The component does not have a resource identified by the uri

This gave me headaches for 3 days! I have a XAML UserControl in a class library and a class (only C#) that derives from the UserControl in my .exe project. In xaml designer of my MainWindow.xaml and when starting the application, I got the error "component does not have a resource identified by the uri". The answer of "Juan Carlos Girón" finally lead me to the solution:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using System.Reflection;
using System.IO.Packaging;
using System.Windows.Markup;

namespace ClassLibrary1
{
    static class Extension
    {
        public static void LoadViewFromUri(this UserControl userControl, string baseUri)
        {
            try
            {
                var resourceLocater = new Uri(baseUri, UriKind.Relative);
                var exprCa = (PackagePart)typeof(Application).GetMethod("GetResourceOrContentPart", BindingFlags.NonPublic | BindingFlags.Static).Invoke(null, new object[] { resourceLocater });
                var stream = exprCa.GetStream();
                var uri = new Uri((Uri)typeof(BaseUriHelper).GetProperty("PackAppBaseUri", BindingFlags.Static | BindingFlags.NonPublic).GetValue(null, null), resourceLocater);
                var parserContext = new ParserContext
                {
                    BaseUri = uri
                };
                typeof(XamlReader).GetMethod("LoadBaml", BindingFlags.NonPublic | BindingFlags.Static).Invoke(null, new object[] { stream, parserContext, userControl, true });
            }
            catch (Exception)
            {
                //log
            }
        }
    }
}

and called that from by UserControl's .cs file:

namespace ClassLibrary1
{
    public partial class MyUserControl : UserControl
    {
        public MyUserControl()
        {
            //InitializeComponent();
            this.LoadViewFromUri("/ClassLibrary1;component/myusercontrol.xaml");
        }
    }
}

Thanks again to "Juan Carlos Girón"!

The reason you are getting this error is because the way InitializeComponent that is implemented (in VS 2010) will always search in the derived class's assembly.

Here is InitializeComponent:

/// <summary>
/// InitializeComponent
/// </summary>
[System.Diagnostics.DebuggerNonUserCodeAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "4.0.0.0")]
public void InitializeComponent() {
    if (_contentLoaded) {
        return;
    }
    _contentLoaded = true;
    System.Uri resourceLocater = new System.Uri("/WpfApplication1;component/mainwindow.xaml", System.UriKind.Relative);

    #line 1 "..\..\..\MainWindow.xaml"
    System.Windows.Application.LoadComponent(this, resourceLocater);

    #line default
    #line hidden
}

The line where it looks up your XAML resource is System.Windows.Application.LoadComponent(this, resourceLocator). And this most probably fails because equivalent of 'this.GetType().Assembly' is used to determine which assembly to search for the resource identified by the relative Uri. And 'this.GetType()' does get the derived type of the object, not the type of the class where the code is implemented.

PS. Is this a bug? I do not know...

Frustratingly, I had exactly this error and spent forever trying to work out the cause. For me, it was once working but then I made some very minor changes to the XAML of the derived control, and the compiler started giving that error message. Short solution, cutting out many hours of trying to figure it out: shut down Visual Studio and re-opened it, recompiled, problem magically went away! (This is VS2012 Pro) Just added this in case anyone reading is going round in circles trying to find a non-existent problem with their code. Might be worth trying the "IT Crowd solution" first.