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;
}
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);
}
}
enum EnumStatus
{
NAO_INFORMADO = 0,
ENCONTRADO = 1,
BLOQUEADA_PELO_ENTREGADOR = 2,
DISPOSITIVO_DESABILITADO = 3,
ERRO_INTERNO = 4,
AGARDANDO = 5
}
...
if (Enum.TryParse<EnumStatus>(item.status, out status)) {
}
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.
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 :)
Here is a custom implementation of EnumTryParse. Unlike other common implementations, it also supports enum marked with the Flags attribute.
/// <summary>
/// Converts the string representation of an enum to its Enum equivalent value. A return value indicates whether the operation succeeded.
/// This method does not rely on Enum.Parse and therefore will never raise any first or second chance exception.
/// </summary>
/// <param name="type">The enum target type. May not be null.</param>
/// <param name="input">The input text. May be null.</param>
/// <param name="value">When this method returns, contains Enum equivalent value to the enum contained in input, if the conversion succeeded.</param>
/// <returns>
/// true if s was converted successfully; otherwise, false.
/// </returns>
public static bool EnumTryParse(Type type, string input, out object value)
{
if (type == null)
throw new ArgumentNullException("type");
if (!type.IsEnum)
throw new ArgumentException(null, "type");
if (input == null)
{
value = Activator.CreateInstance(type);
return false;
}
input = input.Trim();
if (input.Length == 0)
{
value = Activator.CreateInstance(type);
return false;
}
string[] names = Enum.GetNames(type);
if (names.Length == 0)
{
value = Activator.CreateInstance(type);
return false;
}
Type underlyingType = Enum.GetUnderlyingType(type);
Array values = Enum.GetValues(type);
// some enums like System.CodeDom.MemberAttributes *are* flags but are not declared with Flags...
if ((!type.IsDefined(typeof(FlagsAttribute), true)) && (input.IndexOfAny(_enumSeperators) < 0))
return EnumToObject(type, underlyingType, names, values, input, out value);
// multi value enum
string[] tokens = input.Split(_enumSeperators, StringSplitOptions.RemoveEmptyEntries);
if (tokens.Length == 0)
{
value = Activator.CreateInstance(type);
return false;
}
ulong ul = 0;
foreach (string tok in tokens)
{
string token = tok.Trim(); // NOTE: we don't consider empty tokens as errors
if (token.Length == 0)
continue;
object tokenValue;
if (!EnumToObject(type, underlyingType, names, values, token, out tokenValue))
{
value = Activator.CreateInstance(type);
return false;
}
ulong tokenUl;
switch (Convert.GetTypeCode(tokenValue))
{
case TypeCode.Int16:
case TypeCode.Int32:
case TypeCode.Int64:
case TypeCode.SByte:
tokenUl = (ulong)Convert.ToInt64(tokenValue, CultureInfo.InvariantCulture);
break;
//case TypeCode.Byte:
//case TypeCode.UInt16:
//case TypeCode.UInt32:
//case TypeCode.UInt64:
default:
tokenUl = Convert.ToUInt64(tokenValue, CultureInfo.InvariantCulture);
break;
}
ul |= tokenUl;
}
value = Enum.ToObject(type, ul);
return true;
}
private static char[] _enumSeperators = new char[] { ',', ';', '+', '|', ' ' };
private static object EnumToObject(Type underlyingType, string input)
{
if (underlyingType == typeof(int))
{
int s;
if (int.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(uint))
{
uint s;
if (uint.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(ulong))
{
ulong s;
if (ulong.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(long))
{
long s;
if (long.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(short))
{
short s;
if (short.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(ushort))
{
ushort s;
if (ushort.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(byte))
{
byte s;
if (byte.TryParse(input, out s))
return s;
}
if (underlyingType == typeof(sbyte))
{
sbyte s;
if (sbyte.TryParse(input, out s))
return s;
}
return null;
}
private static bool EnumToObject(Type type, Type underlyingType, string[] names, Array values, string input, out object value)
{
for (int i = 0; i < names.Length; i++)
{
if (string.Compare(names[i], input, StringComparison.OrdinalIgnoreCase) == 0)
{
value = values.GetValue(i);
return true;
}
}
if ((char.IsDigit(input[0]) || (input[0] == '-')) || (input[0] == '+'))
{
object obj = EnumToObject(underlyingType, input);
if (obj == null)
{
value = Activator.CreateInstance(type);
return false;
}
value = obj;
return true;
}
value = Activator.CreateInstance(type);
return false;
}
Based on .NET 4.5
Sample code below
using System;
enum Importance
{
None,
Low,
Medium,
Critical
}
class Program
{
static void Main()
{
// The input value.
string value = "Medium";
// An unitialized variable.
Importance importance;
// Call Enum.TryParse method.
if (Enum.TryParse(value, out importance))
{
// We now have an enum type.
Console.WriteLine(importance == Importance.Medium);
}
}
}
Reference : http://www.dotnetperls.com/enum-parse
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?
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).aspxEnum.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
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 :)
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!
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.
Have a look at the Enum class (struct ? ) itself. There is a Parse method on that but I'm not sure about a tryparse.
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.
Source: Stackoverflow.com