value type variable and reference type variable

Use ref keyword, It helps you to pass value by reference.

The ref keyword causes an argument to be passed by reference, not by value. The effect of passing by reference is that any change to the parameter in the method is reflected in the underlying argument variable in the calling method. The value of a reference parameter is always the same as the value of the underlying argument variable.

private void DoSomething(ref int someintByRef)
{
    someintByRef = 4;
}

 int _myinteger=10;
DoSomething(ref _myinteger);
Console.WriteLine(myinteger);//Prints 4

Update: To be more clear, I'd say without the use of ref "value types" and "reference types" are passed by value only. Difference is for "value types" value(complete instance) will be copied and completely new instance is passed in case of reference types "refence of the instance" is copied and passed by value.

When we use ref, no doubt both are passed by reference

For reference types:

class User
{
    public string Name { get; set; }
    public string LastName { get; set; }
}

void ByRef(ref User user)
{
    user = new User();
}

void ByVal(User user)
{
    user = new User();
}

void Test()
{
    var user = null;
    ByVal(user);
    // user is still null
    ByRef(ref user);
    // user is new User()
}

There are diffrent behavior for the diffrent type of variables. if we talk about value types they are consist about the values only. the don't share memory locations. so every value type have there own values and they can play with it as they want. on the other hand refrence type variables are diffrent in nature they work with sharing mode so if i create an object then the value of this object is being shared by their users. I recomand you to have a look for below video for more clearification.

So if you pass a value type variable to a method the copy of values is being assigned to the using variable. so if you want to use the same variable. then you can have ref keywork for refranceing the value of the same object. on the other side object / refrence type variables are not needed to be ref keyword as they are already refranced by default nature.

Try below code tu understand the behavior.

 class Program
    {
        static object reftypedata = "I am Testing. ";
        static int valuetypeData = 10;

        static void Main(string[] args)
        {

            CalcValue(valuetypeData);
            CalcValuewithRef(ref valuetypeData);
            Calcobject(reftypedata);


            Console.ReadKey();

        }

        public static void CalcValue(int i)
        {
            Console.WriteLine("Value Type.....");
            Console.WriteLine("before Processing i={0}", i);
            i = 100;
            Console.WriteLine("After Processing i={0}", i);
            Console.WriteLine("After Processing valueTypeData = {0}", valuetypeData);

        }
        public static void CalcValuewithRef(ref int i)
        {
            Console.WriteLine("Value with Ref Type.....");
            Console.WriteLine("before Processing i={0}", i);
            i = 100;
            Console.WriteLine("After Processing i={0}", i);
            Console.WriteLine("After Processing valueTypeData = {0}", valuetypeData);

        }
        public static void Calcobject(object value)
        {
            Console.WriteLine("Reference Type.....");
            Console.WriteLine("before Processing value={0}", value);
            value = "Default Value Has been Changed.";
            Console.WriteLine("After Processing value={0}", value);
            Console.WriteLine("After Processing reftypedata={0}", reftypedata);


        }
    }

Output:

    Value Type.....
    before Processing i=10
    After Processing i=100
    After Processing valueTypeData = 10

 Value with Ref Type.....
    before Processing i=10
    After Processing i=100
    After Processing valueTypeData = 100

  Reference Type.....
    before Processing value=I am Testing.
    After Processing value=Default Value Has been Changed.
    After Processing reftypedata=I am Testing.

you also can reffer to this youtube video.