Why does this statement throw a null reference exception?

Question

This will throw a null reference exception when InnerException is null.

String s = " inner exception: " + e.InnerException == null ? "None" : e.InnerException.Message;

but this won't:

String s = " inner exception: " + (e.InnerException == null ? "None" : e.InnerException.Message);

Both of the above build fine. I can't figure out what the former is trying to do that would cause it to evaluate e.InnerException.Message. Why aren't they equivalent?

score 22 · Accepted Answer · 2012-01-27T03:30:33.597

22

This is because your first statement is evaluating " inner exception: " + e.InnerException == null to be true or false. It's all about operator precedence, which is why the second works just fine due to the parenthesis (( and )).

See this reference for Operator Precedence. The + operator is evaluated before the equality == operator.

edited Jan 27 '12 at 03:30

answered Nov 29 '11 at 19:31

That's why it is best to include brackets in non-obvious cases so you don't have to think about the operator precedence. – Malcolm Nov 29 '11 at 22:39

score 4 · Answer 2 · answered Nov 29 '11 at 19:32

It is due to operator precedence, in this case the + operator has higher precedence than the == operator so you need to use parenthesis to override the default precedence order so that the code is executed in the correct order.

You can read all about it in C# language specification:

Operator precedence and associativity

When an expression contains multiple operators, the precedence of the operators controls the order in which the individual operators are evaluated.

score 3 · Answer 3 · answered Nov 29 '11 at 19:32

3

This:

String s = " inner exception: " + e.InnerException == null ? "None" : e.InnerException.Message;

Is probably being evaluated like this:

String s = (" inner exception: " + e.InnerException) == null ? "None" : e.InnerException.Message;

answered Nov 29 '11 at 19:32

Brendan Long

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score 2 · Answer 4 · answered Nov 29 '11 at 19:33

2

Order of operations:

String s = " inner exception: " + e.InnerException == null ? "None" : e.InnerException.Message;

Is evaluated as (" inner exception: " + e.InnerException) == null ? which it's not.

answered Nov 29 '11 at 19:33

Ternary

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Why does this statement throw a null reference exception?

4 Answers4