How to add UserControl to a Panel on a WPF Window

I think I'm missing something that should be obvious here, but I'm drawing a blank on this one. I've built a very primitive UserControl containing nothing more than a `TextBox` to use as a log window: ``` <UserControl x:Class="My.LoggerControl" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" x:Name="LoggerView"> <Grid x:Name="LayoutRoot"> <TextBox x:Name="LogWindow" AcceptsReturn="True"/> </Grid> </UserControl> ``` I don't expect that to be the best way to do it, but it should be good enough for a prototype. The code-behind is similarly simple: ``` public partial class LoggerControl : UserControl, ILogger { public LoggerControl() { InitializeComponent(); } private LogLevel level = LogLevel.Warning; #region ILogger public LogLevel Level { get { return level; } set { level = value; } } public void OnError(string s) { if (level >= LogLevel.Error) LogWindow.AppendText("ERROR:::" + s + "\n"); } // ... #endregion } ``` The thing I can't figure out is how to add this control to my `MainWindow.xaml`. Simplifying, lets say my window looks like this: ``` <Window x:Class="My.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:My" Title="Test" Height="350" Width="525"> <Grid> <local:LoggerControl x:Name="LogView" /> </Grid> </Window> ``` Even with something so simple, the Designer in Visual Studio 2010 can't load the main window. The error given is: A value of type 'LoggerControl' cannot be added to a collectionor dictionary of type 'UIElementCollection'. This error message has only one unrelated hit in the major search engines (plus duplicates) so I haven't found any useful help. Microsoft's own documentation seems to imply that this should work. Any idea how to solve this?