DevForce Community Forum : Linq Query question

Linq Query question : IdeaBlade.Linq

Wed, 23 Dec 2009 10:43:11 -700

Author: GregD
Subject: 1462
Posted: 23-Dec-2009 at 10:43am

IdeaBlade.Linq

Linq Query question : Hey, GregD, can you please tell...

Thu, 17 Sep 2009 20:42:07 -700

Author: monkeyking
Subject: 1462
Posted: 17-Sep-2009 at 8:42pm

Hey, GregD, can you please tell me what i should import to get the PredicateBuilder class?

Linq Query question : thanks GregD, that's helpful ...

Thu, 17 Sep 2009 18:17:25 -700

Author: monkeyking
Subject: 1462
Posted: 17-Sep-2009 at 6:17pm

thanks GregD, that's helpful

Linq Query question : Here's a second variation...

Thu, 10 Sep 2009 15:07:36 -700

Author: GregD
Subject: 1462
Posted: 10-Sep-2009 at 3:07pm

Here's a second variation of the PredicateBuilder approach:

    private void DoItUsingPredicateBuilder() {
      Console.WriteLine(".....Use the PredicateBuilder...");
      List lastNames = new string[] {"Davolio", "Fuller", "Buchanan"}.ToList();
      var expr = PredicateBuilder.False();
      foreach (string lastName in lastNames) {
        // Make a new variable for this iteration's lastName value; otherwise,
        // because of delayed execution, all expressions constructed below
        // will use the last-encountered version of lastName.
        string lastNameThisIteration = lastName;
        expr = expr.Or(e => e.LastName == lastNameThisIteration);
      }
      List employees = _mgr.Employees.Where(expr).ToList();

      //foreach (Employee anEmployee in employees) {
      // Console.WriteLine(anEmployee.LastName);
      //}
      //PromptToContinue();
    }

Linq Query question : You can do this with the DevForce...

Thu, 10 Sep 2009 14:07:44 -700

Author: GregD
Subject: 1462
Posted: 10-Sep-2009 at 2:07pm

You can do this with the DevForce PredicateBuilder. I'll show you that in a second, but just to cover the other alternatives:

1. You can bring the complete list of entities to the client and filter it there:

      List lastNames = new List();
      lastNames.Add("Davolio");
      lastNames.Add("Fuller");
      lastNames.Add("Buchanan");
      var employees = _mgr.Employees.ToList().Where(e => lastNames.Contains(e.LastName));

2.   You can use a Remote Service Method call to do the above on the server, then ship down only the results;

3.   You can hand-code the query with lots of ORs in the Where clause

      List employees = _mgr.Employees.Where(
        e => e.LastName == "Davolio" ||
        e.LastName == "Fuller" ||
        e.LastName == "Buchanan").ToList();

Choices 1 and 3, of course, are either impossible or unacceptable in many scenarios; and choice 2 is maybe more trouble than you'd like to go to. So here's how you do it with the PredicateBuilder, which basically automates the process of constructing a query like #3:

    private void DoItUsingPredicateBuilder() {
      List lastNames = new List();
      lastNames.Add("Davolio");
      lastNames.Add("Fuller");
      lastNames.Add("Buchanan");

      var tests = EmployeeLastNameTests(lastNames).ToArray();
      if (0 == tests.Length) return;
      var employeeLastNamePredicate = PredicateBuilder.Or(tests);
      List employees = _mgr.Employees.Where(employeeLastNamePredicate).ToList();

      //foreach (Employee anEmployee in employees) {
      // Console.WriteLine(anEmployee.LastName);
      //}
      //PromptToContinue();
    }

    private IEnumerable>> EmployeeLastNameTests(IEnumerable words) {
      foreach (var each in words) {
        var word = each;
        yield return e => e.LastName.Contains(word);
      }
    }

Linq Query question : i have a list of stock names,...

Wed, 09 Sep 2009 23:00:05 -700

Author: monkeyking
Subject: 1462
Posted: 09-Sep-2009 at 11:00pm

i have a list of stock names, let's call it 'stockNames' (List stockNames) and i have a 'Stock' table, which are a set of stock entities. now I want to use Linq to query the entities which their names are listed in 'stockName' list.

I used query as that, 'var query = entityManager.Stocks.where(o=>(stockNames.Contains(o.name)));'. but List.Contains(string) can't be used in the Linq 'where' clause. is there any other way to resolve this?

Regards
John