title: Get the value of a InterpolatedStringExpressionSyntax
body: <p>Using Roslyn, I'm to get the constant value of the following string parameter in this method call:</p>
  
  <pre><code>inst.someMethod($"{Constants.SomeValue}");
  </code></pre>
  
  <pre><code>static class Constants
  {
      public static readonly string SomeValue= "some value";
  }
  </code></pre>
  
  <p>I got an instance of <code>InterpolatedStringExpressionSyntax</code> type for the <code>$"{Constants.SomeValue}"</code> parameter.</p>
  
  <p>Next, I need to get its value (which is the <code>"Some value"</code> string), and I tried to call <code>semanticModel.GetConstantValue(expr);</code> but it returns null</p>
  
  <p>How can I get that value?</p>