manually cast unit tests for full framework

IOrderedEnumerable<T> isn't covariant in full framework. Resort to manual casting to workaround the issue

tests/Avalonia.Controls.DataGrid.UnitTests/Collections/DataGridSortDescriptionTests.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using Avalonia.Collections;
 using Xunit;
 
@@ -42,21 +43,28 @@ namespace Avalonia.Controls.DataGrid.UnitTests.Collections
             
             Assert.Equal(expectedResult, result);
         }
-        
+
         [Fact]
         public void ThenBy_Orders_Correctly_When_Ascending()
         {
+            // Casting nonsense below because IOrderedEnumerable<T> isn't covariant in full framework and we need an
+            // object of type IOrderedEnumerable<object> for DataGridSortDescription.ThenBy
             var items = new[]
             {
-                new Item("a", "b"),
+                (object)new Item("a", "b"),
+                        new Item("a", "a"),
+                        new Item("a", "c"), 
+            }.OrderBy(i => ((Item)i).Prop1);
+            var expectedResult = new[]
+            {
                 new Item("a", "a"),
-                new Item("a", "c"), 
-            }.OrderBy(i => i.Prop1);
-            var expectedResult = items.ThenBy(i => i.Prop2).ToList();
+                new Item("a", "b"),
+                new Item("a", "c"),
+            };
             var sortDescription = DataGridSortDescription.FromPath(nameof(Item.Prop2), @descending: false);
             
             sortDescription.Initialize(typeof(Item));
-            var result = sortDescription.ThenBy((IOrderedEnumerable<object>)items).ToList();
+            var result = sortDescription.ThenBy(items).ToList();
             
             Assert.Equal(expectedResult, result);
         }
@@ -64,22 +72,29 @@ namespace Avalonia.Controls.DataGrid.UnitTests.Collections
         [Fact]
         public void ThenBy_Orders_Correctly_When_Descending()
         {
+            // Casting nonsense below because IOrderedEnumerable<T> isn't covariant in full framework and we need an
+            // object of type IOrderedEnumerable<object> for DataGridSortDescription.ThenBy
             var items = new[]
             {
+                (object)new Item("a", "b"),
+                        new Item("a", "a"),
+                        new Item("a", "c"), 
+            }.OrderBy(i => ((Item)i).Prop1);
+            var expectedResult = new[]
+            {
+                new Item("a", "c"),
                 new Item("a", "b"),
                 new Item("a", "a"),
-                new Item("a", "c"), 
-            }.OrderBy(i => i.Prop1);
-            var expectedResult = items.ThenByDescending(i => i.Prop2).ToList();
+            };
             var sortDescription = DataGridSortDescription.FromPath(nameof(Item.Prop2), @descending: true);
             
             sortDescription.Initialize(typeof(Item));
-            var result = sortDescription.ThenBy((IOrderedEnumerable<object>)items).ToList();
+            var result = sortDescription.ThenBy(items).ToList();
             
             Assert.Equal(expectedResult, result);
         }
-        
-        private class Item
+
+        private class Item : IEquatable<Item>
         {
             public Item(string prop1, string prop2)
             {
@@ -89,6 +104,29 @@ namespace Avalonia.Controls.DataGrid.UnitTests.Collections
 
             public string Prop1 { get; }
             public string Prop2 { get; }
+
+            public bool Equals(Item other)
+            {
+                if (ReferenceEquals(null, other)) return false;
+                if (ReferenceEquals(this, other)) return true;
+                return Prop1 == other.Prop1 && Prop2 == other.Prop2;
+            }
+
+            public override bool Equals(object obj)
+            {
+                if (ReferenceEquals(null, obj)) return false;
+                if (ReferenceEquals(this, obj)) return true;
+                if (obj.GetType() != this.GetType()) return false;
+                return Equals((Item) obj);
+            }
+
+            public override int GetHashCode()
+            {
+                unchecked
+                {
+                    return ((Prop1 != null ? Prop1.GetHashCode() : 0) * 397) ^ (Prop2 != null ? Prop2.GetHashCode() : 0);
+                }
+            }
         }
     }
 }