C＃条件运算符的返回类型

Question

I have an object called EmailMessage, which has a nullable System.DateTime field called Timestamp. In my C# code I have the following line:

var TS = EmailMessage.Timestamp == null ? System.DateTime.Now : EmailMessage.Timestamp;

Why does .NET 4 infer the data type of TS to be System.DateTime? rather than System.DateTime (In other words, why does .NET 4 think TS is nullable?) It seems really obvious to me that TS is decidedly not nullable.

Thanks in advance for your help.

Answer 1

Because the C# compiler simply looks at the types of DateTime.Now and EmailMessage.Timestamp :-)

And I'll tell you something: I could break your assumption. Let's say that there are two threads. One thread has your code, the other thread has EmailMessage.Timestamp = null . The other thread executes between the EmailMessage.Timestamp == null and the TS = EmailMessage.Timestamp . Break :-)

I'll add that your code normally is written this way:

var TS = EmailMessage.Timestamp ?? System.DateTime.Now;

using the ?? Operator This way the compiler knows that TS isn't nullable.

Answer 2

If EmailMessage.Timestamp is Nullable<T> , then that is the only valid option. In the true case, it becomes a Nullable<DateTime> that always has a value; in the false case it assumes the value of Timestamp, which may or may not have a value.

It does not do any further analysis. You could, however, use EmailMessage.Timestamp.Value , which is non-nullable.

In the general case, properties can change value between calls, so it is not safe to check against null and assume it will be null next time. That won't happen here, but that is why the c# compiler doesn't make any assumptions.

A simple formation here is probably:

var ts = EmailMessage.Timestamp ?? DateTime.Now;

Answer 3

Because there is an implicit conversion from DateTime to DateTime? (as opposed to going the other way, where you did something like condition ? EmailMessage.TimeStamp : null .

But you should be using the null-coalescing operator here anyway:

var TS = EmailMessage.TimeStamp ?? DateTime.Now

Answer 4

What you might be after is something like this instead:

var TS = EmailMessage.TimeStamp == null ? DateTime.Now : EmailMessage.TimeStamp.Value;

or perhaps

var TS = (EmailMessage.TimeStamp ?? DateTime.Now).Value;

Answer 5

EmailMessage.Timestamp is always DateTime? . It does not matter if you check before if it is null or not. The compiler does not care and should not care. In a multi threaded application EmailMessage.Timestamp can change between the check and where you use the value. Form a compiler perspective EmailMessage.Timestamp can be null when you use it later and should have the type DateTime? .