NH-3963 - More explicit error on MappedAs invalid usage

Author: fredericDelaporte

src/NHibernate.Test/NHSpecificTest/NH3963/Entity.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.NH3963
+{
+	class Entity
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/NH3963/MappedAsFixture.cs

@@ -0,0 +1,38 @@
+using System.Linq;
+using NHibernate.Linq;
+using NUnit.Framework;
+using NHibernate.Type;
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.NH3963
+{
+	[TestFixture]
+	public class MappedAsFixture : BugTestCase
+	{
+		[Test]
+		public void ShouldThrowExplicitErrorOnInvalidUsage()
+		{
+			using (ISession session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var result = session.Query<Entity>()
+					.Where(e => e.Name.MappedAs(NHibernateUtil.AnsiString) == "Bob");
+
+				Assert.Throws<HibernateException>(() => { result.ToList(); });
+			}
+		}
+
+		[Test]
+		public void ShouldThrowExplicitErrorOnUnsupportedTypeUsage()
+		{
+			using (ISession session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var result = session.Query<Entity>()
+					.Where(e => e.Name == "Bob".MappedAs(e.Id == Guid.Empty ? (IType)NHibernateUtil.AnsiString : NHibernateUtil.StringClob));
+
+				Assert.Throws<HibernateException>(() => { result.ToList(); });
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/NH3963/Mappings.hbm.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test" namespace="NHibernate.Test.NHSpecificTest.NH3963">
+
+	<class name="Entity">
+		<id name="Id" generator="guid.comb" />
+		<property name="Name" />
+	</class>
+
+</hibernate-mapping>

src/NHibernate.Test/NHibernate.Test.csproj

@@ -741,6 +741,8 @@
     <Compile Include="NHSpecificTest\NH3386\Fixture.cs" />
     <Compile Include="NHSpecificTest\NH3961\Entity.cs" />
     <Compile Include="NHSpecificTest\NH3961\DateParametersComparedTo.cs" />
+    <Compile Include="NHSpecificTest\NH3963\Entity.cs" />
+    <Compile Include="NHSpecificTest\NH3963\MappedAsFixture.cs" />
     <Compile Include="NHSpecificTest\NH3950\Entity.cs" />
     <Compile Include="NHSpecificTest\NH3950\Fixture.cs" />
     <Compile Include="NHSpecificTest\NH3952\Entity.cs" />
@@ -3214,6 +3216,7 @@
   <ItemGroup>
     <EmbeddedResource Include="NHSpecificTest\NH3386\Mappings.hbm.xml" />
     <EmbeddedResource Include="NHSpecificTest\NH3961\Mappings.hbm.xml" />
+    <EmbeddedResource Include="NHSpecificTest\NH3963\Mappings.hbm.xml" />
     <EmbeddedResource Include="NHSpecificTest\NH3950\Mappings.hbm.xml" />
     <EmbeddedResource Include="NHSpecificTest\NH3952\Mappings.hbm.xml" />
     <EmbeddedResource Include="NHSpecificTest\NH2204\Mappings.hbm.xml" />

src/NHibernate/Linq/Visitors/ExpressionParameterVisitor.cs

@@ -47,10 +47,19 @@ internal static IDictionary<ConstantExpression, NamedParameter> Visit(ref Expres
 
 		protected override Expression VisitMethodCallExpression(MethodCallExpression expression)
 		{
-			if (expression.Method.Name == "MappedAs" && expression.Method.DeclaringType == typeof(LinqExtensionMethods))
+			if (expression.Method.Name == nameof(LinqExtensionMethods.MappedAs) && expression.Method.DeclaringType == typeof(LinqExtensionMethods))
 			{
-				var parameter = (ConstantExpression)VisitExpression(expression.Arguments[0]);
-				var type = (ConstantExpression)expression.Arguments[1];
+				var rawParameter = VisitExpression(expression.Arguments[0]);
+				var parameter = rawParameter as ConstantExpression;
+				var type = expression.Arguments[1] as ConstantExpression;
+				if (parameter == null)
+					throw new HibernateException(
+						$"{nameof(LinqExtensionMethods.MappedAs)} must be called on an expression which can be evaluated as " +
+						$"{nameof(ConstantExpression)}. It was call on {rawParameter?.GetType().Name ?? "null"} instead.");
+				if (type == null)
+					throw new HibernateException(
+						$"{nameof(LinqExtensionMethods.MappedAs)} type must be supplied as {nameof(ConstantExpression)}. " +
+						$"It was {expression.Arguments[1]?.GetType().Name ?? "null"} instead.");
 
 				_parameters[parameter].Type = (IType)type.Value;