References:

A Criminal who uses two Names is said to have an Alias. To create a second Name for a variable in a program, we can also generate an Alias, or an alternative Name. in C#, a variable that acts as an alias for another variable is called a Reference variable, or simply a reference.
Reference is actually another Name for an already existing variable. Once a reference is initialized with a variable, either the variable Name or the reference name may be used to refer to the variable.

The Reference type variable holds the reference of memory address instead of Value. When we pass parameters by reference, unlike value parameters, a New storage location is not created for these Parameters. The reference parameters represent the same memory location as the Actual parameters that are supplied to the Method.

Whenever we pass a arguments by reference, The method copies the Address of an argument into the formal parameter. Inside the Method, the Address is used to access the Actual argument used in the call. This mean that changes made to the parameter affect the passed argument.

Declaring Reference Parameters:

We can Declare the Reference parameters using the ref Keyword. The following Example demonstrates this:

/*Example: Reference in C#: InfoBrother:*/

using System;

namespace reference
{
    class Refer
    {
        //swap method:
        public void swap(ref int p, ref int q)
        {
            int temp = p;  //save the value of p;
            p = q;         //put q into p;
            q = temp;      //put temp into q;
        }

        //main method:
        public unsafe static void Main()
        {
            Refer obj = new Refer();

            int x = 10;
            int y = 20;

            Console.WriteLine("Before Swap: X:{0}, Y: {1}", x, y);

            obj.swap(ref x, ref y);  //method called:
            Console.WriteLine("After Swap: X:{0}, Y: {1}", x, y);
            Console.ReadKey();
        }
    }
}

Out Keyword:

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.

/*Example: Reference vs out in C#: InfoBrother:*/

using System;

namespace reference
{
    class ReferVsout
    {
        public static void Main() //Main method
        {
            int val1 = 0; //must be initialized 
            int val2; //optional

            Example1(ref val1);
            Console.WriteLine(val1); // val1=1

            Example2(out val2);
            Console.WriteLine(val2); // val2=2

            Console.ReadKey();
        }

        static void Example1(ref int value) //method: by Reference:
        {
            value = 1;
        }

        static void Example2(out int value) //method: by Out:
        {
            value = 2; //must be initialized 
        }
    }
    
}

• More about Reference:
  

  » Do not be confused with the concept of passing by reference and the concept of reference type. These two concepts are not the same.
  » A value type or a reference type can be passed to method parameter by using ref keyword. There is no boxing of a value type when it is passed by reference.
  » Properties cannot be passed to ref or out parameters since internally they are functions and not members/variables.