Set canvas property default values to NaN.

And add unit tests. Fixes #704.

src/Avalonia.Controls/Canvas.cs

@@ -22,25 +22,25 @@ namespace Avalonia.Controls
         /// Defines the Left attached property.
         /// </summary>
         public static readonly AttachedProperty<double> LeftProperty =
-            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Left");
+            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Left", double.NaN);
 
         /// <summary>
         /// Defines the Top attached property.
         /// </summary>
         public static readonly AttachedProperty<double> TopProperty =
-            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Top");
+            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Top", double.NaN);
 
         /// <summary>
         /// Defines the Right attached property.
         /// </summary>
         public static readonly AttachedProperty<double> RightProperty =
-            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Right");
+            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Right", double.NaN);
 
         /// <summary>
         /// Defines the Bottom attached property.
         /// </summary>
         public static readonly AttachedProperty<double> BottomProperty =
-            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Bottom");
+            AvaloniaProperty.RegisterAttached<Canvas, Control, double>("Bottom", double.NaN);
 
         /// <summary>
         /// Initializes static members of the <see cref="Canvas"/> class.

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -95,6 +95,7 @@
   </Choose>
   <ItemGroup>
     <Compile Include="AppBuilderTests.cs" />
+    <Compile Include="CanvasTests.cs" />
     <Compile Include="ClassesTests.cs" />
     <Compile Include="LayoutTransformControlTests.cs" />
     <Compile Include="Presenters\ItemsPresenterTests_Virtualization_Simple.cs" />

tests/Avalonia.Controls.UnitTests/CanvasTests.cs

@@ -0,0 +1,112 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using Avalonia.Controls.Shapes;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class CanvasTests
+    {
+        [Fact]
+        public void Left_Property_Should_Work()
+        {
+            Rectangle rect;
+            var target = new Canvas
+            {
+                Width = 400,
+                Height = 400,
+                Children =
+                {
+                    (rect = new Rectangle
+                    {
+                        MinWidth = 20,
+                        MinHeight = 25,
+                        [Canvas.LeftProperty] = 30,
+                    })
+                }
+            };
+
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(30, 0, 20, 25), rect.Bounds);
+        }
+
+        [Fact]
+        public void Top_Property_Should_Work()
+        {
+            Rectangle rect;
+            var target = new Canvas
+            {
+                Width = 400,
+                Height = 400,
+                Children =
+                {
+                    (rect = new Rectangle
+                    {
+                        MinWidth = 20,
+                        MinHeight = 25,
+                        [Canvas.TopProperty] = 30,
+                    })
+                }
+            };
+
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(0, 30, 20, 25), rect.Bounds);
+        }
+
+        [Fact]
+        public void Right_Property_Should_Work()
+        {
+            Rectangle rect;
+            var target = new Canvas
+            {
+                Width = 400,
+                Height = 400,
+                Children =
+                {
+                    (rect = new Rectangle
+                    {
+                        MinWidth = 20,
+                        MinHeight = 25,
+                        [Canvas.RightProperty] = 30,
+                    })
+                }
+            };
+
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(350, 0, 20, 25), rect.Bounds);
+        }
+
+        [Fact]
+        public void Bottom_Property_Should_Work()
+        {
+            Rectangle rect;
+            var target = new Canvas
+            {
+                Width = 400,
+                Height = 400,
+                Children =
+                {
+                    (rect = new Rectangle
+                    {
+                        MinWidth = 20,
+                        MinHeight = 25,
+                        [Canvas.BottomProperty] = 30,
+                    })
+                }
+            };
+
+            target.Measure(new Size(400, 400));
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(0, 345, 20, 25), rect.Bounds);
+        }
+    }
+}