c# - Why strings behave like ValueType

Question

I was perplexed after executing this piece of code, where strings seems to behave as if they are value types. I am wondering whether the assignment operator is operating on values like equality operator for strings.

Here is the piece of code I did to test this behavior.

using System;

namespace RefTypeDelimma
{
    class Program
    {
        static void Main(string[] args)
        {
            string a1, a2;

            a1 = "ABC";
            a2 = a1; //This should assign a1 reference to a2
            a2 = "XYZ";  //I expect this should change the a1 value to "XYZ"

            Console.WriteLine("a1:" + a1 + ", a2:" + a2);//Outputs a1:ABC, a2:XYZ
            //Expected: a1:XYZ, a2:XYZ (as string being a ref type)

            Proc(a2); //Altering values of ref types inside a procedure 
                      //should reflect in the variable thats being passed into

            Console.WriteLine("a1: " + a1 + ", a2: " + a2); //Outputs a1:ABC, a2:XYZ
            //Expected: a1:NEW_VAL, a2:NEW_VAL (as string being a ref type)
        }

        static void Proc(string Val)
        {
            Val = "NEW_VAL";
        }
    }
}

In the above code if I use a custom classes instead of strings, I am getting the expected behavior. I doubt is this something to do with the string immutability?

welcoming expert views on this.

Answer 1

You're not changing anything about the object a1 points to, but instead changing which object a1 points to.

_{(source: morethannothing.co.uk)}

Your example replaces "new Person { … }" with a string literal, but the principle is the same.

The difference comes when you're changing properties of the object. Change the property of a value type, and it's not reflected in the original.

_{(source: morethannothing.co.uk)}

Change the property of a reference type, and it is reflected in the original.

p.s. Sorry about the size of the images, they're just from something I had lying around. You can see the full set at http://dev.morethannothing.co.uk/valuevsreference/, which covers value types, reference types, and passing value types by value and by reference, and passing reference types by value and by reference.