There is often some confusion about this, even in the chandlers shop.
Firstly, do not confuse diodes included within a solar array that isolates the separate elements with the diodes required to stop the solar panel discharging the battery when there is no sun. The later is unlikely to be built into a panel, the former usually are.
Some controllers include blocking diodes to prevent discharge, some do not, and further confusion can arise because a controller that supports a wind generator and Solar may have blocking diodes to isolate the two from each other and the alternator but does not have diodes to prevent discharge through the solar panel, this is the case with my Marlec HDRi controller and the fact that the product description on the box mentioned the former caused confusion at the chandlers where the sales person assumed the diodes were to prevent discharge, they are not! Fortunately I had already read the manual and knew differently, from the manual:
"Note : Solar panels must be fitted with appropriate blocking diodes."
Blocking diodes are cheap and should be well over-specified for the current to be carried, if not they will get very hot and you are loosing power. Multiple diodes can be put in parallel to increase current handling capacity (2 approximately doubles the rating, three trebles it etc.) but the better option is find a seriously big one.
Very small trickle charging panels are sometimes connected directly to the battery, perhaps plugged in through a 12 volt power point so they can be removed. Whilst you can get away with this for very small units you can't for a panel of any size as the continuous charge will gas the battery (see battery types by technology) so you need a controller to manage the process.
Generally it is worth paying more for a good controller which should do a better job and will also give you some information about the system such as battery voltage, instantaneous current or power being supplied, power supplied since reset etc. Some, like the Marlec HDRi, have dual outputs so that charging the starter and service batteries can be controlled independently, a major benefit in my view although it would be nice if the starter battery was given priority.
Some will support just solar, others Wind Generators and Solar, the latter are specific to a model or make (or makes) of wind generator but OK with generic solar panels up to a certain power.