Parameters are always passed by value to a method by default.If we want to pass them by reference then we can use either out or ref keyword.

Reference parameters basically never pass the value of a variable used in the method calling, instead they use the variable themselves. Rather than creating a new storage for that variable in the method declaration, the very same storage space is used, so the value of the variable in the member method and the value of the reference parameter will always be the same. Reference parameters require ref modifier as part of both the declaration and the calling.

Output parameters are very much like reference parameters. The variable specified at the time of calling doesn't need to have been assigned a value before it is passed to the called method. When the method is invoked completely we can read that variable as it is assigned by now.

Like reference parameters, output parameters don't create a new storage location, but use the storage location of the variable specified on the invocation. Output parameters need the out modifier as part of both the declaration and the invocation - that means it's always clear when you're passing something as an output parameter.

Ref

The ref keyword is used to pass an argument as a reference. This means that when value of that parameter is changed in the method, it gets reflected in the calling method. An argument that is passed using a ref keyword must be initialized in the calling method before it is passed to the called method.

If you want to pass a variable as ref parameter you need to initialize it before you pass it as ref parameter to method. Ref keyword will pass parameter as a reference this means when the value of parameter is changed in called method it get reflected in calling method also

Declaration of Ref Parameter

 int val1 = 0; //must be initialized
 int val2; //optional
 
 Example1(ref val1); -- Calling through ref

static void Example1(ref int value) //called method
 {
 value = 1;
 }

Out

The out keyword is also used to pass an argument like ref keyword, but the argument can be passed without assigning any value to it. An argument that is passed using an out keyword must be initialized in the called method before it returns back to calling method.

If you want to pass a variable as out parameter you don’t need to initialize it before you pass it as out parameter to method. Out keyword also will pass parameter as a reference but here out parameter must be initialized in called method before it return value to calling method.

Declaration of Out Parameter

 int val2; //optional
 
 Example1(out val1); -- Calling through ref

static void Example1(out int value) //called method
 {
 value = 1; //must be initialized
 }