Possible bug in C# 7.3 handling of generic Enum constraints [duplicate]

Question

The following code does not compile in C# 7.3 even though it does support generics constrained to be enums:

using System;
public class Test<T> where T: Enum
{
    public void Method()
    {
        if (!Enum.TryParse<T>("something", out var value))
            throw new Exception("Oops");
    }
}

My other code that uses Enum constraints does work, so I have the right versions of everything, it just doesn't seem to be able to call another method that also is constrained to be an Enum.

Is this a bug or did I misunderstand how this is is supposed to work.

Can you add the compiler error to your question please.

Ashigore
– Ashigore

2019-08-23 08:16:36 +00:00
Commented Aug 23, 2019 at 8:16 — Ashigore
– Ashigore, Commented Aug 23, 2019 at 8:16
What error you are getting?

Chetan
– Chetan

2019-08-23 08:17:32 +00:00
Commented Aug 23, 2019 at 8:17 — Chetan
– Chetan, Commented Aug 23, 2019 at 8:17

canton7 · Accepted Answer · 2019-08-23 08:22:11Z

7

You need an extra constraint:

public class Test<T> where T: struct, Enum
{
    public void Method()
    {
        if (!Enum.TryParse<T>("something", out var value))
            throw new Exception("Oops");
    }
}

With just where T : Enum, you're allowed to call new Test<Enum>().Method(); -- i.e. pass in the Enum type, rather than any specific type of enum. Adding struct means you have to pass in a specific type of enum.

More specifically, Enum.TryParse<T> has the constraint where T : struct, so you need to match this constraint in your method.

edited Aug 23, 2019 at 8:22

answered Aug 23, 2019 at 8:16

canton7

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6 Comments

ProgrammingLlama Over a year ago

TryParse Code

bikeman868 Over a year ago

That works. Can you explain why. Since Enum.TryParse<T> has the same where T: Enum constraint I would expect this to work and find this result very surprising.

canton7 Over a year ago

@bikeman868 I've edited my answer. Enum.TryParse<T> has the constraint where T : struct, not where T : Enum see here

bikeman868 Over a year ago

I did some reading, and to clarify: Enum is a class but a specific type of enum is a struct. So the Enum constraint constrains it to the Enum class or a class that inherits from Enum and the additional struct constraint means that it must also be a struct and only specific enum types have these characteristics.

canton7 Over a year ago

@bikeman868 Yep, that's what my first paragraph tries to explain

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