Can named arguments be used with the params modifier?

Question

I am trying to invoke Invoke on a Dispatcher using the following overload:

public object Invoke(Delegate method, params object[] args);

I want to use named arguments, but I can't seem to find the syntax for the argument with the params modifier. All of the following won't compile:

dispatcher.Invoke(method: () => { }, args: {});
dispatcher.Invoke(method: () => { }, args: new object[0]);
dispatcher.Invoke(method: () => { }, args: null);
dispatcher.Invoke(method: () => { }, args: new object[] {});
object[] foo = {};
dispatcher.Invoke(method: () => { }, args: foo);
dispatcher.Invoke(method: () => { }, args: new[] {"Hello", "World!"});

I found these two questions to which there seems to be no definite answers:

Named parameters with params

How to set named argument for string.Format?

So my question is: can it be done or not? If yes, how?

UDPATE

Daniel Hilgarth's shows that yes params can be used with named parameters. I integrated his answer using this pattern:

Action method = () => { };
if (_dispatcher != null)
    _dispatcher.Invoke(method: method, args: null);
else
    method();

First question is mine and seems there is no way. You must go without params. — Chuck Norris, Apr 13 '12 at 13:46
You say "seems"; it would be nice to have a definite answer ;-) — , Apr 13 '12 at 13:52
Please specify the exact compiler error. It works for me when I change `Delegate` to `Action` so it is not a problem of the `params` keyword. — Daniel Hilgarth, Apr 13 '12 at 13:52
@DanielHilgarth: can you post how you made it work as an answer? — , Apr 13 '12 at 13:53
Not the way he wanted. He wanted to do something like this: `dispatcher.Invoke(method: method, args: "Hello", "World");`. Note that there is no array passed to the named parameter `args`, but a list of comma separated values. — Daniel Hilgarth, Apr 13 '12 at 14:06

score 2 · Answer 1 · answered Apr 13 '12 at 13:49

2

Have a try;

dispatcher.Invoke(method: () => { }, args: new[] {"Hello", "World!"});

answered Apr 13 '12 at 13:49

daryal

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Actually yes :) and it doesn't work either; I'll add it to my list. – Apr 13 '12 at 13:50
@MartinLiversage I expect no problems. Like the following; object[] a = new object[] { "x", "y" }; – daryal Apr 13 '12 at 14:00
@MartinLiversage: Not correct. This works. arrays in C# have always been covariant. – Daniel Hilgarth Apr 13 '12 at 14:00

Daniel Hilgarth · Accepted Answer · 2012-04-13T14:11:59.887

The following code compiles without problems:

void Main()
{
    Invoke(method: () => {}, args: new object[] {});
}

public object Invoke(Action method, params object[] args)
{
    return null;
}

I had to change the type of the first parameter from Delegate to Action to make it compile, because () => {} can't be converted to Delegate.

Alternatively, the following does also compile:

void Main()
{
    Action method = () => {};
    Invoke(method: method, args: new object[] {});
}

public object Invoke(Delegate method, params object[] args)
{
    return null;
}

My assumption is that you never got a compile error about the args parameter but about the method parameter saying "Cannot convert from 'lambda expression' to 'System.Delegate'". This problem is solved by either casting the lambda to an Action (Invoke(method: (Action)(() => {}) ...) or by defining a variable of type Action that is passed as parameter to the method (see above), because Action can implicitly be converted to Delegate.

+1 Not only does this word fine, this helped me create what I think is more readable code. I will update to demosntrate. — , Apr 13 '12 at 14:03
@sixfeetsix: Yes, the new code looks very nice and clean, aside from the unnecessary use of named parameters in the first place ;-) — Daniel Hilgarth, Apr 13 '12 at 14:11

Can named arguments be used with the params modifier?

2 Answers2