Problems with dynamic parameter

Question

I have this function

string F(dynamic a)
{
    return "Hello World!";
}

later when i say

dynamic a = 5;
var result = F(a);

result must be in compilation time a string type, but that not happened, why? In fact, the compilar pass this

int result2 = F(a);

and not this

int result3 = F(5);

Anything help please?

Something is not adding up here. You've declared a string return type and are, in fact, returning a string. So I do not see how it is possible that you are getting an int result. — Brian Driscoll, Nov 06 '14 at 13:53
Recreating his program, the intellisense tells me that `result` is dynamic too — Jonesopolis, Nov 06 '14 at 13:54
@Chris wouldn't overloads have to have the same return type? — Brian Driscoll, Nov 06 '14 at 13:55
here we go, [look here](http://stackoverflow.com/questions/7829518/c-sharp-dynamic-parameter-causes-compiler-to-think-method-return-is-dynamic). Mainly, `dynamic stops the compiler from knowing the type on any parameters, properties, or method return types` — Jonesopolis, Nov 06 '14 at 13:57
@Brian No - overloads can return different types, so long as the compiler is able to differentiate them through different arguments — James Thorpe, Nov 06 '14 at 13:57
Thanks @JamesThorpe, and I realized what you said after I asked the question as well. — Brian Driscoll, Nov 06 '14 at 14:02
What do you mean by _"result must be in compilation time a string type, but that not happened, why?"_? What does your compiler say exactly? — CodeCaster, Nov 06 '14 at 14:06
Possible duplicate of http://stackoverflow.com/questions/7829518/c-sharp-dynamic-parameter-causes-compiler-to-think-method-return-is-dynamic. — Patrick Hofman, Nov 06 '14 at 14:07

score 7 · Accepted Answer · edited Nov 06 '14 at 14:25

7

It is explained in here:

Overload resolution occurs at run time instead of at compile time if one or more of the arguments in a method call have the type dynamic, or if the receiver of the method call is of type dynamic.

Now in the case of F(a) since a is dynamic, compiler doesn't check for the overloads at compile-time. But when you say:

F(2);

2 is an integer and not dynamic. That's why the overload resolution occurs at compile time and you get the error.If you cast the integer literal to dynamic you won't get any error at compile time (but you do on run-time):

int x = F((dynamic)2);

edited Nov 06 '14 at 14:25

Patrick Hofman

153,850
22
249
325

answered Nov 06 '14 at 14:02

Selman Genç

100,147
13
119
184

@BlueMonkMN if there is no overload that returns an int then an exception will occur at runtime. – Selman Genç Nov 06 '14 at 14:22
As I expected. I'm just being a stickler over the phrase "you won't get any error." This looks like good evidence to support the position that dynamic typing leads to more error-prone code. – BlueMonkMN Nov 06 '14 at 14:24

Problems with dynamic parameter

1 Answers1