Why can't I use System.ValueType as a generics constraint?
C#GenericsC# Problem Overview
- Why can't I use a constraint of
where T : System.ValueType
? - Why does Microsoft prevent this type from being a constraint?
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); }
C# Solutions
Solution 1 - C#
There are two differences between using
where T : struct
and
where T : ValueType
- the latter would allow
T
to beValueType
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.
Solution 2 - C#
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.
Solution 3 - C#
Using struct
as a generic constraint is functionally equivalent to a "ValueType" constraint. In .NET, a struct is a value type.
Solution 4 - C#
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