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Posted 17 May 2011

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See if a Flags enum is valid

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17 May 2011CPOL
You can use System.Linq.Expressions to avoid converting the Enum value to an integer type.Here's a solution inspired by the MiscUtil project[^]:using System;using System.Collections.Generic;using System.Linq;using System.Linq.Expressions; public static class...
You can use System.Linq.Expressions to avoid converting the Enum value to an integer type.

Here's a solution inspired by the MiscUtil project[^]:

C#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
  
public static class EnumExtensions
{
   private static class GenericOperator<T>
   {
      public static readonly Func<T, T> Not = Operator(Expression.Not);
      public static readonly Func<T, T, T> Or = Operator<T>(Expression.Or);
      public static readonly Func<T, T, T> And = Operator<T>(Expression.And);
      public static readonly Func<T, T, bool> Equal = Operator<bool>(Expression.Equal);
 
      private static Func<T, T> Operator(Func<Expression, UnaryExpression> body)
      {
         try
         {
            Type typeT = typeof(T);
            var param = Expression.Parameter(typeT, "p");
 
            if (typeT.IsEnum)
            {
               Type enumType = Enum.GetUnderlyingType(typeT);
               var x = Expression.Convert(param, enumType);
               var op = Expression.Convert(body(x), typeT);
               return Expression.Lambda<Func<T, T>>(op, param).Compile();
            }
 
            return Expression.Lambda<Func<T, T>>(body(param), param).Compile();
         }
         catch (InvalidOperationException ex)
         {
            string message = ex.Message;
            return delegate { throw new InvalidOperationException(message); };
         }
         catch (ArgumentException ex)
         {
            string message = ex.Message;
            return delegate { throw new InvalidOperationException(message); };
         }
      }
 
      private static Func<T, T, TResult> Operator<TResult>(Func<Expression, Expression, BinaryExpression> body)
      {
         try
         {
            Type typeT = typeof(T);
            var left = Expression.Parameter(typeT, "left");
            var right = Expression.Parameter(typeT, "right");
 
            if (typeT.IsEnum)
            {
               Type enumType = Enum.GetUnderlyingType(typeT);
               var x = Expression.Convert(left, enumType);
               var y = Expression.Convert(right, enumType);
               Expression op = body(x, y);
               if (op.Type == enumType) op = Expression.Convert(op, typeT);
               return Expression.Lambda<Func<T, T, TResult>>(op, left, right).Compile();
            }
 
            return Expression.Lambda<Func<T, T, TResult>>(body(left, right), left, right).Compile();
         }
         catch (InvalidOperationException ex)
         {
            string message = ex.Message;
            return delegate { throw new InvalidOperationException(message); };
         }
         catch (ArgumentException ex)
         {
            string message = ex.Message;
            return delegate { throw new InvalidOperationException(message); };
         }
      }
   }
 
   private sealed class Cache<T> where T : struct, IFormattable, IComparable
   {
      public static readonly bool IsFlags = Attribute.IsDefined(typeof(T), typeof(FlagsAttribute));
      public static readonly T[] Values = (T[])Enum.GetValues(typeof(T));
      private static readonly T _valuesCombined;
      private static readonly T _invalidMask;
 
      static Cache()
      {
         _valuesCombined = Values.Aggregate(GenericOperator<T>.Or);
         _invalidMask = GenericOperator<T>.Not(_valuesCombined);
      }
 
      public static bool IsValid(T value)
      {
         bool result;
         if (GenericOperator<T>.Equal(default(T), value))
         {
            result = true;
         }
         else if (IsFlags)
         {
            T masked = GenericOperator<T>.And(value, _invalidMask);
            result = GenericOperator<T>.Equal(default(T), masked);
         }
         else
         {
            result = -1 != Array.IndexOf(Values, value);
         }
         return result;
      }
   }
 
   public static IEnumerable<T> GetValues<T>() where T : struct, IComparable, IFormattable
   {
      var values = Cache<T>.Values;
      if (null == values || 0 == values.Length) return Enumerable.Empty<T>();
      return values.Select(x => x);
   }
 
   public static bool IsDefined<T>(T value) where T : struct, IComparable, IFormattable
   {
      return Cache<T>.IsValid(value);
   }
 
   public static bool IsFlags<T>() where T : struct, IComparable, IFormattable
   {
      return Cache<T>.IsFlags;
   }
}

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Richard Deeming
Software Developer CodeProject
United Kingdom United Kingdom
I started writing code when I was 8, with my trusty ZX Spectrum and a subscription to "Input" magazine. Spent many a happy hour in the school's computer labs with the BBC Micros and our two DOS PCs.

After a brief detour into the world of Maths, I found my way back into programming during my degree via free copies of Delphi and Visual C++ given away with computing magazines.

I went straight from my degree into my first programming job, at Trinet Ltd. Eleven years later, the company merged to become ArcomIT. Three years after that, our project manager left to set up Nevalee Business Solutions, and took me with him. Since then, we've taken on four more members of staff, and more work than you can shake a stick at. Smile | :)

Between writing custom code to integrate with Visma Business, developing web portals to streamline operations for a large multi-national customer, and maintaining RedAtlas, our general aviation airport management system, there's certainly never a dull day in the office!

Outside of work, I enjoy real ale and decent books, and when I get the chance I "tinkle the ivories" on my Technics organ.

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