Default Value in a C# Function

Question

is it possible to do this in C#? (in C++ its not)

function sum ( int a = 9, int b = 4){

}

and then call the function like :

int someValue = sum(, 14) // so 14 is for the second value

Answer 1

C# 4 allows named arguments and optional parameters¹:

int Sum (int a = 9, int b = 4)
{
    return a + b;
}

Then:

Sum(10, 5);       // Positional arguments as "normal"
Sum(b: 5);        // Use the default value for a
Sum(a: 5);        // Use the default value for b
Sum();            // Default both parameters
Sum(b: 1, a: 10); // Arguments can be reordered

EDIT: For overloaded methods, if there are multiple matches the compiler checks whether any of those candidates are only valid due to giving default values. If that's the case, those candidates are effectively given priority. This is one of the tie-breaking rules listed in section 7.5.3.2. There's no preference for just using "fewer" default values; it's an all or nothing approach. I give an example of this in my article on overloading.

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¹ It's unfortunate that the terminology is often confused, even by Microsoft. Parameters have always have names - what's new is that you can specify the name in the calling code, for the argument. On the other hand, you make the parameter optional by specifying a default.

Answer 2

With C# 4 you can do int someValue = sum (b: 14);

EDIT - as per comments:

C++ can have default values for parameters... these parameters can be "skipped" when calling a method/function BUT the rule in C++ is that only the right-most parameters can be skipped... which makes it possible in C++ to call sum() and sum (7) refering to a but NOT the call you give as an example...