Question: I'm trying to find missing dates between two DateTime variables for a collection of DateTimes.

For example.

```
Collection
2010-01-01
2010-01-02
2010-01-03
2010-01-05

DateRange
2010-01-01 -> 2010-01-06

```

would give me a `List<DateTime>` of

```
2010-01-04
2010-01-06

```

I can think of a few was of implementing this but nothing clean and simple

Any ideas?

Answer: 
I can think of a lot of ways of implementing this, e.g.:

```
DateTime[] col = { new DateTime(2010, 1, 1),
                   new DateTime(2010, 1, 2),
                   new DateTime(2010, 1, 3),
                   new DateTime(2010, 1, 5)};

var start = new DateTime(2010, 1, 1);
var end = new DateTime(2010, 1, 6);

var range = Enumerable.Range(0, (int)(end - start).TotalDays + 1)
                      .Select(i => start.AddDays(i));

var missing = range.Except(col);

```

And you could put the range-stuff into an Extension-Method

```
public static class extensions
{
    public static IEnumerable<DateTime> Range(this DateTime startDate, DateTime endDate)
    {
        return Enumerable.Range(0, (int)(endDate - startDate).TotalDays + 1)
                         .Select(i => startDate.AddDays(i));
    }
}

```

Then it would be simply

```
DateTime[] col = { new DateTime(2010, 1, 1),
                   new DateTime(2010, 1, 2),
                   new DateTime(2010, 1, 3),
                   new DateTime(2010, 1, 5)};

var start = new DateTime(2010, 1, 1);
var end = new DateTime(2010, 1, 6);
var missing = start.Range(end).Except(col);

```

But maybe this is not a high-performance-solution :-)
Depending on exactly what you are looking for and the sizes of the sets of data. A simple way would be to load the dates into a collection, then use a simple loop. I'll add a code sample here in a second.

```
DateTime currentDate = new DateTime(2010, 1, 1);
DateTime endDate = new DateTime(2010, 1, 6);
List<DateTime> existingDates = new List<DateTime>; //You fill with values
List<DateTime> missingDates = new List<DateTime>;

while(currentDate <= endDate)
{
    if(existingDates.contains(currentDate))
        missingDates.Add(currentDate);

    //Increment date
    currentDate = currentDate.AddDays(1);
}

```

Using this example you just need to load "existingDates" with the proper values, then the "missingDates" list will have your results