Generic Enum string value parser in c#?

Question

I have a weird question about parsing enumerations from strings. As it is, my app needs to handle parsing of few enumerations from a config file. However, I don't want to write parsing routines for each enumeration type (as there are many).

The problem I am facing is that the following code is showing some weird error - The type of T has to be a non-nullable value type or something of the sort. I thought that enums are by default non-nullable?

If I restrict the type of T using where T : enum, everything else inside the method body (other than the if Enum.TryParse statement) is underlined as error.

Can anyone help with this weird minuscule issue?

Thanks, Martin

public static T GetConfigEnumValue<T>(NameValueCollection config,
                                      string configKey, 
                                      T defaultValue) // where T : enum ?
{
    if (config == null)
    {
        return defaultValue;
    }

    if (config[configKey] == null)
    {
        return defaultValue;
    }

    T result = defaultValue;
    string configValue = config[configKey].Trim();

    if (string.IsNullOrEmpty(configValue))
    {
        return defaultValue;
    }

    //Gives me an Error - T has to be a non nullable value type?
    if( ! Enum.TryParse<T>(configValue, out result) )
    {
        result = defaultValue;
    }

    //Gives me the same error:
    //if( ! Enum.TryParse<typeof(T)>(configValue, out result) )  ...

    return result;
}

A user requested to post the text of the error (it is at code time, not compile/runtime), so here it goes:

The type 'T' must be a non-nullable value type in order to use it as parameter TEnum in the generic type or method 'System.Enum.TryParse(string, out TEnum)'

Can you attach the error? without the where condition, it looks like your code should work. — Nathan Tregillus, Jun 14 '11 at 17:17
@jdv-Jan de Vaan so does this work? I mean i know enum's are really ints and ints are of type System.Int32 struct... but does it really work? — bleepzter, Jun 14 '11 at 17:28
Have a look at http://stackoverflow.com/questions/3811042/adding-constraints-for-nullable-enum, it might help. — Nair, Jun 14 '11 at 17:30
@bleep: An enum can derive from any integral type (`byte`, `long`, etc.). — Gabe, Jun 14 '11 at 17:33
@bleepzter: Now that you added the error message, I see confirmed that this is the solution. So in retrospect I should have posted it as an answer. — , Jun 14 '11 at 18:28

score 1 · Answer 1 · edited Jun 14 '11 at 19:29

1

Ah ok, with that information in mind, I see what the Enum.TryParse method is complaining about.

put a generic constraint on the method like so:

public static T GetConfigEnumValue<T>(NameValueCollection config, 
                                      string configKey, 
                                      T defaultValue) // where T : ValueType

Or just place the same constraint that is on the Enum.TryParse method.

where T : struct, new()

you can find this definition here:

http://msdn.microsoft.com/en-us/library/dd783499.aspx

edited Jun 14 '11 at 19:29

Gustavo Mori

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answered Jun 14 '11 at 19:15

Nathan Tregillus

6,006
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Note that `where T : ValueType` doesn't work, `ValueType` is nullable, it has to be a `where T : struct`. – Greg Bair Sep 10 '14 at 18:29

score 1 · Accepted Answer · answered Jun 16 '11 at 20:39

public static T GetConfigEnumValue<T>(NameValueCollection config, string configKey, T defaultValue)
{
    if (config == null)
    {
        return defaultValue;
    }

    if (config[configKey] == null)
    {
        return defaultValue;
    }

    T result = defaultValue;
    string configValue = config[configKey].Trim();

    if (string.IsNullOrEmpty(configValue))
    {
        return defaultValue;
    }

    try
    {
        result = (T)Enum.Parse(typeof(T), configValue, true);
    }
    catch
    {
        result = defaultValue;
    }

    return result;
}

score 0 · Answer 3 · edited May 23 '17 at 12:20

0

Since C# won't let you do where T : enum, you have to use where T : struct.

Note that there are ways around that restriction as Michael suggested.

edited May 23 '17 at 12:20

Community

1
1

answered Jun 14 '11 at 17:31

Gabe

84,912
12
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Generic Enum string value parser in c#?

3 Answers3