Guid == null should not be allowed by the compiler

Mark is correct. Value types that define their own equality operators automatically get lifted-to-nullable versions defined as well, for free. The nullable equality operator that takes two nullable guids is applicable in this situation, will be called, and will always return false.

In C# 2, this produced a warning, but for some reason, this stopped producing a warning for guid-to-null but continues to produce a warning for int-to-null. I don't know why; I haven't had time to investigate yet.

I apologize for the error; I probably screwed up one of the warning-detection code paths when rewriting the nullable logic in C# 3. The addition of expression trees to the language majorly changed the order in which nullable arithmetic operations are realized; I made numerous mistakes moving that code around. It's some complicated code.

The comparison is valid because the compiler converts the Guid to a Nullable<Guid> and then it makes sense.

There is a bug report on the warning not being issued here.

See here here for a fuller explanation from Eric Lippert.

Actually there is a case when Guild == null will return true.

However it is kinda hard to explain.

In ORM mapping frameworks (openAccess for example) when you have a Guid field which will have a default value of Guid.Empty of course it is possible to have the fallowing scenario :

• You add a new Guid field + a Property
• You upgrade the old database schema.. in this case all values will be NULL in the database.
• If you populate an object having this null column of type Guild of course the Object WILL get an Guid.Empty value HOWEVER if you use an LINQ query ... in the LINQ query it looks the Guid is not yet populated so you need to use == null. Maybe it is a bug but this is the way it is.

In short (using OpenAccess but probably not only) :

var item = GetItems().Where(i => i.SomeGuidField == null); will work and u will get items with null guid this is after an schema update. item.First().SomeGuidField will return Empty Guid

var item = GetItems().Where(i => i.SomeGuidField == Guid.Empty); will not work even if after the population of the item it will be Guid.Empty and will return empty result.