Fix missing cast when comparing a Guid and String columns in Sql Server (#2042)

Author: maca88

src/NHibernate.Test/Async/NHSpecificTest/NH3426/Fixture.cs

@@ -84,5 +84,18 @@ public async Task WhereGuidToStringAsync()
 				Assert.That(list, Has.Count.EqualTo(1));
 			}
 		}
+
+		[Test]
+		public async Task CompareStringColumnWithGuidToStringAsync()
+		{
+			using (var session = OpenSession())
+			{
+				var list = await (session.Query<Entity>()
+				                  .Where(x => x.Id.ToString() == x.Name)
+				                  .ToListAsync());
+
+				Assert.That(list, Has.Count.EqualTo(0));
+			}
+		}
 	}
 }

src/NHibernate.Test/NHSpecificTest/NH3426/Fixture.cs

@@ -72,5 +72,18 @@ public void WhereGuidToString()
 				Assert.That(list, Has.Count.EqualTo(1));
 			}
 		}
+
+		[Test]
+		public void CompareStringColumnWithGuidToString()
+		{
+			using (var session = OpenSession())
+			{
+				var list = session.Query<Entity>()
+				                  .Where(x => x.Id.ToString() == x.Name)
+				                  .ToList();
+
+				Assert.That(list, Has.Count.EqualTo(0));
+			}
+		}
 	}
 }

src/NHibernate/Dialect/MsSql2000Dialect.cs

@@ -352,6 +352,7 @@ protected virtual void RegisterFunctions()
 			// Casting to CHAR (without specified length) truncates to 30 characters. 
 			// A longer version would be safer, but 50 is enough to prevent errors when casting uniqueidentifer to a string representation (NH-2858)
 			RegisterFunction("str", new SQLFunctionTemplate(NHibernateUtil.String, "cast(?1 as nvarchar(50))"));
+			RegisterFunction("strguid", new SQLFunctionTemplate(NHibernateUtil.String, "cast(?1 as char(36))"));
 
 			RegisterFunction("substring", new EmulatedLengthSubstringFunction());
 

src/NHibernate/Dialect/MsSqlCeDialect.cs

@@ -170,6 +170,7 @@ protected virtual void RegisterFunctions()
 		{
 			RegisterFunction("substring", new EmulatedLengthSubstringFunction());
 			RegisterFunction("str", new SQLFunctionTemplate(NHibernateUtil.String, "cast(?1 as nvarchar)"));
+			RegisterFunction("strguid", new SQLFunctionTemplate(NHibernateUtil.String, "cast(?1 as nvarchar)"));
 
 			RegisterFunction("current_timestamp", new NoArgSQLFunction("getdate", NHibernateUtil.DateTime, true));
 			RegisterFunction("date", new SQLFunctionTemplate(NHibernateUtil.DateTime, "dateadd(dd, 0, datediff(dd, 0, ?1))"));