Structure is an value type, Then why don't compiler gives any error when string is declared within struct

Why below written code doesn't gives error even when string is a reference type.

public struct example
{
   public int a;
   public string name;
};

public void usestruct()
{
example objExample= new example();
MessageBox.Show(objExample.name);
}

EDIT

Modifying Jon Answer, I have few more Questions.

            public struct Example 
            { 
               public int a; 
               public string name; 
            } 

            class Test 
            { 
                static void Main() 
                { 
   //Question 1
                    Example t1 = new Example(); 
                    t1.name = "First name"; 
                    Example t2 = t1; 
                    t2.name = "Second name"; 

                    Console.WriteLine(t1.name); // Prints "First name" 
                    Console.WriteLine(t2.name); // Prints "Second name"
                    Console.WriteLine(t1.name); // What will it Print ????

   //Question 2
                    Example t1 = new Example(); 
                    t1.name = "First name"; 
                    Console.WriteLine(sizeof(t1)); // What will it Print ???? 
                       // I think it will print 4 + 4 bytes. 
                       //4 for storing int, 4 for storing address of string reference.
    //Considering 
    //int of 4 bytes 
    //Memory Address of 4 byte 
    //Character of 1 byte each.
                      //My collegue says it will take 4+10 bytes 
                      //i.e. 4 for storing int, 10 bytes for storing string.
                } 
            } 
开发者_开发百科

How many bytes will second case take.

The struct just contains a reference to a string. Why would that cause a problem? A reference is just a value. When you copy the structure's value (e.g. with assignment, or passing it to a method) the reference will be copied too. Note that if you change the value of the field in one copy of the struct, that won't change the value in a different copy:

using System;

public struct Example
{
   public int a;
   public string name;
}

class Test
{
    static void Main()
    {
        Example t1 = new Example();
        t1.name = "First name";
        Example t2 = t1;
        t2.name = "Second name";

        Console.WriteLine(t1.name); // Prints "First name"
        Console.WriteLine(t2.name); // Prints "Second name"        
    }
}

If Example were a class instead, these would both print "Second name" as the values of t1 and t2 would be references to the same instance.

This isn't specific to strings, either - any reference type will work.

However, I would strongly advise against creating mutable structs or exposing public fields.