c# Creating an unknown generic type at runtime

I think you're looking for the MakeGenericType method:

// Assuming that Property.PropertyType is something like List<T>
Type elementType = Property.PropertyType.GetGenericArguments()[0];
Type repositoryType = typeof(GenericRepository<>).MakeGenericType(elementType);
var repository = Activator.CreateInstance(repositoryType);

Activator.CreateInstance(typeof(GenericRepository<>).MakeGenericType(new Type[] { Property.GetTYpe() }))