Why can anonymous delegates omit arguments, but la

问题:

//ok
Action<int> CallbackWithParam1 = delegate { };    

//error CS1593: Delegate 'System.Action<int>' does not take 0 arguments
Action<int> CallbackWithParam2 = () => { };   

Just wondered why the discrepancy really. :-/

回答1:

Jared is of course correct. To add a couple more details:

• Almost no one uses the "skip the parameter list" syntax.
• We have no scenario for lambdas that requires that feature.
• The feature complicates type inference and overload resolution, and makes it more likely that both will fail.
• What syntax would you like for the feature? Action<int> c = => {}; ??? I have no desire whatsoever to make => into a unary prefix operator.

So on the one hand we have the list of pros:

• Lambdas gain consistency with an unnecessary C# 2.0 feature that hardly anyone knows about or uses -- a feature that frankly, we wish we'd never done in the first place

and the cons:

• implementation complicates already-complex type inference and overload resolution algorithms
• feature leads to more bug possibilities for users with no corresponding gain in representational power.
• no obviously nice syntax

If you were given that list of pros and cons, what would you do? I hope "implement the feature" would not be your choice; it was not ours.

回答2:

It's essentially as simple as they are different features with different sets of supported scenarios. It's almost like asking

Why can lambdas be expressions but delegates can only be blocks?