How to TryParse for Enum value?

2019-01-06 10:02发布

I want to write a function which can validate a given value (passed as a string) against possible values of an enum. In the case of a match, it should return the enum instance; otherwise, it should return a default value.

The function may not internally use try/catch, which excludes using Enum.Parse, which throws an exception when given an invalid argument.

I'd like to use something along the lines of a TryParse function to implement this:

public static TEnum ToEnum<TEnum>(this string strEnumValue, TEnum defaultValue)
{
   object enumValue;
   if (!TryParse (typeof (TEnum), strEnumValue, out enumValue))
   {
       return defaultValue;
   }
   return (TEnum) enumValue;
}

标签: c# enums
14条回答
劫难
2楼-- · 2019-01-06 10:33

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 :)

Summer. ? 凉城
3楼-- · 2019-01-06 10:33

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

贪生不怕死
4楼-- · 2019-01-06 10:35

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

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我只想做你的唯一
5楼-- · 2019-01-06 10:36

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.

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地球回转人心会变
6楼-- · 2019-01-06 10:37

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);
    }
}
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你好瞎i
7楼-- · 2019-01-06 10:39

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.

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