System.Nullable<T> What is the value of null * int value?

Question

Consider the following statements:

int? v1 = null;

int? v2 = 5 * v1;

What is the value of v2? (null or empty string?)

How can I prevent the compiler to mark it as invalid operation? Do I need to follow custom exception handling?

Answer 1

It's null.

C# Language Specification 3.0 (Section §7.2.7: Lifted operators)

For the binary operators + - * / % & | ^ << >> :

a lifted form of an operator exists if the operand and result types are all non-nullable value types. The lifted form is constructed by adding a single ? modifier to each operand and result type. The lifted operator produces a null value if one or both operands are null (an exception being the & and | operators of the bool? type, as described in §7.10.3). Otherwise, the lifted operator unwraps the operands, applies the underlying operator, and wraps the result.

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How can I prevent the compiler to mark it as invalid operation? Do I need to follow custom exception handling?

It's not an invalid operation. It won't throw an exception so you don't need to handle exceptions in this case.