group by - Count on Groupby & update column value C# DataTable

Question

I have a Datatable like this:

Id	Amount1	total
6	30	30
8	22	12
7	11	31
8	15	51

question from:https://stackoverflow.com/questions/65541048/count-on-groupby-update-column-value-c-sharp-datatable

Answer 1

You could use a lookup to count the ids:

// copy structure and data into new DataTable
DataTable tblCount = yourOriginalDataTable.Copy();
// add the count column
tblCount.Columns.Add("IdCount", typeof(int)); 
// create a dictionary like collection where the key is the id
var idCounter = tblCount.AsEnumerable().ToLookup(row => row.Field<int>("Id"));
// loop the rows and update the column with the correct count value
foreach(DataRow row in tblCount.Rows)
    row.SetField("IdCount", idCounter[row.Field<int>("Id")].Count());