Not able to figure out exceptions in the below code

Question

ToUpper() is working when null reference is assigned to an Object type but failing for a null string. Please clarify the fundamentals of it :

Object obj = null;
string str1 = Convert.ToString(obj).ToUpper(); //No exception 

string str2 = null;
string str3 = Convert.ToString(str2).ToUpper(); // Throws exception

Answer 1

This is because Convert.ToString(object) returns string.Empty when object is null and Convert.ToString(string) returns the string unchanged (ie: null). You can not call .ToUpper() on null.

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