How to get the 'text' value return from 'Textbox'?

Question

I am writing scripts using White for UI automation testing. I need to get the actual text value return from a'Textbox'.

I used the following line to search the TextBox first, the AutomationId ("vwSearched_Number") represent the actual text value (e.g. 123456). I want to get this text value (123456) returned to be used for next method. But the script failed with the error msg from Assert validation which means no 'SearchedNumber' value returned.

  TextBox SearchedNumber = MainWindow.Get<TextBox>(SearchCriteria.ByAutomationId("vwSearched_Number"));

   Assert.IsNotNull(SearchedNumber, "SearchedNumber is not valid");