Are ILists passed by value?

Question

Passing Value Type parameters to functions in c# is by value unless you use the ref or out keyword on the parameter. But does this also apply to Reference Types?

Sp

Answer 1

You're not alone; this confuses a lot of people.

Here's how I like to think of it.

A variable is a storage location.

A variable can store something of a particular type.

There are two kinds of types: value types and reference types.

The value of a variable of reference type is a reference to an object of that type.

The value of a variable of value type is an object of that type.

A formal parameter is a kind of variable.

There are three kinds of formal parameters: value parameters, ref parameters, and out parameters.

When you use a variable as an argument corresponding to a value parameter, the value of the variable is copied into the storage associated with the formal parameter. If the variable is of value type, then a copy of the value is made. If the variable is of reference type, then a copy of the reference is made, and the two variables now refer to the same object. Either way, a copy of the value of the variable is made.

When you use a variable as an argument corresponding to an out or ref parameter the parameter becomes an alias for the variable. When you say:

void M(ref int x) { ...}
...
int y = 123;
M(ref y);

what you are saying is "x and y now are the same variable". They both refer to the same storage location.

I find that much easier to comprehend than thinking about how the alias is actually implemented -- by passing the managed address of the variable to the formal parameter.

Is that clear?