title: How to Create Custom MarkupExtension like StaticResource?
body: <p>please help me how to create a MarkupExtension looks like StaticResource of wpf,
  i have:</p>
  
  <p>my own class:</p>
  
  <pre><code>public class Item{
  
  public string Value{get; set;}
  public string Title{get;set;}
  }
  </code></pre>
  
  <p>and in a Resource Dictionary i have:</p>
  
  <pre><code>// ...
  &lt;gpf:Item x:Key="firstone" Value="Hi" Title="Welcome"/&gt; 
  //...
  </code></pre>
  
  <p>i want to use my Item looks like:</p>
  
  <pre><code>// ...
  &lt;TextBlock Text="{MyEX firstone}"/&gt;
  //...
  </code></pre>
  
  <p>i tired to do this but i do not know how to finish my work:</p>
  
  <pre><code>    //...
      [Localizability(LocalizationCategory.NeverLocalize)]
      [MarkupExtensionReturnType(typeof(string))]
      public class MyEX : MarkupExtension
      {
          public MyEX () { 
          }
  
          public override object ProvideValue(IServiceProvider serviceProvider)
          {
              return ??? ;
          }
      }
  </code></pre>