如何从MarkupExtension返回强类型对象？

Question

Trying to make my first ever MarkupExtension, as a Service Locator and use it to get the DataContext in my XAML :

ViewModel & Interface

public interface IMainViewModel
{
    ICommand OpenProjectCommand { get; }
}

public class MainViewModel : IMainViewModel
{
    public ICommand OpenProjectCommand { get; private set; }
    ...
}

Service Locator :

public static class ServiceLocator
{
    public static void Map<T>(object concreteType)
    {
        // store type
    }

    public static T GetInstance<T>() where T : class
    {
        // get, instantiate and return
    }
}

App.xaml

protected override void OnStartup(StartupEventArgs e)
{
    ServiceLocator.Map<IMainViewModel>(typeof(MainViewModel));
    base.OnStartup(e);  
}

MarkupExtension:

public class ServiceLocatorExtension : MarkupExtension
{
    public Type ServiceType { get; set; }

    public ServiceLocatorExtension(Type type)
    {
        ServiceType = type;
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (ServiceType == null)
            throw new ArgumentException("Type argument is not specified");

        var instance = ServiceLocator.GetInstance<IMainViewModel>(); // how to use this.ServiceType?
        return instance;
    }
}

XAML:

<Window ... DataContext="{loc:ServiceLocator {x:Type loc:IMainViewModel}}">
...
<Button ... Command="{Binding OpenProjectCommand}"/> // problem complains cannot resolve in datacontext type of "object"

Questions:

1) How can I use this.ServiceType property in the MarkupExtension instead of the explicit interface?

2) The command binding on the button in XAML complains it cannot resolve from a datacontext of type object, so I get a warning, which I do not want. How to I make it know its correct type?

Answer 1

Not sure if this is the best solution, still looking for alternatives.

1:

Using reflection:

public override object ProvideValue(IServiceProvider serviceProvider)
{
    if (ServiceType == null)
        throw new ArgumentException("Type argument is not specified");

    var serviceLocatorMethod = typeof(ServiceLocator).GetMethod("GetInstance").MakeGenericMethod(ServiceType);
    return serviceLocatorMethod.Invoke(null, null);
}

2:

As its a designer issue, this solves the problem:

d:DataContext="{d:DesignInstance loc:MainViewModel}"