Question

Class is a reference types while Struct is a value type.

This means that I do not need to explicitly specify ref for every parameter which is a type of class.

My Question is,

Why do we have to specify a ref keyword for a string type which is basically a type of class ?

Please find the example below

public partial class StringManupulation : Form
{
    public StringManupulation()
    {
        InitializeComponent();

        String value = "Hello ";

        Concatenate( value );
        MessageBox.Show( value );

        Concatenate( ref value );
        MessageBox.Show( value );
    }

    /// <summary>
    /// Does not work
    /// </summary>
    /// <param name="value"></param>
    public void Concatenate( String value )
    {
        value = string.Concat( value, "StackOverflow" );
    }

    /// <summary>
    /// Works! 
    /// </summary>
    /// <param name="value"></param>
    public void Concatenate( ref String value )
    {
        value = string.Concat( value, "StackOverflow" );
    }
 }

Answer 1

System.String is immutable – That means you cannot change the content of string once it is assigned. You may try System.Text.StringBuilder (mutable object) instead of System.String class.

static void ParamTest(System.Text.StringBuilder  paramSb)
{
  paramSb.Append("World");
}

static void Main()
{
  System.Text.StringBuilder sb = new StringBuilder();
  sb.Append("Hello");

  ParamTest(sb);
  Console.WriteLine(sb);
}

Have a look at Strings in C# and .NET and article – chapter from C# in Depth.

Quote to Jon Skeet

It is a reference type?

It’s a common misconception that string is a
value type. That’s because its immutability (see next point) makes it
act sort of like a value type. It actually acts like a normal
reference type. See my articles on parameter passing and memory for
more details of the differences between value types and reference
types.