C#: Parse IIS log-files efficient, create statistics

I agree with SLaks, LogParser is your top bet. Most, if not all of its functionality is exposed via a COM API which you could import into your project via COM interop:

For good open source alternatives, check out awstats. Analog is another good option.

Are there free c# classes I can use or is there a little example project?

I wrote a little parser class in C# (.NET Core). See source here https://github.com/alexnolasco/32120528/

Example,

// List requests by hour
var q = new W3CReader(textReader).Read()
             .GroupBy(r => r.UtcTime().RoundUp(TimeSpan.FromHours(1)))
             .Select(g => new
             {
                    Hour = g.Key,
                    Count = g.Count()
             });
foreach (var r in q)
{
    Console.WriteLine("{0}\t{1}", r.Hour, r.Count);
}