var activityBeingValidated = new DelegateInArgument<ActivityBuilder>();
var validationContext = new DelegateInArgument<ValidationContext>();
var assertResult = new Variable<bool>();

return new Constraint<ActivityBuilder>
{
    DisplayName = "CheckIfRootIsCustomTypeActivity",
    Body = new ActivityAction<ActivityBuilder, ValidationContext>
    {
        Argument1 = activityBeingValidated,
        Argument2 = validationContext,

        Handler = new If
        {
            Condition = new InArgument<bool>()
            {
                Expression = new Equal<Type, Type, bool>()
                {
                    Left = ExpressionServices.Convert<Type>(c => activityBeingValidated.Get(c).Implementation.GetType()),
                    Right = ExpressionServices.Convert<Type>(c => typeof(CustomActivity))
                }
            },
            Then = new AssertValidation
            {
                Assertion = true,
                Message = new InArgument<string>("Root activity should be a CustomActivity")
        }
    }
}