Use recommended ContentControl pattern.

src/Avalonia.Controls/ContentControl.cs

@@ -97,7 +97,19 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         void IContentPresenterHost.RegisterContentPresenter(IContentPresenter presenter)
         {
-            Presenter = presenter;
+            RegisterContentPresenter(presenter);
+        }
+
+        /// <summary>
+        /// Called when an <see cref="IContentPresenter"/> is registered with the control.
+        /// </summary>
+        /// <param name="presenter">The presenter.</param>
+        protected virtual void RegisterContentPresenter(IContentPresenter presenter)
+        {
+            if (presenter.Name == "PART_ContentPresenter")
+            {
+                Presenter = presenter;
+            }
         }
     }
 }

src/Avalonia.Controls/Primitives/HeaderedContentControl.cs

@@ -1,6 +1,8 @@
 // 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 Avalonia.Controls.Mixins;
+using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 
 namespace Avalonia.Controls.Primitives
@@ -20,7 +22,18 @@ namespace Avalonia.Controls.Primitives
         /// Defines the <see cref="HeaderTemplate"/> property.
         /// </summary>
         public static readonly StyledProperty<IDataTemplate> HeaderTemplateProperty =
-            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate>(nameof(HeaderTemplate));          
+            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate>(nameof(HeaderTemplate));
+
+        /// <summary>
+        /// Initializes static members of the <see cref="ContentControl"/> class.
+        /// </summary>
+        static HeaderedContentControl()
+        {
+            ContentControlMixin.Attach<HeaderedContentControl>(
+                HeaderProperty,
+                x => x.LogicalChildren,
+                "PART_HeaderPresenter");
+        }
 
         /// <summary>
         /// Gets or sets the header content.
@@ -29,7 +42,16 @@ namespace Avalonia.Controls.Primitives
         {
             get { return GetValue(HeaderProperty); }
             set { SetValue(HeaderProperty, value); }
-        }     
+        }
+
+        /// <summary>
+        /// Gets the header presenter from the control's template.
+        /// </summary>
+        public IContentPresenter HeaderPresenter
+        {
+            get;
+            private set;
+        }
 
         /// <summary>
         /// Gets or sets the data template used to display the header content of the control.
@@ -39,5 +61,16 @@ namespace Avalonia.Controls.Primitives
             get { return GetValue(HeaderTemplateProperty); }
             set { SetValue(HeaderTemplateProperty, value); }
         }
+
+        /// <inheritdoc/>
+        protected override void RegisterContentPresenter(IContentPresenter presenter)
+        {
+            base.RegisterContentPresenter(presenter);
+
+            if (presenter.Name == "PART_HeaderPresenter")
+            {
+                HeaderPresenter = presenter;
+            }
+        }
     }
 }

src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Controls.Primitives
     /// <summary>
     /// Represents an <see cref="ItemsControl"/> with a related header.
     /// </summary>
-    public class HeaderedItemsControl : ItemsControl
+    public class HeaderedItemsControl : ItemsControl, IContentPresenterHost
     {
         /// <summary>
         /// Defines the <see cref="Header"/> property.
@@ -40,17 +40,28 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the header presenter from the control's template.
         /// </summary>
-        public ContentPresenter HeaderPresenter
+        public IContentPresenter HeaderPresenter
         {
             get;
             private set;
         }
 
         /// <inheritdoc/>
-        protected override void OnTemplateApplied(TemplateAppliedEventArgs e)
+        void IContentPresenterHost.RegisterContentPresenter(IContentPresenter presenter)
         {
-            HeaderPresenter = e.NameScope.Find<ContentPresenter>("PART_HeaderPresenter");
-            base.OnTemplateApplied(e);
+            RegisterContentPresenter(presenter);
+        }
+
+        /// <summary>
+        /// Called when an <see cref="IContentPresenter"/> is registered with the control.
+        /// </summary>
+        /// <param name="presenter">The presenter.</param>
+        protected virtual void RegisterContentPresenter(IContentPresenter presenter)
+        {
+            if (presenter.Name == "PART_HeaderPresenter")
+            {
+                HeaderPresenter = presenter;
+            }
         }
     }
 }

src/Avalonia.Controls/Primitives/HeaderedSelectingControl.cs → src/Avalonia.Controls/Primitives/HeaderedSelectingItemsControl.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Controls.Primitives
     /// <summary>
     /// Represents a <see cref="SelectingItemsControl"/> with a related header.
     /// </summary>
-    public class HeaderedSelectingItemsControl : SelectingItemsControl
+    public class HeaderedSelectingItemsControl : SelectingItemsControl, IContentPresenterHost
     {
         /// <summary>
         /// Defines the <see cref="Header"/> property.
@@ -40,17 +40,28 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the header presenter from the control's template.
         /// </summary>
-        public ContentPresenter HeaderPresenter
+        public IContentPresenter HeaderPresenter
         {
             get;
             private set;
         }
 
         /// <inheritdoc/>
-        protected override void OnTemplateApplied(TemplateAppliedEventArgs e)
+        void IContentPresenterHost.RegisterContentPresenter(IContentPresenter presenter)
         {
-            base.OnTemplateApplied(e);
-            HeaderPresenter = e.NameScope.Find<ContentPresenter>("PART_HeaderPresenter");
+            RegisterContentPresenter(presenter);
+        }
+
+        /// <summary>
+        /// Called when an <see cref="IContentPresenter"/> is registered with the control.
+        /// </summary>
+        /// <param name="presenter">The presenter.</param>
+        protected virtual void RegisterContentPresenter(IContentPresenter presenter)
+        {
+            if (presenter.Name == "PART_HeaderPresenter")
+            {
+                HeaderPresenter = presenter;
+            }
         }
     }
 }

tests/Avalonia.Controls.UnitTests/HeaderedItemsControlTests .cs

@@ -37,7 +37,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.Header = "Foo";
             target.ApplyTemplate();
-            target.HeaderPresenter.UpdateChild();
+            ((ContentPresenter)target.HeaderPresenter).UpdateChild();
 
             var child = target.HeaderPresenter.Child;
 

tests/Avalonia.UnitTests/TestTemplatedRoot.cs

@@ -18,7 +18,10 @@ namespace Avalonia.UnitTests
 
         public TestTemplatedRoot()
         {
-            Template = new FuncControlTemplate<TestTemplatedRoot>(x => new ContentPresenter());
+            Template = new FuncControlTemplate<TestTemplatedRoot>(x => new ContentPresenter
+            {
+                Name = "PART_ContentPresenter",
+            });
         }
 
         public event EventHandler<NameScopeEventArgs> Registered