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 :-)
In .NET 2.0 :)

```
    static void Main(string[] args)
    {
        List<DateTime> dates = new List<DateTime>();
        dates.Add(new DateTime(2010, 01, 27));
        dates.Add(new DateTime(2010, 01, 30));
        dates.Add(new DateTime(2010, 01, 31));
        dates.Add(new DateTime(2010, 02, 01));

        DateTime startDate = new DateTime(2010, 01, 25);
        DateTime endDate = new DateTime(2010, 02, 02);

        List<DateTime> missingDates = new List<DateTime>(GetMissingDates(dates, startDate, endDate));

    }

    private static IEnumerable<DateTime> GetMissingDates(IList<DateTime> dates, DateTime startDate, DateTime endDate)
    {
        TimeSpan _timeStamp = endDate - startDate;
        DateTime _tempDateTime = startDate;
        IList<DateTime> _dateTimeRange = new List<DateTime>();
        IList<DateTime> _missingDates = new List<DateTime>();

        for (int i = 0; i <= _timeStamp.Days; i++)
        {
            _dateTimeRange.Add(_tempDateTime);
            _tempDateTime = _tempDateTime.AddDays(1);
        }

        foreach (DateTime dt in _dateTimeRange)
        {
            if (!dates.Contains(dt))
                yield return dt;
        }
    }

```