Question Mark (?) after session variable reference - What does that mean

Question

I have had a code snippet comes to modify. In there i found this such syntax.

Session(\"LightBoxID\")?.ToString()

I didn\'t understand what

Answer 1

It's the Null-Conditional Operator It's a syntactic sugar for null checking:

return str?.ToString();

will become

if (str == null)
{
    return null;
}
return str.ToString();

Answer 2

It performs a null-check on Session("LightBoxID") before attempting to call .ToString() on it.

MSDN: Null-conditional Operators (C# and Visual Basic)