Why can't I use System.ValueType as a generics constraint?

Question

• Why can't I use a constraint of where T : System.ValueType?
• Why does Microsoft prevent this type from being a constraint?

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Example:

Why can't I do the following?

// Defined in a .Net class public void bar<T>(T a) where T : ValueType {...}  // Defined in my class public void foo<T>(T a) where T : ValueType  { bar<T>(a); } 

What is the difference in using struct over ValueType?

// Defined in my class public void foo<T>(T a) where T : struct  { bar<T>(a); } 

Answer 1

There are two differences between using

where T : struct 

and

where T : ValueType 

• the latter would allow T to be ValueType itself, which is a reference type.
• the latter would also allow T to be a nullable value type

The first of these differences is almost never what you want. The second could occasionally be useful; Nullable<T> is slightly odd in that it satisfies neither the where T : struct nor where T : class constraint.

More useful would be the constraint

where T : struct, System.Enum 

which is prohibited by C# for no good reason that I can tell. See my blog post and the Unconstrained Melody project for more on this.

Answer 2

ValueType is not the base class of value types, it is simply a container for the value when it is boxed. Since it is a container class and not in any sort of hierarchy for the actual types you're wanting to use, it is not useful as a generic constraint.