Question

I have a CustomControl derived class in a dll (MyNamespace.UI). How can I use this control in my main application? I’ve tried adding a reference to the project and then using a custom XAML namespace to point to my namespace, but it can’t find it:

<Page ...
    xmlns:ui="using:MyNamespace.UI"
    ...>
<Canvas>
    <ui:MyControl />
</Canvas>
</Page>

I get an error:

The name “MyControl” does not exist in the namespace “using:MyNamespace.UI”.

Searching around, I managed to find the following quote:

The name of the assembly that defines the backing types for a XAML
namespace is not specified in the mapping. The logic for which
assemblies are available is controlled at the app-definition level and
is part of basic app deployment and security principles. Declare any
assembly that you want included as a code-definition source for XAML
as a dependent assembly in project settings.

http://msdn.microsoft.com/en-gb/library/windows/apps/jj150588.aspx

Can someone point me in the direction of the “dependent assembly” setting in the project settings?

Edit After making sure the root namespace project setting matched the namespace of my control (MyNamespace.UI) I could get the project to compile. But I can’t get it to run because I get loads of crashes with the following information:

WinRT information: Cannot create instance of type ‘MyNamespace.UI.MyControl’ [Line: 44 Position: 37]

No information on why it couldn’t make the control though… I stepped through the constructor of MyControl and the exception is thrown in InitializeComponent(). This is the XAML of my UserControl (MyControl):

<UserControl
    x:Class="MyNamespace.UI.MyControl"
    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"
    mc:Ignorable="d"
    d:DesignHeight="300"
    d:DesignWidth="400">

    <Grid>

    </Grid>
</UserControl>

Edit 2 I deleted the Grid control, built and ran, and it worked, so then I put the Grid control back, and it still worked. Very weird.

Answer 1

You need to make sure the MyNamespace.UI assembly has a RootNamespace property that matches MyNamespace.UI. This property can’t be changed in the project settings pages, but you can do it via notepad or equivalent. My RootNamespace was set to UI, and when I changed it, it started working.