Fix correctly type nulls (and other constants) (#354)

Author: rockResolve

src/System.Linq.Dynamic.Core/Parser/ExpressionParser.cs

@@ -1518,10 +1518,12 @@ Expression ParseTypeAccess(Type type)
             {
                 Expression[] args = shorthand ? new[] { ParseStringLiteral() } : ParseArgumentList();
 
-                // If only 1 argument, and the arg is ConstantExpression, just return the ConstantExpression
-                if (args.Length == 1 && args[0] is ConstantExpression)
+                // If only 1 argument, and the arg is ConstantExpression, return the conversion
+                // If only 1 argument, and the arg is null, return the conversion (Can't use constructor)
+                if (args.Length == 1 
+                    && (args[0] == null || args[0] is ConstantExpression))
                 {
-                    return args[0];
+                    return GenerateConversion(args[0], type, errorPos);
                 }
 
                 // If only 1 argument, and if the type is a ValueType and argType is also a ValueType, just Convert

test/System.Linq.Dynamic.Core.Tests/Parser/ExpressionParserTests.cs

@@ -71,10 +71,10 @@ public void Parse_ParseAndOperator(string expression, string result)
         }
 
         [Theory]
-        [InlineData("string(null)", null)]
         [InlineData("string(\"\")", "")]
         [InlineData("string(\"a\")", "a")]
-        public void Parse_CastStringShouldReturnConstantExpression(string expression, object result)
+        [InlineData("int(42)", 42)]
+        public void Parse_CastStringIntShouldReturnConstantExpression(string expression, object result)
         {
             // Arrange
             ParameterExpression[] parameters = { ParameterExpressionHelper.CreateParameterExpression(typeof(bool), "x") };
@@ -88,20 +88,27 @@ public void Parse_CastStringShouldReturnConstantExpression(string expression, ob
         }
 
         [Theory]
-        [InlineData("int?(null)", null)]
-        [InlineData("int?(5)", 5)]
-        [InlineData("int(42)", 42)]
-        public void Parse_CastIntShouldReturnConstantExpression(string expression, object result)
+#if NET452
+        [InlineData("int?(5)", typeof(int?), "Convert(5)")]
+        [InlineData("int?(null)", typeof(int?), "Convert(null)")]
+        [InlineData("string(null)", typeof(string), "Convert(null)")]
+#else
+        [InlineData("int?(5)", typeof(int?), "Convert(5, Nullable`1)")]
+        [InlineData("int?(null)", typeof(int?), "Convert(null, Nullable`1)")]
+        [InlineData("string(null)", typeof(string), "Convert(null, String)")]
+#endif
+        public void Parse_NullableShouldReturnNullable(string expression, object resultType, object result)
         {
             // Arrange
             ParameterExpression[] parameters = { ParameterExpressionHelper.CreateParameterExpression(typeof(bool), "x") };
             var sut = new ExpressionParser(parameters, expression, null, null);
 
             // Act
-            var constantExpression = (ConstantExpression)sut.Parse(null);
+            var unaryExpression = (UnaryExpression)sut.Parse(null);
 
             // Assert
-            Check.That(constantExpression.Value).Equals(result);
+            Check.That(unaryExpression.Type).Equals(resultType);
+            Check.That(unaryExpression.ToString()).Equals(result);
         }
     }
 }