Fixes nhibernate#2621

Special treatment of enums mapped on string columns because all reasons
for checking string lengths or streamlining the length of parameters of
queries on string columns to the length of the column for SQL Server
are also valid for enums because they are sometimes by convention used
to model the possible values of a string column

Author: wvopt

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Fixture.cs

@@ -0,0 +1,22 @@
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH2621Enum
+{
+	public class Fixture : BugTestCase
+	{
+		[Test]
+		public void TestOk()
+		{
+			using (ISession s = OpenSession())
+			{
+				using (ITransaction t = s.BeginTransaction())
+				{
+					var query = s.CreateQuery(@"
+							SELECT Name FROM NHibernate.Test.NHSpecificTest.GH2620Enum.ClassWithString ROOT WHERE ROOT.Kind = :kind");
+					query.SetParameter("kind", Kind.SomeKind);
+					Assert.DoesNotThrow(() => query.List());
+				}
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Mappings.hbm.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
+           namespace="NHibernate.Test.NHSpecificTest.GH2621Enum"
+           assembly="NHibernate.Test"
+>
+  <class name="ClassWithString" >
+    <id name="Id">
+      <generator class="increment"/>
+    </id>
+    <property name="Name"/>
+    <property name="Kind"/>
+  </class>
+
+</hibernate-mapping>

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Model.cs

@@ -0,0 +1,15 @@
+namespace NHibernate.Test.NHSpecificTest.GH2621Enum
+{
+	public abstract class ClassWithString
+	{
+		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual string Kind { get; set; }
+	}
+
+	public enum Kind 
+	{
+		SomeKind,
+		SomeOtherKind
+	}
+}

src/NHibernate/Driver/MicrosoftDataSqlClientDriver.cs

@@ -75,6 +75,9 @@ public MicrosoftDataSqlClientDriver()
 		/// <inheritdoc />
 		public virtual void AdjustParameterForValue(DbParameter parameter, SqlType sqlType, object value)
 		{
+			if (value is Enum)
+				value = Enum.GetName(value.GetType(), value);
+
 			if (value is string stringVal)
 				switch (parameter.DbType)
 				{

src/NHibernate/Driver/SqlClientDriver.cs

@@ -320,6 +320,9 @@ public override bool SupportsMultipleQueries
 
 		public virtual void AdjustParameterForValue(DbParameter parameter, SqlType sqlType, object value)
 		{
+			if (value is Enum)
+				value = Enum.GetName(value.GetType(), value);
+
 			if (value is string stringVal)
 			{
 				switch (parameter.DbType)

src/NHibernate/Type/AbstractStringType.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Data.Common;
 using System.Globalization;
@@ -64,7 +64,7 @@ public override void Set(DbCommand cmd, object value, int index, ISessionImpleme
 			// set the parameter value before the size check, since ODBC changes the size automatically
 			parameter.Value = value;
 
-			if (parameter.Size > 0 && ((string)value).Length > parameter.Size)
+			if (parameter.Size > 0 && value != null && Convert.ToString(value).Length > parameter.Size)
 				throw new HibernateException("The length of the string value exceeds the length configured in the mapping/parameter.");
 		}