Make GridView TextBox ReadOnly (Disabled) in ASP.Net

Here Mudassar Ahmed Khan has explained with an example, how to make GridView TextBox ReadOnly (Disabled) in ASP.Net when Editing using C# and VB.Net. The TextBox in ASP.Net GridView will be made ReadOnly or Disabled during the Edit Mode of GridView inside the Row being currently edited using C# and VB.Net.