In this code:

class TestClass
{
  private string _myString = "hello";

  public TestClass()
  {
      this.ChangeString( "_myString", "SomeNewValue" );
  }
  public void ChangeString( string fldname, string newValue )
  {
      // refer to the field fldname and change it's value to newValue
      ??????
  }
}

How can i refer to the myString field and change it's value from it's name ?