Confused about how to implement extension methods in LINQ to Objects

Question

Hi all,

Fairly new to LINQ and trying to wrap my head around extension methods. What I'm attempting to do:

1) Have a table with three columns, col1 (string), col2 (double), col3 (DateTime)

        table1.Rows.Add("string 1", 1, new DateTime(2009, 01, 01));
        table1.Rows.Add("string 1", 2, new DateTime(2009, 02, 01));
        table1.Rows.Add("string 1",3, new DateTime(2009, 03, 01));
        table1.Rows.Add("string 1", 4, new DateTime(2009, 04, 01));
        table1.Rows.Add("string 2",1, new DateTime(2009, 05, 01));
        table1.Rows.Add("string 2", 1, new DateTime(2009, 06, 01));
        table1.Rows.Add("string 2", 5, new DateTime(2009, 07, 01));
        table1.Rows.Add("string 3", 6, new DateTime(2009, 08, 01));

2) I need to write a LINQ query to group by column 1, and send the grouped rows to a method which returns a value double. Something like this

var query = from t in table1
            group t by t.col1 into g
            select new { r1 = g.Key, r2=mycalc(g))

3) and have an extension function:

public static double Median(this IEnumerable<DataSet1.DataTable1Row> source)
{
  //calc using the grouped row data and return a dobule
}

I've been working on this for a bit and don't quite get it. Can someone please help?

Answer 1

Well, it's not entirely clear which bit is causing the problem. If you've already done the Median method, then you can change query to:

var query = from t in table1
            group t by t.col1 into g
            select new { r1 = g.Key, r2=g.Median() };

Is the Median bit causing you problems? It's probably going to be easiest to do something like:

public static double Median(this IEnumerable<DataSet1.DataTable1Row> source)
{
    List<double> values = source.Select(x => x.col2).ToList();
    values.Sort();
    if ((values.Count % 2) == 1) // Odd number of values
    {
        return values[values.Count/2];
    }
    else // Even number of values: find mean of middle two
    {
        return (values[values.Count/2] + values[values.Count/2 + 1]) / 2;
    }
}

There may be more efficient ways of doing it, but I don't know them...

Answer 2

Hi Jon,

Thanks for your answer - I actually tried that, but then I re-tried your example again and realized my error wasn't exactly what I originally thought it was. The error I get now is:

Extension methods must be defined in a non-generic static class.

Fixed that problem and it works great now - thanks for the help!

Regards, Brad