c# - How do delegate/lambda typing and coercion work? -

i've noticed examples of things work , don't work when dealing lambda functions , anonymous delegates in c#. what's going on here?

class test : control {     void testinvoke() {         // best overloaded method match 'invoke' has invalid arguments         invoke(dosomething);          // cannot convert anonymous method type 'system.delegate' because not delegate type         invoke(delegate { dosomething(); });          // ok         invoke((action)dosomething);          // ok         invoke((action)delegate { dosomething(); });          // cannot convert lambda expression type 'system.delegate' because not delegate type         invoke(() => dosomething());          // ok         invoke((action)(() => dosomething()));     }      void testqueueuserworkitem() {         // best overloaded method match 'queueuserworkitem' has invalid arguments         threadpool.queueuserworkitem(dosomething);          // ok         threadpool.queueuserworkitem(delegate { dosomething(); });          // best overloaded method match 'queueuserworkitem' has invalid arguments         threadpool.queueuserworkitem((action)dosomething);          // no overload 'dosomething' matches delegate 'waitcallback'         threadpool.queueuserworkitem((waitcallback)dosomething);          // ok         threadpool.queueuserworkitem((waitcallback)delegate { dosomething(); });          // delegate 'waitcallback' not take '0' arguments         threadpool.queueuserworkitem(() => dosomething());          // ok         threadpool.queueuserworkitem(state => dosomething());     }      void dosomething() {         // ...     } } 

well that's lot of examples. guess questions following:

1. why invoke refuse lambda function or anonymous delegate, yet threadpool.queueuserworkitem fine?

2. what heck "cannot convert anonymous method type 'system.delegate' because not delegate type" mean anyway?

3. why threadpool.queueuserworkitem accept anonymous delegate no parameters, not lambda expression no parameters?

1. threadpool.queueuserworkitem has specific delegate in signature; invoke has delegate. lambda expressions , anonymous methods can converted specific delegate type.

2. it's bad error message. means, "i don't know delegate type you're trying convert to."

3. you're using anonymous method without parameter list @ all can converted delegate type doesn't use out/ref parameters. if tried delegate() { ... } (i.e. explicitly empty parameter list) wouldn't work. "i don't care parameters" ability of anonymous methods only feature have lambda expressions don't.

it's easiest demonstrate of in context of simple assignments, imo:

// doesn't work: no specific type delegate d = () => console.writeline("bang");  // fine: know exact type convert action = () => console.writeline("yay");  // doesn't work: eventhandler isn't parameterless; we've specified 0 parameters eventhandler e1 = () => console.writeline("bang"); eventhandler e2 = delegate() { console.writeline("bang again"); };  // works: don't care parameter lists eventhandler e = delegate { console.writeline("lambdas can't this"); };