Trying to parse a flag enum to string

Question

I have a class "license" which is a collection of a bunch of enum flags like this:

Public class License
{
    UsageType Usage { get; set; }
    PlatformType Platform { get; set; }

    public enum UsageType { read = 1, write = 2, wipe = 4, all = 7 }
    public enum PlatformType { windows = 1, linux = 2, ios = 4, all = 7 }

    etc...
}

The point is that the various flags of the same category can be OR'd together to form a profile of what the user can do with said license. Now I'm trying to display the values of "Usage" and "Platform" in a human-friendly way so for instance if Usage == UsageType.read | UsageType.write then it should be parsed to "read, write".

I did this successfully with a single enum type by testing the value for each flag and appending enumitem.ToString() for each flag it has to a string. Since I have a lot of these enums and values though, I'd like to come up with a more generic approach.

I came up with this (below) but since I'm not very familiar with template functions in c# so I don't know why this doesn't work but at least it should illustrate what i mean:

private string parseEnum<T>(T value)
{
    string ret = "";
    foreach (var ei in (T[])Enum.GetValues(typeof(T)))
    {
        if (value.HasFlag(ei)) ret += ei.ToString() + ", ";
    }
    ret = ret.substring(0, ret.Length-1);
    return ret;
}

It's saying that T does not contain a definition for "HasFlag" but how could it now that if it doesn't know what T is?

Answer 1

You should use the FlagsAttribute, which causes the built-in ToString and Enum.Parse methods to work just the way you want. Also note that the convention is that flags enum names should be plural, so e.g. UsageTypes instead of UsageType.

[Flags]
public enum UsageTypes { Read = 1, Write = 2, Wipe = 4, All = 7 }
[Flags]
public enum PlatformTypes { Windows = 1, Linux = 2, iOs = 4, All = 7 }

var e1 = License.UsageTypes.Read | License.UsageTypes.Write;
var s = e1.ToString();
Debug.Assert(s == "Read, Write");
var e2 = (License.UsageTypes)Enum.Parse(typeof(License.UsageTypes), s);
Debug.Assert(e1 == e2);