Linq Query question
Printed From: IdeaBlade
Category: DevForce
Forum Name: DevForce 2009
Forum Discription: For .NET 3.5
URL: http://www.ideablade.com/forum/forum_posts.asp?TID=1462
Printed Date: 06-Apr-2025 at 5:24pm
Topic: Linq Query question
Posted By: monkeyking
Subject: Linq Query question
Date Posted: 09-Sep-2009 at 11:00pm
i have a list of stock names, let's call it 'stockNames' (List<string> 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<string>.Contains(string) can't be used in the Linq 'where' clause. is there any other way to resolve this?
Regards
John
|
Replies:
Posted By: GregD
Date 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<string> lastNames = new List<string>();
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<Employee> 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<string> lastNames = new List<string>();
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<Employee> employees = _mgr.Employees.Where(employeeLastNamePredicate).ToList();
//foreach (Employee anEmployee in employees) {
// Console.WriteLine(anEmployee.LastName);
//}
//PromptToContinue();
}
private IEnumerable<Expression<Func<Employee, bool>>> EmployeeLastNameTests(IEnumerable<String> words) {
foreach (var each in words) {
var word = each;
yield return e => e.LastName.Contains(word);
}
}
|
Posted By: GregD
Date 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<string> lastNames = new string[] {"Davolio", "Fuller", "Buchanan"}.ToList();
var expr = PredicateBuilder.False<Employee>();
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<Employee> employees = _mgr.Employees.Where(expr).ToList();
//foreach (Employee anEmployee in employees) {
// Console.WriteLine(anEmployee.LastName);
//}
//PromptToContinue();
}
|
Posted By: monkeyking
Date Posted: 17-Sep-2009 at 6:17pm
|
thanks GregD, that's helpful
|
Posted By: monkeyking
Date Posted: 17-Sep-2009 at 8:42pm
|
Hey, GregD, can you please tell me what i should import to get the PredicateBuilder class?
|
Posted By: GregD
Date Posted: 23-Dec-2009 at 10:43am
|