Linq比较C＃中的两个集合

Question

I want to compare two collections in C# that I'm currently doing using nested "for" loop. is there a way in Linq to do the same which will be quicker and more efficient? here is my current code which works perfectly just looking for an efficient way:

OrgCollection myYears = Org.RetrieveDistinctYear();
if (myYears.Count > 0)
{
AcademicYearCollection allYears = AcademicYear.RetrieveAll();
for (int i = 0; i < myYears.Count; i++)
{
    for (int j = 0; j < allYears.Count; j++)
    {
        if (myYears[i].AcademicYearCode == allYears[j].Code)
        {
        ddlYear.Items.Insert(0, new ListItem(allYears[j].Name,allYears[j].Code));
        break;
        }
    }
}
}

I want to compare "Code" from AcademicYearCollection with the "AcademicYearCode" property in OrgCollection & if it is the same then add it in the Dropdownlist "ddlYear".

Thanks in advance.

Answer 1

You can do it in LINQ, which gives shorter code. To know if it is more efficient or not you would have to profile it. I think that linq's join operator uses some kind of hash buckets inernally which should give better performance, especially if the collections are large. Your current solution is O(N^2) which will quickly degrade if the number of options increases.

OrgCollection myYears = Org.RetrieveDistinctYear();
AcademicYearCollection allYears = AcademicYear.RetrieveAll();

var items = from y in myYears
            join ay in allYears
            on y.Code equals ay.AcademicYearCode
            select new { ay.Name, ay.Code }

Answer 2

OrgCollection myYears = Org.RetrieveDistinctYear();
if (myYears.Count > 0)
{
    AcademicYearCollection allYears = AcademicYear.RetrieveAll();
    for (int i = 0; i < myYears.Count; i++)
    {
         if (allYears[j].Any(allY => allY ==  myYears[i].AcademicYearCode ))
            {
                ddlYear.Items.Insert(0, new ListItem(allYears[j].Name, allYears[j].Code));
                break;
            }

    }
}

This could be an option but i think the extension method 'any' works the same way by doing an iteration.

Answer 3

How about this

var allYears = AcademicYear.RetrieveAll().ToDictionary(y => y.Code, y.Name);

ListItem match = null;
foreach(var year in Org.RetrieveDistinctYear())
{
    if (allYears.HasKey(year.AcademicYearCode)
    {
        match = new ListItem(
                       allYears[year.AcademicYearCode], 
                       year.AcademicYearCode);
        break;
    }
}

if (match != null)
{
    ddlYear.Items.Insert(0, match); 
}

Using a Dictionary here provides superior performance and the further down the Org.RetrieveDistinctYear results the match is, the more benefit there will be. If the results of RetrieveDistinctYear are often short or the match is at the top the overhead of creating the dictionary will make the code unesscessarily slow.

---

EDIT

Or this approach

var allYears = AcademicYear.RetrieveAll().ToDictionary(y => y.Code, y.Name);

var matchingCode = Org.RetrieveDistinctYear()
    .Select(y = y.AcademicYearCode)
    .FirstOrDefault(code => allYears.HasKey(code));

if (!string.IsEmptyOrWhitespace(matchingCode))
{
    ddlYear.Items.Insert(0, new ListItem(
                                 allYears[matchingCode], 
                                 matchingCode)); 
}

Answer 4

This solution has similar speed and efficiency to your original solution, but it can be parallelized by changing from y in myYears to from y in myYears.AsParallel() to possibly speed it up.

OrgCollection myYears = Org.RetrieveDistinctYear();
AcademicYearCollection allYears = AcademicYear.RetrieveAll();

var items = from y in myYears
            let match = allYears.FirstOrDefault( ay => y.AcademicYearCode == ay.Code)
            where match != null
            select new ListItem(match.Name, match.Code);