如何在C＃中将不同应用于LINQ查询？

Question

I have this query

from d in db.v_Report_CompanySearches
where d.PersonName.ToLower().Contains(mPersonName)
orderby d.PersonName ascending
group d by d.PersonName into g
select (g);

But it returns all columns, when I just want PersonName . Plus I want to make sure PersonName is distinct, right now it returns duplicate values.

What is wrong here?

Answer 1

Call Dictinct at the end of query using parenthesis, you don't need the grouping thing there.

var result  = (from d in db.v_Report_CompanySearches
               where d.PersonName.ToLower().Contains(mPersonName)
               orderby d.PersonName ascending
               select d.PersonName ).Distinct();

Answer 2

original question: Is there way to use Distinct in LINQ query syntax?

accepted answer:

The Distinct extension method in LINQ does not have a query syntax equivalent.

See http://blogs.msdn.com/b/charlie/archive/2006/11/19/linq-farm-group-and-distinct.aspx for additional information as to why.

answer #2

There is no Distinct() method syntax in the language integrated query syntax. The closest you could do would be to move the current call:

 var q = (from c in tbl select c.TABLE_TYPE).Distinct(); 

Answer 3

Use g.First()

from d in db.v_Report_CompanySearches
where d.PersonName.ToLower().Contains(mPersonName)
orderby d.PersonName ascending
group d by d.PersonName into g
select (g.First().PersonName);

Answer 4

Use select (g.key) to select the grouping key only.

from d in db.v_Report_CompanySearches
where d.PersonName.ToLower().Contains(mPersonName)
orderby d.PersonName ascending
group d by d.PersonName into g
select (g.Key);

in your case the result will be an Enumerable<string> assuming PersonName is a string.

If you use the g.First() method as proposed by another answer, the result will be an IEnumerable<CompanySearch> assuming db.v_Report_CompanySearches is a list of CompanySearch . The result will contain only distinctive values for the PersonName but the other fields are just filled with the first match.

If you just select(g) the result is an Enumerable<IGrouping<string, CompanySearch>> The result will contain the Personnames distinctive as a key (since you group by them) but contains all the results as values. (You can extract them by using a SelectMany statement, see How to get values from IGrouping ).

Since my answer got downvoted I make another example, everyone can execute and check the validiy of it:

class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public override string ToString()
    {
        return $"{Name}: {Age}";
    }
}

static void Main(string[] args) {
    personList = new List<Person>()
    {
        new Person {Name = "Jay", Age = 25},
        new Person {Name = "Hans", Age = 25},
        new Person {Name = "Fritz", Age = 25},
        new Person {Name = "Hello", Age = 26},
        new Person {Name = "World", Age = 27},
        new Person {Name = "Whatever", Age = 26},
        new Person {Name = "ASDF", Age = 24},
        new Person {Name = "Foo", Age = 25},
        new Person {Name = "Bar", Age = 22},
    };
    var ages = from p in personList
               where p.Age > 22
               orderby p.Age ascending
               group p by p.Age into g
               select (g);

    Console.WriteLine($"select(g): {ages.GetType()}");

    var ages2 = from p in personList
        where p.Age > 22
        orderby p.Age ascending
        group p by p.Age into g
        select (g.First());

    Console.WriteLine($"select(g.First()): {ages2.GetType()}");

    var ages3 = ages.First();
    Console.WriteLine($"(select(g)).First(): {ages3.GetType()}");
}

In the output you can see that the first one is an IEnumerable<IGrouping<int,Person>> , the second output is an IEnumerable<Person> and the third one is an IGrouping<int,Person> .