VB.NET & Linq: Sum datatable column and Group by date in another column

Question

I have a typedDataset (ds) and its datatable(dt). The datatable has the columns Date(ttmmyyy) and Worktime. I want to sum the Worktime by month. I already accomplished to sum the Worktime with

Dim sum = dt.AsEnumerable().Sum((Function(x) x.Worktime))

I need help with group this by month.

Solution

I created a table with your fields and the following query worked nice:

 Dim query = From row In dt
          Group row By VarDate= row.Field(Of DateTime)("Date") Into MonthGroup = Group
          Select New With {
              Key VarDate,
              .Worktime = MonthGroup.Sum(Function(r) r.Field(Of Int32)("Worktime"))
         }

Used variable VarDate because Date is reserved for it's own class.

Output:

Vardate: 10/15/2013 Worktime: 150

Vardate: 10/16/2013 Worktime: 180

EDIT:

To get the output just use:

 For Each obj In query
     Dim datet = obj.VarDate
     Dim WorkH = obj.Worktime
 Next

To avoid declarying types when using linq, insert the line: option infer on on the very first line of the vb.net document ( so you can use dim datet = obj.Vardate ) without declaring the type. It is like var in c#