Question

When foreaching through a generic list I often want to do something different for the first element in the list:

List<object> objs = new List<object> {     new Object(),     new Object(),     new Object(),     new Object() };  foreach (object o in objs) {     if (o == objs.First())     {         System.Diagnostics.Debug.WriteLine('First object - do something special');     }     else     {         System.Diagnostics.Debug.WriteLine('object Do something else');     } } 

This will output:

     First object - do something special     object Do something else     object Do something else     object Do something else 

This is all fine and dandy.

However if my generic list is of a value type, this approach will fail.

List<int> ints = new List<int> { 0, 0, 0, 0 }; foreach (int i in ints) {     if (i == ints.First())     {         System.Diagnostics.Debug.WriteLine('First int - do something special');     }     else     {         System.Diagnostics.Debug.WriteLine('int Do something else');     } } 

This will output:

     First int - do something special     First int - do something special     First int - do something special     First int - do something special 

Now I know I could recode this to add a boolean flag variable or traditional for loop, but I am wondering if there’s any way to find out if a foreach loop is on the first iteration of its looping.

Answer 1

Well, you could code it using explicit iteration:

using(var iter = ints.GetEnumerator()) {   if(iter.MoveNext()) {      // do 'first' with iter.Current       while(iter.MoveNext()) {        // do something with the rest of the data with iter.Current      }   } } 

The bool flag option (with foreach) is probably easier though… that is what I (almost) always do!

Another option would be LINQ:

if(ints.Any()) {   var first = ints.First();   // do something with first }  foreach(var item in ints.Skip(1)) {   // do something with the rest of them } 

The downside of the above is that it tries to look at the list 3 times… since we know it is a list, that is fine – but if all we had was an IEnumerable<T>, it would only be sensible to iterate it once (since the source might not be re-readable).