为什么这会引发空引用异常？

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?

Answer 1

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.

Answer 2

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.

Answer 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;

Answer 4

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.