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

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.

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.

Using struct as a generic constraint is functionally equivalent to a "ValueType" constraint. In .NET, a struct is a value type.