Fix 1373, Improve EDDL() Exception message in the [Flags] case

- https://aspnetwebstack.codeplex.com/workitem/1373
- "Error message is not clear when enum that has flags attribute calls EnumDropDownListFor html helper"
- Add new message for case where ModelType has a [Flags] attribute
- Provide EnumHelper.HasFlags(Type) internal method to make determination easy for EDDL()
- Leads to a duplicate call to Nullable.GetUnderlyingType(Type) but only in the failure case

src/System.Web.Mvc/Html/EnumHelper.cs

@@ -2,6 +2,7 @@
 
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.Contracts;
 using System.Linq.Expressions;
 using System.Reflection;
 using System.Web.Mvc.Properties;
@@ -38,8 +39,7 @@ namespace System.Web.Mvc.Html
                 Type checkedType = Nullable.GetUnderlyingType(type) ?? type;
                 if (checkedType.IsEnum)
                 {
-                    FlagsAttribute attribute = checkedType.GetCustomAttribute<FlagsAttribute>(inherit: false);
-                    isValid = attribute == null;
+                    isValid = !HasFlagsInternal(checkedType);
                 }
             }
 
@@ -257,6 +257,22 @@ namespace System.Web.Mvc.Html
             return GetSelectList(metadata.ModelType, value);
         }
 
+        internal static bool HasFlags(Type type)
+        {
+            Contract.Assert(type != null);
+
+            Type checkedType = Nullable.GetUnderlyingType(type) ?? type;
+            return HasFlagsInternal(checkedType);
+        }
+
+        private static bool HasFlagsInternal(Type type)
+        {
+            Contract.Assert(type != null);
+
+            FlagsAttribute attribute = type.GetCustomAttribute<FlagsAttribute>(inherit: false);
+            return attribute != null;
+        }
+
         // Return non-empty name specified in a [Display] attribute for the given field, if any; field's name otherwise
         private static string GetDisplayName(FieldInfo field)
         {

src/System.Web.Mvc/Html/SelectExtensions.cs

@@ -169,8 +169,17 @@ namespace System.Web.Mvc.Html
 
             if (!EnumHelper.IsValidForEnumHelper(metadata.ModelType))
             {
-                throw Error.Argument("expression", MvcResources.SelectExtensions_InvalidExpressionParameterType,
-                    metadata.ModelType.FullName);
+                string formatString;
+                if (EnumHelper.HasFlags(metadata.ModelType))
+                {
+                    formatString = MvcResources.SelectExtensions_InvalidExpressionParameterTypeHasFlags;
+                }
+                else
+                {
+                    formatString = MvcResources.SelectExtensions_InvalidExpressionParameterType;
+                }
+
+                throw Error.Argument("expression", formatString, metadata.ModelType.FullName, "Flags");
             }
 
             // Run through same processing as SelectInternal() to determine selected value and ensure it is included

src/System.Web.Mvc/Properties/MvcResources.Designer.cs

@@ -1,7 +1,7 @@
 //------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by a tool.
-//     Runtime Version:4.0.30319.18051
+//     Runtime Version:4.0.30319.34003
 //
 //     Changes to this file may cause incorrect behavior and will be lost if
 //     the code is regenerated.
@@ -1095,6 +1095,15 @@ namespace System.Web.Mvc.Properties {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Return type &apos;{0}&apos; is not supported. Type must not have a &apos;{1}&apos; attribute..
+        /// </summary>
+        internal static string SelectExtensions_InvalidExpressionParameterTypeHasFlags {
+            get {
+                return ResourceManager.GetString("SelectExtensions_InvalidExpressionParameterTypeHasFlags", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to The SessionStateTempDataProvider class requires session state to be enabled..
         /// </summary>

src/System.Web.Mvc/Properties/MvcResources.resx

@@ -507,6 +507,9 @@
   <data name="SelectExtensions_InvalidExpressionParameterType" xml:space="preserve">
     <value>Return type '{0}' is not supported.</value>
   </data>
+  <data name="SelectExtensions_InvalidExpressionParameterTypeHasFlags" xml:space="preserve">
+    <value>Return type '{0}' is not supported. Type must not have a '{1}' attribute.</value>
+  </data>
   <data name="DefaultControllerFactory_DirectRouteAmbiguous" xml:space="preserve">
     <value>Multiple controller types were found that match the URL. This can happen if attribute routes on multiple controllers match the requested URL.{1}{1}The request has found the following matching controller types: {0}</value>
   </data>

test/System.Web.Mvc.Test/Html/Test/SelectExtensionsTest.cs

@@ -1137,7 +1137,7 @@ namespace System.Web.Mvc.Html.Test
             // Act & Assert
             Assert.ThrowsArgument(() => helper.EnumDropDownListFor(model => model), paramName: "expression",
                 exceptionMessage: "Return type 'System.Web.Mvc.Html.Test.SelectExtensionsTest+EnumWithFlags' is not " +
-                "supported." + Environment.NewLine +
+                "supported. Type must not have a 'Flags' attribute." + Environment.NewLine +
                 "Parameter name: expression");
         }
 
@@ -1151,7 +1151,7 @@ namespace System.Web.Mvc.Html.Test
             // Act & Assert
             Assert.ThrowsArgument(() => helper.EnumDropDownListFor(m => m.WithFlags), paramName: "expression",
                 exceptionMessage: "Return type 'System.Web.Mvc.Html.Test.SelectExtensionsTest+EnumWithFlags' is not " +
-                "supported." + Environment.NewLine +
+                "supported. Type must not have a 'Flags' attribute." + Environment.NewLine +
                 "Parameter name: expression");
         }