C#

xlApp.Visible = true;  // make the application visible
xlWorkSheet.Visible = XlSheetVisibility.xlSheetVisible; // select the current worksheet

for(int i = 0; i < dt.Rows.Count; i++)

Action<int> DoSomething = (i) => dt.Rows[i][3];   // Create an action with 'i' as an argument and dt as an enclosing scope variable
for (int i = 0; i < dt.Rows.Count; i++
   DoSomething(i);  // Call the action passing the current value of i as a parameter

dt.Rows[i][7].ToString()

Quote:

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class ActivityTimeSum
    {
        public string settings { get; set; }
        public string dataTableName { get; set; }
        public DataTable selectDT { get; set; }
        public CheckEdit checkBox1 { get; set; }
        public CheckEdit checkBox2 { get; set; }
        public CheckEdit checkBox3 { get; set; }
        public CheckEdit checkBox4 { get; set; }

        Dictionary<string, int> myDic = new Dictionary<string, int>();
        List<string> keys = new List<string>();
        List<int> values = new List<int>();
        List<CheckEdit> checkBoxes = new List<CheckEdit>();
        List<string> selectedCheckBox = new List<string>();

        public Dictionary<string, int> CalculateTime()
        {
            checkBoxes.Add(checkBox1);
            checkBoxes.Add(checkBox2);
            checkBoxes.Add(checkBox3);
            checkBoxes.Add(checkBox4);

            foreach (var c in checkBoxes)
            {
                if (c.Checked)
                {
                    selectedCheckBox.Add(c.Tag.ToString());
                }
            }
            for (int i = 0; i < selectDT.Rows.Count; i++)
            {
                switch (selectedCheckBox.Count)
                {
                    case 1:
                        keys.Add(selectDT.Rows[i][Int32.Parse(selectedCheckBox[0])].ToString());
                        break;
                    case 2:
                        keys.Add(string.Join(", ", selectDT.Rows[i][Int32.Parse(selectedCheckBox[0])].ToString(), 
                            selectDT.Rows[i][Int32.Parse(selectedCheckBox[1])].ToString()));
                        break;
                    case 3:
                        keys.Add(string.Join(", ", selectDT.Rows[i][Int32.Parse(selectedCheckBox[0])].ToString(),
                            selectDT.Rows[i][Int32.Parse(selectedCheckBox[1])].ToString(), selectDT.Rows[i][Int32.Parse(selectedCheckBox[2])].ToString()));
                        break;
                    case 4:
                        keys.Add(string.Join(", ", selectDT.Rows[i][Int32.Parse(selectedCheckBox[0])].ToString(),
                            selectDT.Rows[i][Int32.Parse(selectedCheckBox[1])].ToString(), selectDT.Rows[i][Int32.Parse(selectedCheckBox[2])].ToString(),
                            selectDT.Rows[i][Int32.Parse(selectedCheckBox[3])].ToString()));
                        break;
                }
                values.Add(Int32.Parse(selectDT.Rows[i][11].ToString()));
            }

            for (int i = 0; i < selectDT.Rows.Count; i++)
            {
                if (myDic.ContainsKey(keys[i]))
                {
                    myDic[keys[i]] += values[i];
                }
                else
                {
                    myDic[keys[i]] = values [i];
                }
            }
            return myDic;

List<string> words = source.Split(',').Select(s => s.Trim()).ToList();

string.Split(", ", StringSplitOptions.None);

var jobStartDates = dc.JobStartDates.AsNoTracking().Include(jsd => jsd.Revisions).Where(jsd => jsd.JobId == jobId).ToList();
var results = jobStartDates.Select(jsd => new JobStartDateEntity
{
    Id = jsd.Id,
    ...
    StartDateRevisions = jsd.Revisions.Select(sdr => new JobStartDateRevisionEntry
    {
        Id = sdr.Id,
        ...
    }).OrderByDescending(sdr => sdr.Revision).ToList(),
});

var results = (from jsd in dc.JobStartDates
               where jsd.JobId == jobId
               select new JobStartDateEntity
               {
                   ...
               }).ToList();

var jobStartDateIds = results.Select(jsd => jsd.Id).ToList();

var revisions = dc.JobStartDateRevisions
    .Where(sdr => jobStartDateIds.Contains(sdr.Id))
    .Select(sdr => new JobStartDateRevisionEntity { ... })
    .ToList();

forech (var result in results)
{
    result.StartDateRevisions = revisions
        .Where(sdr => sdr.Id == result.Id)
        .OrderByDescending(sdr => sdr.Revision)
        .ToList();
}