C# Generics: Constraining T where T : Object doesn't compile; Error: Constraint cannot be special class 'object'

Question

When I constrain T with : Object like this:

public interface IDoWork<T> where T : Object {     T DoWork(); } 

I get the error:

Constraint cannot be special class 'object'

Does that mean there is an implied difference with the following that does compile?

public interface IDoWork<T> // where T : Object {     T DoWork(); } 

Answer 1

If you want to constrain a generic type to be a reference type, use : class.

public interface IDoWork<T> where T : class {     T DoWork(); } 

This will forbid the generic type from being a value type, such as int or a struct.

Answer 2

There is no difference between the two constraints, except for that one is disallowed for being useless to explicitly state.

The C# 4.0 language specification (10.1.5 Type parameter constraints) says two things about this:

The type must not be object. Because all types derive from object, such a constraint would have no effect if it were permitted.

...

If T has no primary constraints or type parameter constraints, its effective base class is object.

In your comment, you said that you were trying to make T be of type Void. Void is a special type that indicates that there is no return type and cannot be used in place of T, which requires an appropriate concrete type. You will have to create a void version of your method and a T version if you want both.