将Linq查询结果分组为字符串

Question

Here is what I am trying to accomplish:

I have a list of companies that offer different services. I am trying to group the services together of a company in a string format so when I can export to excel it shows up in one column.

Right now if a company has 4 services, they will show up 4 different times in query. Which is logical, just want to group them together.

Here is what I have tried and get "A lambda expression with a statement body cannot be converted to an expression tree"

Services = (from cc in CharityCategories join c in Cao_Categories on cc.CategoryID equals c.CategoryID
                        join chy in CharityYears on cc.CharityYearID equals chy.CharityYearID
                        where chy.CampYearID == 5 && chy.StatusID == 1
                        group c by c.Category into cg
                        select new { Categories = cg.Key.Trim()}).Aggregate(new StringBuilder(), (a, b) =>
                       {
                               if (a.Length > 0)
                                      a.Append(",");
                               a.Append(b.ToString().Split('=')[1].Replace(" }", ""));
                               return a;
                        }).ToString() ,

LinqPad shows the error on the line, right after StringBuilder() , " Aggregate(new StringBuilder(), (a, b) "

I can get them to group in a link and when clicked, that link lists them in a format like { myservice = 1} - This is why I am using .Append

Answer 1

use below code then change your logic accordingly, hopefully this will work

Correction in cg.Key.ToString().Trim()

Services = (from cc in CharityCategories
                            join c in Cao_Categories on cc.CategoryID equals c.CategoryID
                            join chy in CharityYears on cc.CharityYearID equals chy.CharityYearID
                            where chy.CampYearID == 5 && chy.StatusID == 1
                            group c by c.Category into cg
                            select new { Categories = cg.Key.ToString().Trim() }).Aggregate(new StringBuilder(), (a, b) =>
                            {
                                if (a.Length > 0)
                                    a.Append(",");
                                a.Append(b.ToString().Split('=')[1].Replace(" }", ""));
                                return a;
                            }).ToString();

I don't know your requirement, we can correct this if you can provide exact expected result

Answer 2

Well as the error says you cannot convert a lambda expression with a statement body to an expression tree. Entity framework uses expression trees to convert to sql statements. However, you can materialize the results first with ( AsEnumerable ) and then use your code with linq to objects.

Services = (from cc in CharityCategories
                        join c in Cao_Categories on cc.CategoryID equals c.CategoryID
                        join chy in CharityYears on cc.CharityYearID equals chy.CharityYearID
                        where chy.CampYearID == 5 && chy.StatusID == 1
                        group c by c.Category into cg
                        select new { Categories = cg.Key.ToString().Trim() })
                        .AsEnumerable()
                        .Aggregate(new StringBuilder(), (a, b) =>
                        {
                            if (a.Length > 0)
                                a.Append(",");
                            a.Append(b.ToString().Split('=')[1].Replace(" }", ""));
                            return a;
                        }).ToString();

Important : The aggregation will take place after all the rows were retrieved from the DB.

Answer 3

Assuming your result set can be represented in the form of the following class

public class Category
{
     public string Name{get;set;}
     public string Product{get;set;}
}

You could write the following LINQ query to get the result in the way you wished to:

var testList = new List <Category> {
new Category {Name = "A",Product = "decks"},
new Category {Name = "B",Product = "cards"},
new Category {Name = "C",Product = "paper"},
new Category {Name = "A",Product = "scissor"},
new Category {Name = "B",Product = "crates"},
new Category {Name = "C",Product = "rocks"}
};

var finalList = testList
    .GroupBy(x => x.Name)
    .Select(x => new {
        Category =x.Key,
        Items = String.Join(",", x.Select(y => y.Product))
     });

Thus the result set would be in the form of:

1. Category A -> decks, scissor
2. Category B-> cards,crates
3. Category C-> paper, rocks

Do mark as answer if this helps.