C# Enum.TryParse parses invalid number strings

Question

C# .NET 4.5, Windows 10, I have the following enum:

private enum Enums
{
    A=1, B=2, C=3
}

And this program behaves in a very strange way:

public static void Main()
{
    Enums e;
    if (Enum.TryParse("12", out e))
    {
        Console.WriteLine("Parsed {0}", e);
    }
    else
    {
        Console.Write("Not parsed");
    }
    Console.ReadLine();
}

I would expect the result of the TryParse method to be false, but to my surprise the console shows "Parsed 12". In the Watch window it even shows that the value is "12" and it is of the Enums type!

This is true for any number string that I tried (e.g. "540"), but not for strings that include letters ("A12", "12A").

I can easily overcome this by first checking if it's a number-only string, but why is this the behaviour? Is it by design?

Thanks! Ido

Answer 1

Internally, enums are stored as integers so that's likely why TryParse is returning true for integers being passed in.

Regarding why any integer is working, it's by design. From MSDN (emphasis mine):

When this method returns, result contains an object of type TEnum whose value is represented by value if the parse operation succeeds. If the parse operation fails, result contains the default value of the underlying type of TEnum. Note that this value need not be a member of the TEnum enumeration. This parameter is passed uninitialized.

Answer 2

A variable or field of an enumeration type can hold any values of its underlying type, so storing the value of 12 in a variable of type Enums in your case is entirely legal:

var e = (Enums) 12;
var i = (int) e; // i is 12

Therefore, Enum.TryParse must be able to parse any value of type int (or whichever underlying integer type used in your enumeration).

If you want to reject values having no representation in your enumeration, check them with Enum.IsDefined.