In my web app the user is shown a list of products which can be marked and in a later step be combined. Only products of the same type can be combined.
The question is, should the user be allowed to mark products of more than one type and be informed about this when she tries to combine them, or should she be informed when she tries to mark a second type item?
I think you can handle this a couple ways.
The system can disable or filter out the oranges when the user picks an apple. That way the system guides the user. We use a similar approach in a app where a buyer picks print options and some combinations can be incompatible, such as stapling and coil binding. If the buyer picks stapling and then tries to pick a binding, we show the binding as disabled with a small message informing the buyer that she already added stapling.
The system can warn the user when she tries to pick an orange after picking an apple. The warning could be on-click or on submit.
The system can warn the user that she has both an apple and an orange and if there are two or more of either, give the user a choice to group one or more.
I personally like option 1 because it informs the buyer in an non-intrusive way and makes it simpler to pick another apple. My example is somewhat a cross between 1 and 2 - we disable and also give a message if the user tries to pick the orange.
I'll propose an alternative approach. Warn the user as soon as they mark multiple products of different types that they won’t be able to combine them but still allow them to be marked anyway. Assuming the process or marking and unmarking a product is quick and easy, this is better than preventing me from marking the second product.
For example, if I mark two products of type A and then try to mark a third product of type B, you should warn me that I won’t be able to combine those products (and explain why) but still allow the third product to be marked. That way, I am free to decide which product(s) to unmark in my own time (I might decide that I actually want to combine type B products and so remove the two type A products).
Does the user need to do other things in this step? If not, you can make sure the user only selects one product by continuing the process when the user selects a product. Make sure the user knows that he will be forwarded to the next step when selecting a product (I think copy will do the trick). It's always a good think to keep the back-button enable so users can change their mind.
sounds like a product filter where the users initial choice filters possible future choices to only the type of the first choice.
so the answer is to filter available options to only the type of the initial choice
To inform a user that he did something wrong is good. To prevent the user from doing the wrong thing is always better.
Highlight the items that can be selected after the first selection has been made to make it inevitable to select the "correct" items. I would try a design where the other items are selectable (not disabled), since if you leave them enabled you have the chance of explaining to the user why it shouldn't be selected if that choice is made.
