CODEWITHSUNDEEP
c#.netdebuggingforeach
Gleb
Gleb

Reputation: 1432

While debugging, how can I start a foreach loop at an arbitrary position in the enumeration?

I am debugging my program. Can I set a start object for the foreach loop in the debug mode? For instance, I want the foreach loop to start from the 5th element of my collection.

Upvotes: 5

Views: 1666

Answers (5)

einord
einord

Reputation: 2325

@Shaggy's answer is probably the easiest, but it's good to know that the old fashioned for loop is the fastest, and enables you to skip the 0 index.

for(int x = 4; x < numbers.Length; x++)
{
    // Do anything with numbers[x]
}

Even though I would not recommend it (It's really bad code), you could make this loop behave different when debugging, as you asked:

#if DEBUG
for(int x = 4; x < numbers.Length; x++)
#else
for(int x = 0; x < numbers.Length; x++)    
#endif
{
    // Do anything with numbers[x]
}

Upvotes: 0

Steve Guidi
Steve Guidi

Reputation: 20200

An easy way to set the start object is to overwrite the variable that you are enumerating in the loop.

To illustrate, consider the following foreach loop.

IEnumerable<int> numbers = Enumerable.Range(1, 100);
foreach (int n in numbers)
{
    Console.WriteLine(n);
}
  1. Set your breakpoint on the numbers statement in the loop initializer, or step to the loop but don't run the numbers statement.

  2. Use the immediate window to override the value of numbers:

    numbers = numbers.Skip(5);  // or Enumerable.Skip(numbers, 5)

  3. Continue debugging; loop runs from the sixth element.

If your loop uses an inline-computed enumeration as follows, then you're out of luck. Consider using a hit-count breakpoint instead.

foreach (int n in Enumerable.Range(1, 100))  // no way to change enumeration.
{
    Console.WriteLine(n);
}

Note: once the numbers statement is run, the debugger will cache your enumeration so it may no longer be changed and affect the loop. You may observe this while stepping through the loop construct.

Upvotes: 1

ths
ths

Reputation: 2942

foreach does not necessarily operate on a collection which does even have a defined order. consider a Dictionary<T> or a ConcurrentBag<T>. so, no, this is not possible in the general case, especially without code changes (which could change the problem you are trying to debug).

Upvotes: 0

kjbartel
kjbartel

Reputation: 10581

Considering you are talking about debugging I'm assuming what you want to do is to break from the 5th element on etc. If this is the case then you can use a break point specifying a hit count which will break on the nth hit and so on.

See MSDN here.

Upvotes: 1

Shaggy
Shaggy

Reputation: 73

No, you cant. Foreach loop uses IEnumerable<T> where T is the type of object. So unlike for loop you cannot set initial or start index for the iteration. So you will always start from the 0th location object.

Other option is to use linq as shown below.

//Add using System.Linq  statement at top.

int[] numbers = new int[] { 1,2,3,4,5,6,7,8};

foreach(var num in numbers.Skip(5))
{
     Console.WriteLine(num);
}

Upvotes: 7

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