ref

The ref keyword explicitly specifies that a variable should be passed by reference rather than by value.

A developer may wish to pass a variable by reference particularly in case of value types. If a variable is passed by reference, only a pointer is sent to a function in reality, reducing the cost of a method call in case it would involve copying large amounts of data, something C# does when normally passing value types.

Another common reason to pass a variable by reference is to let the called method modify its value. Because this is allowed, C# always enforces specifying that a value is passed by reference even in the method call, something many other programming languages don't. This let developers reading the code easily spot places that can imply a type has had its value changed in a method, which is useful when analyzing the program flow.

Passing a value by reference does not imply that the called method has to modify the value; see the out keyword for this.

Passing by reference requires the passed variable to be initialized.

An example of passing a variable by reference follows:

void CallingMethod()
{
    int i = 24;
    if (DoubleIfEven(ref i))
        Console.WriteLine("i was doubled to {0}", i); // outputs "i was doubled to 48"
}

bool DoubleIfEven(ref int iValue)
{
    if (iValue%2 == 0)
    {
        iValue *= 2;
        return true;
    }
    return false;
}