Understanding Values and References in Microsoft Visual C#

Copy a value type variable

Simply make the copy. Because the variable is a value type, you will have two copies of the same value. For example:

int i = 42;
int copyi = i;

Copy a reference type variable

Simply make the copy. Because the variable is a reference type, you will have two references to the same object. For example:

Circle c = new Circle(42);
Circle refc = c;

Declare a variable that can hold a value type or the null value

Declare the variable by using the ? modifier with the type. For example:

int? i = null;

Pass an argument to a ref parameter

Prefix the argument with the ref keyword. This makes the parameter an alias for the actual argument rather than a copy of the argument. The method may change the value of the parameter, and this change is made to the actual argument rather than to a local copy. For example:

static void Main()
{
    int arg = 42;
    doWork(ref arg);
    Console.WriteLine(arg);
}

Pass an argument to an out parameter

Prefix the argument with the out keyword. This makes the parameter an alias for the actual argument rather than a copy of the argument. The method must assign a value to the parameter, and this value is made to the actual argument. For example:

static void Main()
{
    int arg;
    doWork(out arg);
    Console.WriteLine(arg);
}

Box a value

Initialize or assign a variable of type object with the value. For example:

object o = 42;

Unbox a value

Cast the object reference that refers to the boxed value to the type of the value variable. For example:

int i = (int)o;

Cast an object safely

Use the is operator to test whether the cast is valid. For example:

WrappedInt wi = new WrappedInt();
...
object o = wi;
if (o is WrappedInt)
{
    WrappedInt temp = (WrappedInt)o;
    ...
}

Alternatively, use the as operator to perform the cast, and test whether the result is null. For example:

WrappedInt wi = new WrappedInt();
...
object o = wi;
WrappedInt temp = o as WrappedInt;
if (temp != null)
    ...