title: what is "int?" in C# types
body: <p>I've seen this code sample question in a exam and it works perfectly.</p>
  
  <pre><code>namespace Trials_1
  {
      class Program
      {
          static void Main(string[] args)
          {
              int? a = 9;
              Console.Write("{0}", a);
          }
      }
  }
  </code></pre>
  
  <p>But the below code throws an error CS0266.</p>
  
  <pre><code>namespace Trials_1
  {
      class Program
      {
          static void Main(string[] args)
          {
              int? a = 9;
              int b = a;
              Console.Write("{0},{1}", a, b);
          }
      }
  }
  </code></pre>
  
  <p>Can somebody explain me in detail? </p>