When I was adding localization to my first Windows Phone 7 app, I initially got following exception:

XamlParseException was unhandled
No matching constructor found on type 'MyApp.Resources.MyResources'. [Line: 15 Position: 34]

with an InnerException:

MissingMethodException
   at System.Activator.InternalCreateInstance(Type type, Boolean nonPublic, StackCrawlMark& stackMark)
   at System.Activator.CreateInstance(Type type)
   at MS.Internal.XamlManagedRuntimeRPInvokes.CreateInstance(XamlTypeToken inXamlType, XamlQualifiedObject& newObject)
   at MS.Internal.XcpImports.Application_LoadComponentNative(IntPtr pContext, IntPtr pComponent, UInt32 cUriStringLength, String uriString, UInt32 cXamlStrLength, Byte* pXamlStr, UInt32 cAssemblyStrLength, String assemblyStr)
   at MS.Internal.XcpImports.Application_LoadComponent(IManagedPeerBase componentAsDO, String resourceLocator, UnmanagedMemoryStream stream, UInt32 numBytesToRead, String assemblyString)
   at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
   at MyApp.App.InitializeComponent()
   at MyApp.App..ctor()
   at System.Reflection.RuntimeConstructorInfo.InternalInvoke(RuntimeConstructorInfo rtci, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
   at System.Reflection.RuntimeConstructorInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
   at System.Reflection.ConstructorInfo.Invoke(Object[] parameters)
   at MS.Internal.TypeProxy.<>c__DisplayClass30.<GetCreateObjectDelegate>b__2a()
   at MS.Internal.TypeProxy.CreateInstance(UInt32 customTypeId)
   at MS.Internal.FrameworkCallbacks.CreateKnownObject(IntPtr nativeRootPeer, UInt32 customTypeId, String initializationString, IntPtr& nativePeer, UInt32 isCreatedByParser)
   at MS.Internal.FrameworkCallbacks.CreateUnknownObject(String assemblyName, String typeName, IntPtr nativeRootPeer, String initializationString, UInt32& customTypeId, UInt32& coreTypeId, UInt32& typeFlags, IntPtr& nativePeer)

If you break on all thrown CLR Exceptions, you'll get following error instead:

The problem here is that the access modifier of resource files added to your WP7 project are internal by default. Changing the access modifier in the designer, changes the class definition. However, the constructor stays internal. You can change the modifier manually in the xxResources.Designer.cs file. Just keep attention that it will return to internal when changing your resource file through the designer.

[global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "4.0.0.0")]
[global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
[global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
public class MyResources {
    
    private static global::System.Resources.ResourceManager resourceMan;
    
    private static global::System.Globalization.CultureInfo resourceCulture;
    
    [global::System.Diagnostics.CodeAnalysis.SuppressMessageAttribute("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
    public MyResources() {
}