XamlParseException when adding a usercontrol into a project

Question

I created a user control in a project that consists of just a MainWindow.xaml and the code behind. I added the .dll to the toolbox of VS, and dropped it onto a window in a new project. This created the following:

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication5"
        xmlns:ThinkGeoClean="clr-namespace:ThinkGeoClean;assembly=ThinkGeoClean" x:Class="WpfApplication5.MainWindow"
        mc:Ignorable="d"
        Title="MainWindow" >
    <Grid>
        <ThinkGeoClean:ListBoxCustom x:Name="listBoxCustom" />
    </Grid>
</Window>

The ThinkGeoClean is the name of the .dll I added which is the usercontrol. ListBoxCustom is just a public class in the control, but is NOT what I want to show. I want to show the main window of the usercontrol (not a window), but it doesn't show as an option after typing <ThinkGeoClean:. The only thing that shows up is ListBoxCustom. If I go ahead and type <ThinkGeoClean.MainWindow>, it gives a XamlParseException error on that line.

Now, if I go into the code-behind and do:

ThinkGeoClean.MainWindow newWin = new ThinkGeoClean.MainWindow();
newWin.Show();

It will pop up the usercontrol in a new window and it works fine.

Here's the beginning of usercontrol code behind:

namespace ThinkGeoClean
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {

And the xaml for it is just a single grid containing some buttons and a map control.

Edit: In addition to the answer below, my user control was originally just a normal WPF project. I thought changing the output type to a class library would change it into a user control automatically, but I actually had to go into the xaml and change the the into .

Answer 1

This XAML:

<Grid>
    <ThinkGeoClean:ListBoxCustom x:Name="listBoxCustom" />
</Grid>

...is not equivalent to creating an instance of a window and call the Show() method on it programmatically.

Instead the XAML processor will try to add the window to the Children collection of the Grid and this is not possible since a window cannot be child of another control. That's why you get an exception.

Also, a UserControl must be hosted in a window or page. It is not a top-level control that you can display without any parent host.