title: how indexer is called in Idataerrorinfo?
body: <p>In MVVM <code>IDataErrorInfo</code> is used for validation. In this on which time indexer check value can not get. and after calling it how it check value i can not get.the code is here. what is the value passed in column name.</p>
  
  <pre><code>public string this[string columnName]
  {
      get { return GetValidationError(columnName); }
  }
  </code></pre>
  
  <p>why use indexer for validation in wpf .can any one say about it.</p>