DesiredSize doesn't need to be null.

Perspex.Controls/Border.cs

@@ -85,7 +85,7 @@ namespace Perspex.Controls
             if (content != null)
             {
                 content.Measure(availableSize.Deflate(padding));
-                return content.DesiredSize.Value.Inflate(padding);
+                return content.DesiredSize.Inflate(padding);
             }
 
             return new Size();

Perspex.Controls/Decorator.cs

@@ -80,7 +80,7 @@ namespace Perspex.Controls
             if (content != null)
             {
                 content.Measure(availableSize.Deflate(padding));
-                return content.DesiredSize.Value.Inflate(padding);
+                return content.DesiredSize.Inflate(padding);
             }
 
             return new Size();

Perspex.Controls/Grid.cs

@@ -359,7 +359,7 @@ namespace Perspex.Controls
                     }
 
                     child.Measure(new Size(childSizeX, childSizeY));
-                    Size desired = child.DesiredSize.Value;
+                    Size desired = child.DesiredSize;
 
                     // Elements distribute their height based on two rules:
                     // 1) Elements with rowspan/colspan == 1 distribute their height first

Perspex.Controls/Presenters/ContentPresenter.cs

@@ -65,7 +65,7 @@ namespace Perspex.Controls.Presenters
             if (child != null)
             {
                 child.Measure(availableSize);
-                return child.DesiredSize.Value;
+                return child.DesiredSize;
             }
 
             return new Size();

Perspex.Controls/Presenters/DeckPresenter.cs

@@ -14,10 +14,8 @@ namespace Perspex.Controls.Presenters
     using Perspex.Styling;
     using System;
     using System.Collections;
-    using System.Collections.Specialized;
     using System.Linq;
     using System.Reactive.Linq;
-    using System.Threading.Tasks;
 
     public class DeckPresenter : Control, IVisual, IPresenter, ITemplatedControl
     {
@@ -87,7 +85,7 @@ namespace Perspex.Controls.Presenters
         protected override Size MeasureOverride(Size availableSize)
         {
             this.Panel.Measure(availableSize);
-            return this.Panel.DesiredSize.Value;
+            return this.Panel.DesiredSize;
         }
 
         protected override Size ArrangeOverride(Size finalSize)

Perspex.Controls/Presenters/ItemsPresenter.cs

@@ -64,7 +64,7 @@ namespace Perspex.Controls.Presenters
         protected override Size MeasureOverride(Size availableSize)
         {
             this.Panel.Measure(availableSize);
-            return this.Panel.DesiredSize.Value;
+            return this.Panel.DesiredSize;
         }
 
         protected override Size ArrangeOverride(Size finalSize)

Perspex.Controls/Presenters/ScrollContentPresenter.cs

@@ -76,7 +76,7 @@ namespace Perspex.Controls.Presenters
                 }
 
                 content.Measure(measureSize);
-                var size = content.DesiredSize.Value;
+                var size = content.DesiredSize;
                 this.measuredExtent = size;
                 return size.Constrain(availableSize);
             }
@@ -95,7 +95,7 @@ namespace Perspex.Controls.Presenters
 
             if (child != null)
             {
-                child.Arrange(new Rect((Point)(-this.Offset), child.DesiredSize.Value));
+                child.Arrange(new Rect((Point)(-this.Offset), child.DesiredSize));
                 return finalSize;
             }
 

Perspex.Controls/Primitives/AdornerLayer.cs

@@ -60,7 +60,7 @@ namespace Perspex.Controls.Primitives
                 }
                 else
                 {
-                    child.Arrange(new Rect(child.DesiredSize.Value));
+                    child.Arrange(new Rect(child.DesiredSize));
                 }
             }
 

Perspex.Controls/Primitives/TemplatedControl.cs

@@ -166,7 +166,7 @@ namespace Perspex.Controls.Primitives
             if (child != null)
             {
                 child.Measure(availableSize);
-                return child.DesiredSize.Value;
+                return child.DesiredSize;
             }
 
             return new Size();

Perspex.Controls/Primitives/Track.cs

@@ -103,11 +103,11 @@ namespace Perspex.Controls.Primitives
 
                 if (this.Orientation == Orientation.Horizontal)
                 {
-                    return new Size(0, thumb.DesiredSize.Value.Height);
+                    return new Size(0, thumb.DesiredSize.Height);
                 }
                 else
                 {
-                    return new Size(thumb.DesiredSize.Value.Width, 0);
+                    return new Size(thumb.DesiredSize.Width, 0);
                 }
             }
 

