Question

I am having a list myTask, in which I want to do a group by Date and calculate the total per date on the Amount.

How can I do that?

public class Task{
public DateTime Date{get;set;}
public string Item {get;set;}
public decimal Amount {get;set;}    
}

 public List<Task> taskList{ get; set; }

Answer 1

I suspect you want:

var totals = taskList.GroupBy(x => x.Date)
                     .Select(g => new { Date = g.Key,
                                    Total = g.Sum(x => x.Amount) });

Or in one go:

var totals = taskList.GroupBy(x => x.Date, (date, values) => 
                                 new { Date = date,
                                       Total = values.Sum(x => x.Amount) });

Note that this is assuming the Date property really is a date rather than a timestamp. Otherwise you might want x => x.Date.Date as the first argument.

If this isn’t what you’re after, please say how it differs from what you want.

EDIT: Okay, if you want the items as well – but presumably not the date with each item – you can write:

var totals = taskList.GroupBy(x => x.Date, (date, values) => 
                       new { Date = date,
                             Total = values.Sum(x => x.Amount),
                             Items = values.Select(x => x.Item).ToList(),
                             Amounts = values.Select(x => x.Amount).ToList() });