Update: fixed the title per comments and I noticed Rico chimed in.



I had two separate interactions today where someone told me they opted for ugly\error prone for-loop code because foreach was “too slow”.  There are so many things wrong with that analysis where do I start??


  1. In one of the cases we were iterating over an array… Google helped me quickly find this post and Kevin is right, C# and VB make foreach of arrays just peachy. 
  2. The rationale for why foreach is “too slow” was simply “Rico said so”.  Again a quick google search puts the direct facts to rest.  But the meta-issue, which I am sure Rico would agree with, is that there are very few “laws” of perf.  Know your scenarios and measures are the only two I think.  Because rico said didn’t work for me in 2nd grade and it doesn’t work for you now either.
  3. Yes, fine there is something to be aware of wrt foreach over ArrayList, which I will not repeat here but to say they very often are a wash in real world scenarios and developer productivity (on-time, less bugs) is almost always a better tradeoff.  In addition, you can mitigate this in your own collection types today and we have addressed it with List<T> in Whidbey.  
  4. It is my belief most of our customers are not having significant perf issues on the platform (but I am always open to new data).  That is not to say the CLR doesn’t have some perf issue we need to address (we do!), I am just saying that on balance, for the kinds of apps customers are building today the value we give far outweighs the cost.  Where there are perf issues, I seriously doubt they are fixed by moving from foreach to for!
  5. Eric tells me Java is adding foreach because it is such a useful feature… it can’t be that bad ;-)


Go, use foreach and be proud!