Non-nullable enum as generic parameter [duplicate]

Question

I am trying to implement this helper function in C# 7.3:

public static T? ToEnum<T>(this string enumName)
    where T : Enum 
    => Enum.TryParse<T>(enumName, out T val) ? val : null;

But got a compiler error:

Error CS8370 Feature 'nullable reference types' is not available in C# 7.3

Why does the compiler think I am using a nullable reference type while T is constrained to be an Enum which is a value type already?

Answer 1

This problem is not specific to nullable enums. Even if you use a non-nullable enum, it won't work. That's because while specific enum types are value types, System.Enum is a reference type as explained here. So, you need to add a struct constraint in addition to the Enum constraint.

The following should work:

public static T? ToEnum<T>(this string enumName) 
    where T : struct, Enum
    => Enum.TryParse<T>(enumName, out T val) ? val : default(T?);

Also, note that you can't use null in the second branch of the ternary operator because the compiler can't infer the type. Use default(T?) instead.

Example of usage:

enum MyEnum { a, b, c }

static void Main(string[] args)
{
    var e1 = "b".ToEnum<MyEnum>();
    var e2 = "d".ToEnum<MyEnum>();

    Console.WriteLine("e1 = " + (e1?.ToString() ?? "NULL"));  // e1 = b
    Console.WriteLine("e2 = " + (e2?.ToString() ?? "NULL"));  // e2 = NULL
}