Friday, 1 February 2013

Using enum as a generic type

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Unfortunately if you want to use an enum as a generic type, the obvious way of doing it doesn't work.

private void Method<TEnum>()
    where TEnum : enum

enum is treated as a special type and Microsoft haven't implemented this (yet). However, it is possible to use enums in generics. The MSDN article for Enum gives the following type definition for the class Enum.

[SerializableAttribute]
[ComVisibleAttribute(true)]
public abstract class Enum : ValueType,
 IComparable, IFormattable, IConvertible

This definition can be used to get enums working as generic types by constraining the generic type to those of Enum. Note that we can not constrain to type ValueType due to a 'special class' rule in .NET, but we can use struct instead to get around this.

private void Method<TEnum>()
    where TEnum : struct, IConvertible, IComparable, IFormattable

The above still allows some errors to get through the compilation process, as we could specify the type of a struct that implements the IComparable, IFormattable and IConvertable interfaces. We can check the type of TEnum and confirm at runtime if it is an enum before doing any work.

private void Method<TEnum>()
    where TEnum : struct, IConvertible, IComparable, IFormattable
{
    if (!typeof(TEnum).IsEnum)
    {
        throw new ArgumentException("TEnum must be an enum.");
    }

    // ...
}

I'm not aware of a way around this that could guarantee 100% type-safety at runtime but it isn't really a big problem. After all, what are the chances that we are going to accidentally pass in a struct that implements the three interfaces.

Further reading