Question

I just started learning C# today, and I am trying to make a generic method which can operate on different enums. I got some code that I found in another question on paper, but the compiler keeps complaining. I was wondering if someone could give me a hand. Here is what I have so far:

    static void ReadMenuInput<T>(out T menuInput)
    {
        while (true)
        {
            if (enum<T>.TryParse(Console.ReadLine(), out menuInput) && menuInput < sizeof(T))
            {
                break;
            }
            Console.WriteLine("Please enter a valid input.");
        } 
    }

Thank you for your help!

Answer 1

There are a couple of different issues with that particular code snippet, some of them easier to deal with than others. A few of them are simply showing your lack of experience:

• enum and Enum are not interchangable; one is a class name and one is a C# keyword. In some cases (say, string and String) the keyword is just an alias for the type, but enum is not a type name, it is used to define type names. To call class methods you need to use Enum.
• You are trying to call the generic TryParse<> method with the wrong syntax; it should be Enum.TryParse<T>.
• Enumerations and integers are not the same type, and you cannot just compare them. They are, however, convertible to one another via an explicit typecast. Since sizeof returns an int, you need such a typecast (but see below).

The more complex issues with your code sample:

• sizeof(enum) doesn’t do quite what you expect, I think; it returns the size in bytes of the enum, which is typically going to be 4. You probably want the IsDefined method, which lets you know if a particular integer value is defined in an enumerated type
• As the compiler will tell you, you can only use generic types in this context that are non-nullable. The way to define this is with a where T: struct constraint on your generic type. Note that, despite its name, this doesn’t force your type to be a structure; it simply forces it to be a value type.
• There is no way for you specify a constraint on the generic type that it must be an enumerated type; if you pass some other value type into the method, it will throw an exception at run-time. It’s up to you to handle this case properly.

Here’s a working (as in — it compiles — I haven’t actually tested it) version of the code snippet you want. However, I will point out that every one of the problems in your original code would be worked out just by reading and understanding the error messages; this is one of the most important skills you should get good at as a C# developer.

static void ReadMenuInput<T>(out T menuInput) where T : struct
{           
  while (true)
  {
    if (Enum.TryParse<T>(Console.ReadLine(), out menuInput)
     && Enum.IsDefined(typeof(T), menuInput))
    {
      break;
    }

    Console.WriteLine("Please enter a valid input.");
  } 
}