Using VB.NET IIF I get NullReferenceException

IIf is an actual function, so all arguments get evaluated. The If keyword was added to VB.NET 2008 to provide the short-circuit functionality you're expecting.

Try

logLine = "e.Value: " + If(e.Value Is Nothing, "", e.Value.ToString())

VB does not do short-circuiting evaluation in Iif. In your case, e.Value.ToString() is being evaluated no matter whether e.Value is nothing.

This is the expected behaviour.

IIF is a function; therefore the parameters for the function will be evaluated before sending it to the function.

In contrast, the ternary operator in C# is a language construct that prevents the evaluation of the second parameter if the expression of the ternary is true.