Fixed: error setting default sort for non-string properties

src/JsonApiDotNetCore/Internal/ResourceGraph.cs

@@ -84,7 +84,9 @@ private IEnumerable<IResourceField> Getter<T>(Expression<Func<T, dynamic>> selec
 
             var targeted = new List<IResourceField>();
 
-            if (selector.Body is MemberExpression memberExpression)
+            var selectorBody = RemoveConvert(selector.Body);
+
+            if (selectorBody is MemberExpression memberExpression)
             {   // model => model.Field1
                 try
                 {
@@ -97,8 +99,7 @@ private IEnumerable<IResourceField> Getter<T>(Expression<Func<T, dynamic>> selec
                 }
             }
 
-
-            if (selector.Body is NewExpression newExpression)
+            if (selectorBody is NewExpression newExpression)
             {   // model => new { model.Field1, model.Field2 }
                 string memberName = null;
                 try
@@ -119,11 +120,17 @@ private IEnumerable<IResourceField> Getter<T>(Expression<Func<T, dynamic>> selec
                 }
             }
 
-            throw new ArgumentException($"The expression returned by '{selector}' for '{GetType()}' is of type {selector.Body.GetType()}"
-                        + " and cannot be used to select resource attributes. The type must be a NewExpression.Example: article => new { article.Author };");
-
+            throw new ArgumentException(
+                $"The expression '{selector}' should select a single property or select multiple properties into an anonymous type. " + 
+                $"For example: 'article => article.Title' or 'article => new {{ article.Title, article.PageCount }}'.");
         }
 
+        private static Expression RemoveConvert(Expression expression)
+            => expression is UnaryExpression unaryExpression
+               && unaryExpression.NodeType == ExpressionType.Convert
+                ? RemoveConvert(unaryExpression.Operand)
+                : expression;
+
         private void ThrowNotExposedError(string memberName, FieldFilterType type)
         {
             throw new ArgumentException($"{memberName} is not an json:api exposed {type:g}.");

test/UnitTests/Models/ResourceDefinitionTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using JsonApiDotNetCore.Builders;
 using JsonApiDotNetCore.Internal.Query;
@@ -23,7 +24,7 @@ public void Property_Sort_Order_Uses_NewExpression()
             // Assert
             Assert.Equal(2, sorts.Count);
 
-            Assert.Equal(nameof(Model.Prop), sorts[0].Attribute.PropertyInfo.Name);
+            Assert.Equal(nameof(Model.CreatedAt), sorts[0].Attribute.PropertyInfo.Name);
             Assert.Equal(SortDirection.Ascending, sorts[0].SortDirection);
 
             Assert.Equal(nameof(Model.Password), sorts[1].Attribute.PropertyInfo.Name);
@@ -62,7 +63,7 @@ public class Model : Identifiable
     {
         [Attr] public string AlwaysExcluded { get; set; }
         [Attr] public string Password { get; set; }
-        [Attr] public string Prop { get; set; }
+        [Attr] public DateTime CreatedAt { get; set; }
     }
 
     public sealed class RequestFilteredResource : ResourceDefinition<Model>
@@ -72,19 +73,21 @@ public sealed class RequestFilteredResource : ResourceDefinition<Model>
         public RequestFilteredResource(bool isAdmin) : base(new ResourceGraphBuilder(new JsonApiOptions(), NullLoggerFactory.Instance).AddResource<Model>().Build())
         {
             if (isAdmin)
-                HideFields(m => m.AlwaysExcluded);
+                HideFields(model => model.AlwaysExcluded);
             else
-                HideFields(m => new { m.AlwaysExcluded, m.Password });
+                HideFields(model => new { model.AlwaysExcluded, model.Password });
         }
 
         public override QueryFilters GetQueryFilters()
             => new QueryFilters {
                 { "is-active", (query, value) => query.Select(x => x) }
             };
+
         public override PropertySortOrder GetDefaultSortOrder()
-            => new PropertySortOrder {
-                (t => t.Prop, SortDirection.Ascending),
-                (t => t.Password, SortDirection.Descending)
+            => new PropertySortOrder
+            {
+                (model => model.CreatedAt, SortDirection.Ascending),
+                (model => model.Password, SortDirection.Descending)
             };
     }
 }