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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Answer 1

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

Answer 2

I think the example below covers a lot of use cases that one would expect from ValueType. The parameter type of T? allows nullables, and the type constraints restrict it to structs that implement IFormattable which is true for the common value types I can think of.

public void foo<T>(T? a) where T : struct, IFormattable

Note that this allows types such as decimal, datetime, timespan.

https://docs.microsoft.com/en-us/dotnet/api/system.iformattable?view=netcore-3.1

Answer 3

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.

Answer 4

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.