What is the best way to strictly parse string to enum?

Question

Let's say I have an enum:

public enum MyEnum
{
    OptionOne = 0,
    OptionTwo = 2,
    OptionThree = 4
}

Like it was said in the How should I convert a string to an enum in C#? question, I parse enum from string, using Enum.Parse method:

public class Enumer
{
    public static MyEnum? ParseEnum(string input)
    {
        try
        {
            return (MyEnum) Enum.Parse(typeof (MyEnum), input);
        }
        catch (ArgumentException)
        {
            return null;
        }
    }

}

Unfortunately, it doesn't work as expected with integer numbers, represented as strings.
I do not expect Parse.Enum() to convert int from string, but actually it does.
A simple test:

[TestClass]
public class Tester
{
    [TestMethod]
    public void TestEnum()
    {
        Assert.AreEqual(MyEnum.OptionTwo, Enumer.ParseEnum("OptionTwo"));
        Assert.IsNull(Enumer.ParseEnum("WrongString"));
        Assert.IsNull(Enumer.ParseEnum("2")); // returns 2 instead of null
        Assert.IsNull(Enumer.ParseEnum("12345")); // returns 12345 instead of null
    }
}

Only two checks of four can be passed.
Enumer.ParseEnum("2") returns MyEnum.OptionTwo instead of null.
Moreover, Enumer.ParseEnum("12345") returns 12345, regardless it's out of the scope.

What is the best way to parse:

Only string values of "MyEnum.OptionOne", "MyEnum.OptionTwo", "MyEnum.OptionThree" into their enum counterparts.
Strings "MyEnum.OptionOne", "MyEnum.OptionTwo", "MyEnum.OptionThree" AND the values of 0, 2 and 4 into MyEnum.OptionOne, MyEnum.OptionTwo, MyEnum.OptionThree respectively?

The link to the similar question How should I convert a string to an enum in C#? doesn't help with the test provided - it still converts strings as integers, even when they are out of the enum scope.

Possible duplicate of [How do I convert a string to an enum in C#?](http://stackoverflow.com/questions/16100/how-do-i-convert-a-string-to-an-enum-in-c) — Erkan Demirel, Apr 11 '16 at 15:32
@ErkanDemirel, as far as I understand, that question was about Enum.Parse() syntax itself. — enkryptor, Apr 11 '16 at 15:33
@Kolky you've misunderstood the question. I do NOT want to parse "12345" as 12345, but Enum.Parse does it. — enkryptor, Apr 11 '16 at 15:37

Toxantron · Accepted Answer · 2016-04-11T16:00:18.143

You can do this yourself using Enum.GetValues(Type enumType).

public static MyEnum? ParseEnum(string input)
{
    return (MyEnum?) Enum.GetValues(typeof(MyEnum)).OfType<object>()
                         .FirstOrDefault(v => v.ToString() == input);
}

Adding support for integers you can also add.

public static MyEnum? ParseEnum(string input)
{
    int value;
    var isInt = int.TryParse(input, out value);
    return (MyEnum?) Enum.GetValues(typeof(MyEnum)).OfType<object>()
                         .FirstOrDefault(v => v.ToString() == input
                                           || (isInt & (int)v == value));
}

Since I am not 100% sure about your last requirements I add one that includes the name as well and is more generic

public static T? ParseEnum<T>(string input)
    where T : struct 
{
    int value;
    var isInt = int.TryParse(input, out value);
    return (T?)Enum.GetValues(typeof(T)).OfType<object>()
                   .FirstOrDefault(v => v.ToString() == typeof(T).Name + input
                                    || (isInt & (int)v == value));
}

Wouldn't it be more efficient first convert into the enum and then check the scope? — enkryptor, Apr 12 '16 at 09:07
AFAIK the enum methods `Parse()` and `IsDefined()` use reflection since an enum is nothing else than a class with `public const int OptionOne = 1` fields. Therefore this should have identical performance. If you want to do this efficiently you should create a `IDictionary` and `IDictionary` and use this for the conversion. — Toxantron, Apr 12 '16 at 09:12

What is the best way to strictly parse string to enum?

1 Answers1