Can I give a C# parameter a default value?

Question

In C# can we do the following like C++?

public void myMethod(int i, MyClass obj, int value=100){

}

Another question is MyClass is a reference type, if there is no ref before it, it will pass a copy of the MyClass into the method but not the reference?

thanks,

Answer 1

Not in C# up to C# 3 (which is the current version). In C# 4 you can have optional parameters with default values.

Regarding the question about MyClass and ref, parameters are passed by value. For reference types, you could say that the "value" of a variable (or argument) is the reference to the instance, so if you modify properties on the MyClass instance, you modify the same instance as the caller has a reference to.

Jon Skeet has written a good article on the subject: "Parameter passing in C#"

Answer 2

Others have correctly answered the optional parameter part: you can specify a default value for a parameter in C# 4. (There are various restrictions, e.g. mandatory parameters have to come before optional ones, and the default value has to be a compile-time constant.)

<gratuitous plug>See C# in Depth, 2nd edition, chapter 13 for more details</gratuitous plug>

For the "parameter passing" aspect - all arguments are passed by value by default, but in the case of reference type the argument is a reference, not the object. Changes to the object will be visible to the caller, but if you change the parameter itself to refer to a different object, that change won't be visible to the caller. (It won't change the value of the caller's variable.) See my article on parameter passing for details.