Should I eat protein bars when dieting?

This is a much more complex question than it appears. Fortunately you narrowed it down by giving me the carb and fat amounts (I'll assume the protein is 30 grams because I'm pretty sure I know the bar you mean). The answer is, "it's relative" - relative to what you would have eaten otherwise. If your alternative was a low-carb, low-fat protein shake with a dash of virgin olive oil or fish oil capsule along with an apple, then the bar was a bad choice. If the alternative was a couple slices of cheese and a Mars bar then it was a good choice.

To clarify, here are the advantages of quality protein bars:
1) they usually have a good dose of quality protein - try to find one with primarily whey, casein, egg and to a lesser extent soy and avoid bars that have collagen, hydrolysed collagen or "hydrolysed protein" high on their list of ingredients and 20 or more grams of protein
2) the carbs in protein bars are often largely sugar alcohols, which have 2 advantages - they are lower in calories than other carbs (2-3 calories per gram vs 4) and they don't spike insulin (insulin is the hormone that puts the brakes on fat burning and causes rebound hunger - you get hungry sooner after insulin-spiking foods).
3) the good ones taste great (although most aren't so great).

Disadvantages:
1) they can have over 400 calories - so more than you want from a snack.
2) they usually don't have much fibre.
3) if you eat them too often in place of a snack that would usually contain vegetables and/or fruit, you compromise your health and don't get the myriad of healthy phytochemicals found in these foods.

So the bottom line is if you choose a good bar (less than 8 grams of sugar or so, etc) and ensure the amount of calories in the bar is in keeping with your daily calorie plan (you do have a daily calorie plan, right?), then a bar is a perfectly good choice. However, for most people if they are dieting aggressively, they are probably better off with other snack choices that are lower in calories and higher in fibre.

thought the bars might be to good too be true. oatmeal and a protein shake a better option then?

Merci beaucoup!

Really great answer to that question! I recently went skiing in Canada and I took a stack of protein bars that I ate as a snack whilst on the slopes. I now understand a lot more about the nutritional relevance of these bars so thanks for that!

if it helps look at the ingredients and follow this for general guidlines;
avoid hydrolysed collagen protein(this is gelatin,cheap incomplete protein)not good
look for erythritol (a sugar that doesnt seem to spike blood sugar/insulin so much)good
look for cla,mct and sesame oil in gram dosages.good
hopes this helps