T3, T4> // (4 arg, no return value)

I don't know why they stopped at 4 args each, but it has always been enough for me.

回答2:

There's the Action delegate you could use, like so:

private void RefreshForm()
{
    if (InvokeRequired) Invoke(new Action(Refresh));
    else Refresh();
}

Or, with lambda syntax:

private void RefreshForm()
{
    if (InvokeRequired) Invoke((Action)(() => Refresh()));
    else Refresh();
}

Finally there's anonymous delegate syntax:

private void RefreshForm()
{
    if (InvokeRequired) Invoke((Action)(delegate { Refresh(); }));
    else Refresh();
}

回答3:

In this specific case you can (and should) just use MethodInvoker to do this... that is why it exists.

if (InvokeRequired)
    Invoke(new MethodInvoker(Refresh));
else
    Refresh();

If you were doing something else you could, as others have answered use Func<T,...> or Action<T,...> if they fit your use case.

回答4:

Short version:

Invoke((MethodInvoker)delegate { Refresh(); });

Then you can also drop the check of InvokeRequired; you can just call it as it is. Works also if you need to pass parameters, so there is no need for other parameter-specific delegates (works just as well with the parameter-less Action delegate as well):

private void SetControlText(Control ctl, string text)
{
    Invoke((MethodInvoker)delegate { ctl.Text = text; });
}

回答5:

Do I really need an entire delegate dedicated just for this? aren't there any generic delegates at all?

Defining your own delegates can really make debugging easier, if only because Intellisense can tell you the names of your parameters. For example, you write a delegate like this:

public delegate int UpdateDelegate(int userID, string city, string, state, string zip);

When you use it code, .NET will inform you of the parameter names, delegate name, etc, so there's a lot of context right in the delegate definition if you aren't sure exactly how something is used.

However, if you don't mind sacrificing Intellisense, there is already a class of delegates definined in the System namespace which can be used as ad-hoc delegates:

Func<T>
Func<T, U>
Func<T, U, V>
Func<T, U, V, W>
Action, Action<T>
Action<T, U>
Action<T, U, V>
Action<T, U, V, W>

Only Action and Action exist in .NET 2.0, but its easy enough to declare a helper class with the remaining delegates you need for these kind of miscellaneous ad hoc functions.

回答6:

Yes, there are generic delegates. Action<T1, T2...> is a generic delegate that takes some parameters and returns no value, and Func<T1, T2...R> is a generic delegate that takes some parameters and returns a value.