Pass by copy or reference?

Question

I am a bit new to C# and was just wondering if there is a list of classes (commonly used) which are by default passed as copy. How can I identify them?

I know the basic basic object types — int, uint, float, strings, ... — are passed by copy.

Answer 1

All types, by default, are passed by value. The difference between value types (struct) and reference types (class) is that for value types a copy of the value is passed to the method, whereas for reference types, only a reference is passed.

See MSDN for more details.

Also, don't confuse the notion of value/reference types, and the notion of passing parameters by value or by reference. See this article by Jon Skeet for details

Answer 2

In general,

• value types, which includes native types (int, float, ...) as well as structs are passed "by copy" (as you put it), whereas in

• reference types, which includes classes, only a reference is copied instead of the complete object.

Note, though, that string is not passed "by copy"! It's a reference type, but since it's immutable, it behaves similar to a value type.

Note that the ref keyword can be useful in both cases: In the case of value types, it means that the value is copied back. In the case of reference types, it means that the reference can be changed (i.e., that you can assign the variable to a new object rather than just changing the properties of the object.)

Answer 3

In C# / .Net objects can either be classified as value or reference types [1]. Value types are any type which derive from System.ValueType and are defined in C# with the struct type declaration. These are passed by copy / value.

Reference types are types which do not derive from System.ValueType and are defined in C# with the class keyword. Identifiers to instances of reference types are known as references (similar to pointers). These are also passed by value by default but only the reference is passed not the whole object.

Your question also mentioned that string instances are passed by copy. String in .Net is a reference type (derives directly from System.Object) and hence is not passed by full copy.

[1] Pointers may merit their own class here but I'm ignoring them for this discussion.

Answer 4

Note also that struct objects can include reference types:

struct Currency
{
    int Amount {get;set;} // is referenced by value (copy)
    string Code {get;set;} // is referenced by reference
}

When your use that struct like this:

var obj1 = new Currency{Amount = 1, Code = "USD"};
var obj2 = obj1;

Then

object.ReferenceEquals(obj1.Code, obj2.Code); // false - string is reference type, but used inside value type

If you have declared Currency as class then references to Code were the same.