Why Doesn't Modifying a String in a C# Method Change the Original String, and How Can I Make It?

Understanding C# String Behavior and Reference Passing

C# strings, despite being reference types, exhibit unique behavior regarding modification. The following code illustrates this: modifying a string within a method doesn't change the original string variable.

<code class="language-csharp">class Test
{
    public static void Main()
    {
        string test = "before modification";
        Console.WriteLine(test);
        ModifyString(test);
        Console.WriteLine(test); // Still "before modification"
    }

    public static void ModifyString(string test)
    {
        test = "after modification";
    }
}</code>

This happens because, although strings are reference types, the method receives a copy of the string's reference (pass-by-value). Changes made to this copied reference don't affect the original. Furthermore, strings in C# are immutable; you can't directly alter their characters. Instead, assigning a new value to a string variable creates a new string object.

Modifying Strings by Reference

To modify the original string, use the ref keyword:

<code class="language-csharp">class Test
{
    public static void Main()
    {
        string test = "before modification";
        Console.WriteLine(test);
        ModifyString(ref test);
        Console.WriteLine(test); // Now "after modification"
    }

    public static void ModifyString(ref string test)
    {
        test = "after modification";
    }
}</code>

Using ref, the method directly receives a reference to the original string variable. Assigning a new value within the method updates the original variable's reference. This demonstrates true pass-by-reference behavior. Note that even with ref, you're still creating a new string object; the reference is simply being updated to point to this new object.

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