How to TryParse for Enum value?

Question

I want to write a utility wherein can validate a given value (string) against possible values of enum. And in case of match, return the enum instance, else return a default value. PS: I don't want to user Try...catch here.

public static TEnum ToEnum<TEnum>(this string strEnumValue, TEnum defaultValue){
   TEnum enumValue;
   if(!TryParse(typeof(TEnum),strEnumValue, out enumValue)){ //Looking for implementation??????
return defaultValue;
   }
return enumValue;
}

I am looking for some optmized implementation of "TryParse..." here. No try...catch or Enum.GetNames() solution, please.

[Edit] I know about Enum.Parse(typeof(TEnum), strEnumValue) method. It throw ArgumentException if strEnumValue is not valid.

Looking for TryParse........

Thanks

Answer 1

Have a look at the Enum class (struct ? ) itself. There is a Parse method on that but I'm not sure about a tryparse.

Answer 2

The only way to avoid exception handling is to use the GetNames() method, and we all know that exceptions shouldn't be abused for common application logic :)

Answer 3

There's currently no out of the box Enum.TryParse. This has been requested on Connect (Still no Enum.TryParse) and got a response indicating possible inclusion in the next framework after .NET 3.5. You'll have to implement the suggested workarounds for now.

Answer 4

Is caching a dynamically generated function/dictionary permissable?

Because you don't (appear to) know the type of the enum ahead of time, the first execution could generate something subsequent executions could take advantage of.

You could even cache the result of Enum.GetNames()

Are you trying to optimize for CPU or Memory? Do you really need to?

Answer 5

In the end you have to implement this around Enum.GetNames:

public bool TryParseEnum<T>(string str, bool caseSensitive, out T value) where T : struct {
    // Can't make this a type constraint...
    if (!typeof(T).IsEnum) {
        throw new ArgumentException("Type parameter must be an enum");
    }
    var names = Enum.GetNames(typeof(T));
    value = (Enum.GetValues(typeof(T)) as T[])[0];  // For want of a better default
    foreach (var name in names) {
        if (String.Equals(name, str, caseSensitive ? StringComparison.Ordinal : StringComparison.OrdinalIgnoreCase)) {
            value = (T)Enum.Parse(typeof(T), name);
            return true;
        }
    }
    return false;
}

Additional notes:

• Enum.TryParse is included in .NET 4. See here http://msdn.microsoft.com/library/dd991876(VS.100).aspx
• Another approach would be to directly wrap Enum.Parse catching the exception thrown when it fails. This could be faster when a match is found, but will likely to slower if not. Depending on the data you are processing this may or may not be a net improvement.

---

EDIT: Just seen a better implementation on this, which caches the necessary information: http://damieng.com/blog/2010/10/17/enums-better-syntax-improved-performance-and-tryparse-in-net-3-5

Answer 6

Enum.IsDefined will get things done. It may not be as efficient as a TryParse would probably be, but it will work without exception handling.

public static TEnum ToEnum<TEnum>(this string strEnumValue, TEnum defaultValue)
{
    if (!Enum.IsDefined(typeof(TEnum), strEnumValue))
        return defaultValue;

    return (TEnum)Enum.Parse(typeof(TEnum), strEnumValue);
}

Worth noting: a TryParse method was added in .NET 4.0.

Answer 7

As others have said, you have to implement your own TryParse. If you are using bitfield enums, you also have to handle a string like "MyEnum.Val1|MyEnum.Val2". If you just call Enum.IsDefined with this string, it will return false, even though Enum.Parse handles it correctly.

Answer 8

As others already said, if you don't use Try&Catch, you need to use IsDefined or GetNames... Here are some samples...they basically are all the same, the first one handling nullable enums. I prefer the 2nd one as it's an extension on strings, not enums...but you can mix them as you want!

• www.objectreference.net/post/Enum-TryParse-Extension-Method.aspx
• flatlinerdoa.spaces.live.com/blog/cns!17124D03A9A052B0!605.entry
• mironabramson.com/blog/post/2008/03/Another-version-for-the-missing-method-EnumTryParse.aspx
• lazyloading.blogspot.com/2008/04/enumtryparse-with-net-35-extension.html

Answer 9

I have an optimised implementation you could use in UnconstrainedMelody. Effectively it's just caching the list of names, but it's doing so in a nice, strongly typed, generically constrained way :)

Answer 10

This method will convert a type of enum:

  public static TEnum ToEnum<TEnum>(object EnumValue, TEnum defaultValue)
    {
        if (!Enum.IsDefined(typeof(TEnum), EnumValue))
        {
            Type enumType = Enum.GetUnderlyingType(typeof(TEnum));
            if ( EnumValue.GetType() == enumType )
            {
                string name = Enum.GetName(typeof(HLink.ViewModels.ClaimHeaderViewModel.ClaimStatus), EnumValue);
                if( name != null)
                    return (TEnum)Enum.Parse(typeof(TEnum), name);
                return defaultValue;
            }
        }
        return (TEnum)Enum.Parse(typeof(TEnum), EnumValue.ToString());
    } 

It checks the underlying type and get the name against it to parse. If everything fails it will return default value.

Answer 11

There is not a TryParse because the Enum's type is not known until runtime. A TryParse that follows the same methodology as say the Date.TryParse method would throw an implicit conversion error on the ByRef parameter.

I suggest doing something like this:

//1 line call to get value MyEnums enumValue = (Sections)EnumValue(typeof(Sections), myEnumTextValue, MyEnums.SomeEnumDefault);

//Put this somewhere where you can reuse public static object EnumValue(System.Type enumType, string value, object NotDefinedReplacement) { if (Enum.IsDefined(enumType, value)) { return Enum.Parse(enumType, value); } else { return Enum.Parse(enumType, NotDefinedReplacement); } } //======================================================= //Service provided by Telerik (www.telerik.com) //Conversion powered by NRefactory. //Twitter: @telerik, @toddanglin //Facebook: facebook.com/telerik //=======================================================