为什么我不能像这样使用空合并运算符？

Question

According to MSDN :

The ?? operator is called the null-coalescing operator and is used to define a default value for nullable value types or reference types.

But when I run the code below:

Guid test;
Guid otherGuid = test ?? Guid.NewGuid();

I get the error:

Operator '??' cannot be applied to operands of type 'System.Guid' and 'System.Guid'

I thought Guid was a reference type. Is that not the case? Could someone explain to my why this does not work?

Answer 1

Guid is not a reference type, its a value type , you can't assign it null and that is why you can't check it with ??

Following line will give you error.

Cannot convert null to 'System.Guid' because it is a non-nullable value type

Guid test = null;

You may use Nullable

Nullable<Guid> test = null;
Guid otherGuid = test ?? Guid.NewGuid();

Or short form

Guid? test = null;
Guid otherGuid = test ?? Guid.NewGuid();

Answer 2

Guid is a struct and not a class, and can thus not be null.

For this to work, you could do something like this:

Guid? nullableGuid = null;
/* At some point nullableGuid is perhaps assigned... */
Guid otherGuid = nullableGuid ?? Guid.NewGuid();

Answer 3

GUID is a structure. http://msdn.microsoft.com/en-us/library/system.guid.aspx

Try using Guid.Empty to do the check.

Guid otherGuid = test == Guid.Empty ? Guid.NewGuid() : test;

Answer 4

As others have said, Guid is a value type and not a reference type, see here for a more in-depth explanation about the difference between the two. You'd have two options in your scenario.

1. Coalescence operator:

Guid? guid = null;
otherGuid = guid ?? Guid.NewGuid();

2. default comparison

otherGuid = guid == default ? Guid.NewGuid() : guid;

In the second option you could also guse Guid.Empty , but using the default keyword would be better practice.