Check if enum is mapped as any kind of string

Author: gliljas

src/NHibernate/Linq/Visitors/HqlGeneratorExpressionVisitor.cs

@@ -634,7 +634,7 @@ private bool IsCastRequired(IType type, IType toType, out bool existType)
 				return false;
 			}
 
-			if (type.ReturnedClass.IsEnum && sqlTypes[0].DbType == DbType.String)
+			if (type.ReturnedClass.IsEnum && sqlTypes[0].DbType.IsStringType())
 			{
 				existType = false;
 				return false; // Never cast an enum that is mapped as string, the type will provide a string for the parameter value

src/NHibernate/Util/DbTypeExtensions.cs

@@ -0,0 +1,14 @@
+using System.Data;
+
+namespace NHibernate.Util
+{
+	public static class DbTypeExtensions
+	{
+		/// <summary>
+		/// Checks whether the type is a <see cref="DbType.String"/>, <see cref="DbType.AnsiString"/>, <see cref="DbType.StringFixedLength"/> or <see cref="DbType.AnsiStringFixedLength"/>
+		/// </summary>
+		/// <param name="dbType"></param>
+		/// <returns></returns>
+		public static bool IsStringType(this DbType dbType) => dbType == DbType.String || dbType == DbType.AnsiString || dbType == DbType.StringFixedLength || dbType == DbType.AnsiStringFixedLength;
+	}
+}