如何在具有两个以上字段的对象上使用Linq返回两个字段？

Question

What I would like to do is something I find common in MySQL which is to select all of one field distinct, and a sum of another field in that data sub-set for each unique/distinct result.

Following should replicate something simple to use in order to see the type of data I mean :

public class Stock {
   public int id {get;set;}
   public int count {get;set;}
   public string location {get;set;}
   public Stock() { }
}

public class Foo {

   public void Bar() {
        List<Stock> Stocks = new List<Stock>()
        {
            new Stock
            {
                id = 137829,
                count = 5,
                location = "Ottawa"
            },
            new Stock
            {
                id = 137829,
                count = 27,
                location = "Toronto"
            },
            new Stock
            {
                id = 137830,
                count = 8,
                location = "Toronto"
            }

        };

        var result = Stocks.DistincyBy( s => s.id);
   }
}

What I would like to store in result is as follows (using this code as an example. the real project the structure is more complex).

• id = 137829, count = 32
• id = 137830, count = 8

I know I can use .Sum() , but in this scenario I wish to sum where one field is distinct. How can this be achieved (i prefer my Linq to be inline syntax rather than sql style querying)

Answer 1

Use GroupBy to get the distinct groupings, then Select into your objects. For example:

var result = Stocks.GroupBy(s => s.Id).Select(g => new 
{
   Id = g.Key,
   Count = g.Sum(i => i.Count)
};

Better would be to create a named class to select into, but the syntax would be very similar.