Compile lambda for null access expressions (#3157)

Fixes #3156

Author: bahusoid

Diff for: src/NHibernate.Test/Criteria/Lambda/ExpressionProcessorFindValueFixture.cs

@@ -42,6 +42,36 @@ public void StaticPropertyInstanceMethodCall()
 
 			Assert.AreEqual(expected, actual);
 		}
+		
+		[Test]
+		public void NullableNullHasValue()
+		{
+			int? v = null;
+			var actual = GetValue(() => v.HasValue);
+			var expected = v.HasValue;
+
+			Assert.AreEqual(expected, actual);
+		}		
+		
+		[Test]
+		public void NullableNullGetValueOrDefault()
+		{
+			int? v = null;
+			var actual = GetValue(() => v.GetValueOrDefault());
+			var expected = v.GetValueOrDefault();
+
+			Assert.AreEqual(expected, actual);
+		}
+		
+		[Test]
+		public void NullableNullValue()
+		{			
+			int? v = null;
+			Expression<Func<int>> expression = () => v.Value;
+
+			Assert.Throws<InvalidOperationException>(() => GetValue(expression));
+			Assert.Throws<InvalidOperationException>(() => expression.Compile().Invoke());
+		}
 
 		[Test]
 		public void StaticPropertyInstanceMultipleMethodCall()

Diff for: src/NHibernate/Impl/ExpressionProcessor.cs

@@ -261,18 +261,25 @@ public static object FindValue(Expression expression)
 					return constantExpression.Value;
 				case ExpressionType.MemberAccess:
 					var memberExpression = (MemberExpression) expression;
+					var instance = findValue(memberExpression.Expression);
+					if (instance == null && memberExpression.Expression != null)
+						break;
+
 					switch (memberExpression.Member.MemberType)
 					{
 						case MemberTypes.Field:
-							return ((FieldInfo) memberExpression.Member).GetValue(findValue(memberExpression.Expression));
+							return ((FieldInfo) memberExpression.Member).GetValue(instance);
 						case MemberTypes.Property:
-							return ((PropertyInfo) memberExpression.Member).GetValue(findValue(memberExpression.Expression));
+							return ((PropertyInfo) memberExpression.Member).GetValue(instance);
 					}
 					break;
 				case ExpressionType.Call:
 					var methodCallExpression = (MethodCallExpression) expression;
 					var args = methodCallExpression.Arguments.ToArray(arg => FindValue(arg));
 					var callingObject = findValue(methodCallExpression.Object);
+					if (callingObject == null && methodCallExpression.Object != null)
+						break;
+
 					return methodCallExpression.Method.Invoke(callingObject, args);
 				case ExpressionType.Convert:
 					var unaryExpression = (UnaryExpression) expression;