Why can't I use System.ValueType as a generics constraint?
- 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); }
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.
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.