C# string reference type?

The reference to the string is passed by value. There’s a big difference between passing a reference by value and passing an object by reference. It’s unfortunate that the word “reference” is used in both cases.

If you do pass the string reference by reference, it will work as you expect:

using System;

class Test
{
    public static void Main()
    {
        string test = "before passing";
        Console.WriteLine(test);
        TestI(ref test);
        Console.WriteLine(test);
    }

    public static void TestI(ref string test)
    {
        test = "after passing";
    }
}

Now you need to distinguish between making changes to the object which a reference refers to, and making a change to a variable (such as a parameter) to let it refer to a different object. We can’t make changes to a string because strings are immutable, but we can demonstrate it with a StringBuilder instead:

using System;
using System.Text;

class Test
{
    public static void Main()
    {
        StringBuilder test = new StringBuilder();
        Console.WriteLine(test);
        TestI(test);
        Console.WriteLine(test);
    }

    public static void TestI(StringBuilder test)
    {
        // Note that we're not changing the value
        // of the "test" parameter - we're changing
        // the data in the object it's referring to
        test.Append("changing");
    }
}

See my article on parameter passing for more details.