Enum.TryParse not accepted in generic restricted to Enum

Question

I think I'm probably doing something silly but I'm trying to write a generic function that takes a string and converts it into an enum (and then does some other stuff that I've skipped for sake of brevity). The problem is, it complains that Enum.TryParse needs a type that isn't nullable, it complains that T is nullable; Seemingly System.Enum is nullable but actual enums aren't nullable. Is there something I'm doing wrong here or is there a way around the problem.

private T GetEnumFilter<T>(string strValue) where T : Enum
{
     return Enum.TryParse(strValue, out T value) ? value : throw new Exception("Invalid value");
}

I've seen this https://stackoverflow.com/a/8086788/1093406 answer and the sample at the dotnet samples and can't see what I've done wrong.

You shouldn't shadow your argument name with a variable name. Use some different name for `out T value` and `? value :` — Heretic Monkey, Nov 11 '20 at 17:12
Sorry, made a couple of errors when simplifying my code for SO. Thanks to @HereticMonkey for pointing out the mistake — Mog0, Nov 11 '20 at 17:20

Jon Skeet · Accepted Answer · 2020-11-11T17:27:50.413

5

Seemingly System.Enum is nullable but actual enums aren't nullable.

Yes, just like System.ValueType is a reference type, but value types themselves aren't.

You just need to add a struct constraint:

private T GetEnumFilter<T>(string value) where T : struct, Enum

This compiles, for example:

private static T GetEnumFilter<T>(string value) where T : struct, Enum =>
    Enum.TryParse(value, out T result) ? result : throw new Exception("Invalid value");

edited Nov 11 '20 at 17:27

answered Nov 11 '20 at 17:12

Jon Skeet

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Thanks @Jon for the amazingly fast answer. I've fixed the code errors in my question introduced when simplifying for SO but the super simple addition of struct is what I was missing. – Mog0 Nov 11 '20 at 17:22

Enum.TryParse not accepted in generic restricted to Enum

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