NullReferenceException Object reference not set to instance of an object

 In the very simple example below, the string “foo” is set to null.

string foo = null;
foo.ToUpper();

  To fix/avoid this error:

• Build null checking into the code and set default values.
• Use Debug.Assert to catch the problem before the exception occurs.

string foo = null;
if(foo!=null) {foo.ToUpper();}