Perspex.Controls/StackPanel.cs

@@ -116,7 +116,7 @@ namespace Perspex.Controls
             foreach (Control child in this.Children)
             {
                 child.Measure(new Size(childAvailableWidth, childAvailableHeight));
-                Size size = child.DesiredSize.Value;
+                Size size = child.DesiredSize;
 
                 if (Orientation == Orientation.Vertical)
                 {
@@ -150,8 +150,8 @@ namespace Perspex.Controls
 
             foreach (Control child in this.Children)
             {
-                double childWidth = child.DesiredSize.Value.Width;
-                double childHeight = child.DesiredSize.Value.Height;
+                double childWidth = child.DesiredSize.Width;
+                double childHeight = child.DesiredSize.Height;
 
                 if (Orientation == Orientation.Vertical)
                 {

Perspex.Controls/TopLevel.cs

@@ -173,8 +173,8 @@ namespace Perspex.Controls
             using (this.BeginAutoSizing())
             {
                 this.ClientSize = new Size(
-                    double.IsNaN(this.Width) ? this.DesiredSize.Value.Width : this.ClientSize.Width,
-                    double.IsNaN(this.Height) ? this.DesiredSize.Value.Height : this.ClientSize.Height);
+                    double.IsNaN(this.Width) ? this.DesiredSize.Width : this.ClientSize.Width,
+                    double.IsNaN(this.Height) ? this.DesiredSize.Height : this.ClientSize.Height);
             }
 
             this.PlatformImpl.Invalidate(new Rect(this.ClientSize));

Perspex.Layout/ILayoutable.cs

@@ -9,7 +9,7 @@ namespace Perspex.Layout
     // TODO: Probably want to move width/height/etc properties to different interface.
     public interface ILayoutable : IVisual
     {
-        Size? DesiredSize { get; }
+        Size DesiredSize { get; }
 
         double Width { get;  }
 

Perspex.Layout/LayoutManager.cs

@@ -211,7 +211,7 @@ namespace Perspex.Layout
 
                 if (!this.Root.IsArrangeValid && this.Root.IsMeasureValid)
                 {
-                    this.Root.Arrange(new Rect(this.Root.DesiredSize.Value));
+                    this.Root.Arrange(new Rect(this.Root.DesiredSize));
                 }
 
                 if (this.toMeasure.Count > 0)

Perspex.Layout/Layoutable.cs

@@ -131,7 +131,7 @@ namespace Perspex.Layout
             set { this.SetValue(VerticalAlignmentProperty, value); }
         }
 
-        public Size? DesiredSize
+        public Size DesiredSize
         {
             get;
             set;
@@ -246,7 +246,6 @@ namespace Perspex.Layout
             this.IsArrangeValid = false;
             this.previousMeasure = null;
             this.previousArrange = null;
-            this.DesiredSize = null;
 
             if (parent != null && IsResizable(parent))
             {
@@ -307,12 +306,12 @@ namespace Perspex.Layout
 
                 if (this.HorizontalAlignment != HorizontalAlignment.Stretch)
                 {
-                    size = size.WithWidth(Math.Min(size.Width, this.DesiredSize.Value.Width));
+                    size = size.WithWidth(Math.Min(size.Width, this.DesiredSize.Width));
                 }
 
                 if (this.VerticalAlignment != VerticalAlignment.Stretch)
                 {
-                    size = size.WithHeight(Math.Min(size.Height, this.DesiredSize.Value.Height));
+                    size = size.WithHeight(Math.Min(size.Height, this.DesiredSize.Height));
                 }
 
                 size = LayoutHelper.ApplyLayoutConstraints(this, size);
@@ -411,8 +410,8 @@ namespace Perspex.Layout
             foreach (ILayoutable child in this.GetVisualChildren().OfType<ILayoutable>())
             {
                 child.Measure(availableSize);
-                width = Math.Max(width, child.DesiredSize.Value.Width);
-                height = Math.Max(height, child.DesiredSize.Value.Height);
+                width = Math.Max(width, child.DesiredSize.Width);
+                height = Math.Max(height, child.DesiredSize.Height);
             }
 
             return new Size(width, height);