Refering to ?: Operator (C# Reference)
The conditional operator (?:) returns one of two values depending on the value of a Boolean expression. Following is the syntax for the conditional operator.
Refering to ?? Operator (C# Reference)
The ?? operator is called the null-coalescing operator and is used to define a default value for a nullable value types as well as reference types. It returns the left-hand operand if it is not null; otherwise it returns the right operand.
That means:
[Part 1]
return source ?? String.Empty;
[Part 2] is not applicable ...