I am trying to build a project that contains the Rad Rich Text Box and no matter what I do the error:

The type or namespace name 'RadRichTextBox' could not be found (are you missing a using directive or an assembly reference?)

Always appears. I have loaded the following dlls to the GAC:

gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.Navigation.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.RichTextBoxUI.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.RadRichTextBoxRibbonBarUI.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Data.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Documents.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.Input.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Telerik.Windows.Controls.RibbonBar.dll"
gacutil.exe -i "C:\Program Files\Telerik\RadControls for Silverlight Q3 2011\Binaries\Silverlight\Design\Telerik.Windows.Controls.RichTextBoxUI.Design.dll"
gacutil.exe -i "C:\Program Files\Microsoft Silverlight\4.0.60831.0\System.Windows.dll"
gacutil.exe -i "C:\Program Files\Microsoft Silverlight\4.0.60831.0\System.Windows.Browser.dll"

This is a fresh install of Windows and VS2010, SP1, ad all updates. I have tried uninstalling and reinstalling Telerik, which didn't help.
I have confirmed the assemblies are present in C:Windows\Assembly and that the project references are there.

I even get this error when running the Telerik Sample project included with the installation.

Is there something that I need to change on my Visual Studio installation? I can;t figure out why this is happening.

Hello Bruce,
Actually Silverlight runtime doesn't use GAC during assembly probing, so you can safely remove mentioned assemblies from there.
Are experiencing the problem with any Silverlight project, or only such containing RadRichTextBox control? I will send you a sample project to verify at your side in the support ticket you opened.
You can also check if Silverlight 4 and Silverlight 4 SDK are installed on your computer, as RadRichTextBox references System.ComponentModel.Composition, which is part of the SDK.

Greetings,
Boby
the Telerik team

Hi am still facing the same issue.  "The type or namespace name 'RadRichTextBox' could not be found (are you missing a using directive or an assembly reference" . I installed both  Silverlight 4 and Silverlight 4 SDK .  Also in xaml i see this error 

Click here to reload the designer
Details 
System.NullReferenceException
Object reference not set to an instance of an object.
   at MS.Internal.XcpImports.MethodEx(IntPtr ptr, String name, CValue[] cvData)
   at MS.Internal.XcpImports.MethodEx(DependencyObject obj, String name)
   at MS.Internal.XcpImports.UIElement_UpdateLayout(UIElement element)
   at Microsoft.Windows.Design.Platform.SilverlightViewProducer.OnViewUpdated() 


Xaml just contains this...
<UserControl x:Class="RadControlsSilverlightApp7.MainPage"
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:telerik="http://schemas.telerik.com/2008/xaml/presentation"
mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
<Grid x:Name="LayoutRoot">
        <telerik:RadRichTextBox  x:Name="radRichTextBox" IsReadOnly="False" IsSpellCheckingEnabled="False" Margin="0,0,0,47" >
            <telerik:RadDocument LayoutMode="Paged">
                <telerik:Section PageMargin="10,10,10,10">
                    <telerik:Paragraph TextAlignment="Center">
                        
                        <telerik:Span Text="Thnak you for using Telerik"></telerik:Span>
                        
                        <telerik:Span Text="Thnak you for using Telerik"></telerik:Span>
                                      
                    </telerik:Paragraph>
                    
                </telerik:Section>
                
            </telerik:RadDocument>
            
        </telerik:RadRichTextBox>
    </Grid>
</UserControl>

Please suggest to fix this issue .. Thanks 

I removed d:DesignWidth="640" d:DesignHeight="480"  and now i see the below error.


Object reference not set to an instance of an object.

Microsoft.Windows.Design.Platform.InvalidDesignerUpdateException
An unhandled exception was encountered while trying to render the current silverlight project on the design surface. To diagnose this failure, please try to run the project in a regular browser using the silverlight developer runtime.

Hello Rajesh ,

We have answered you in the support ticket you opened for this issue.

Kind regards,
Mihail
the Telerik team