WPF

    internal class Metadata : IRegisterMetadata {

        // Called to register design time metadata
        public void Register() {
            AttributeTableBuilder builder = new AttributeTableBuilder();

            //Task
            builder.AddCustomAttributes(typeof(Button), new FeatureAttribute(typeof(TaskProvider)));
            
            MetadataStore.AddAttributeTable(builder.CreateTable());
        }
    }




// class that will change the property
    class TaskProvider : Microsoft.Windows.Design.Interaction.PrimarySelectionTaskProvider
    {
        public TaskProvider()
        {
          
        }

        protected override void Activate(Microsoft.Windows.Design.Model.ModelItem item)
        {

           if (String.IsNullOrEmpty((String)item.Properties["StringProperty"].ComputedValue))
                item.Properties["StringProperty"].SetValue(Guid.NewGuid().ToString()); //sets the value in XAML

            base.Activate(item);
        }
    }

Michael Sync wrote:

this.Startup = new MyView(); in App

Wes Aday wrote:

Would the Application.MainWindow property work for you?

Vinay Srivastava wrote:

this.StartupUri = new System.Uri("Window1.xaml", System.UriKind.Relative);

this.Startup = container.Resolve<shell>(); 

private void Application_Startup(object sender, StartupEventArgs e) {
            IUnityContainer container = new UnityContainer()
                .RegisterType<ILogger, TraceLogger>()
                .RegisterType<IStoplightTimer, RealTimeTimer>();

            //this.RootVisual = new StoplightView();
            this.RootVisual = (UIElement)container.Resolve<StoplightView>();
        }