Replace the Key of an IGrouping?

Question

Is it possible to replace the Key by which an IGrouping is grouped?

I'm currently grouping by an anonymous type like this:

var groups = orders.GroupBy(o => new { o.Date.Year, o.Date.Month });

But now my grouping key is an anonymous type. I would like to replace this grouping key by a defined Type, "YearMonth", with an overridden ToString Method.

public class YearMonth
{
    public int Year { get; set; }
    public int Month { get; set; }

    public override string ToString()
    {
        return Year + "-" + Month;
    }
}

Is there any way to replace the grouping key? Or to create a new IGrouping out of the existing IGrouping with a new grouping key?

Answer 1

Well personally I would just group on the string value, since that seems to be what you really care about from the key.

Another simple option would be to create a date representing the month by using a constant day:

orders.GroupBy(o => new DateTime (o.Date.Year, o.Date.Month, 1))

Then you have built-in value equality and string formatting.

You could make YearMoth an immutable struct, which will also give you value-equality semantics:

public struct YearMonth
{
    public readonly int Year;
    public readonly int Month;

    public YearMonth(int year, int month)
    {
        Year = year;
        Month = month;
    }

    public override string ToString()
    {
        return Year + "-" + Month;
    }
}