ما هي دالة ToLookup في LINQ؟

دالة ToLookup في LINQ

دالة ToLookup في LINQ بتسمح لك تبني هيكل شبيه بـ Dictionary لكن كل مفتاح مربوط بمجموعة من العناصر بدل عنصر واحد.

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الصيغة العامة:

var lookup = collection.ToLookup(item => item.Key);

الفرق بينها وبين GroupBy:

• GroupBy بتنفيذ مؤجل (Lazy / Deferred Execution)
• ToLookup بتنفيذ فوري (Immediate Execution)

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تعريف الكائن:

public class Student
{
    public int StudentID { get; set; }
    public string Name { get; set; }
    public string Branch { get; set; }
    public string Gender { get; set; }
}

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البيانات:

List<Student> studentList = new List<Student>()
{
    new Student() { StudentID = 1, Name = "John", Branch = "CSE", Gender = "Male"},
    new Student() { StudentID = 2, Name = "Moin", Branch = "CSE", Gender = "Male"},
    new Student() { StudentID = 3, Name = "Sara", Branch = "CSE", Gender = "Female"},
    new Student() { StudentID = 4, Name = "Ram", Branch = "ECE", Gender = "Male"},
    new Student() { StudentID = 5, Name = "Sania", Branch = "ECE", Gender = "Female"},
    new Student() { StudentID = 6, Name = "Ravi", Branch = "EEE", Gender = "Male"},
    new Student() { StudentID = 7, Name = "Anjali", Branch = "EEE", Gender = "Female"}
};

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استخدام ToLookup حسب Branch:

var studentLookup = studentList.ToLookup(s => s.Branch);

foreach (var group in studentLookup)
{
    Console.WriteLine($"Branch: {group.Key}, Count: {group.Count()}");
    foreach (var student in group)
    {
        Console.WriteLine($"  {student.Name} ({student.Gender})");
    }
}

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النتيجة:

Branch: CSE, Count: 3
  John (Male)
  Moin (Male)
  Sara (Female)
Branch: ECE, Count: 2
  Ram (Male)
  Sania (Female)
Branch: EEE, Count: 2
  Ravi (Male)
  Anjali (Female)

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استخدام ToLookup بمفتاح مركب (Branch + Gender):

var studentLookup = studentList.ToLookup(s => new { s.Branch, s.Gender });

foreach (var group in studentLookup)
{
    Console.WriteLine($"Branch: {group.Key.Branch}, Gender: {group.Key.Gender}, Count: {group.Count()}");
    foreach (var student in group)
    {
        Console.WriteLine($"  {student.Name}");
    }
}

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النتيجة:

Branch: CSE, Gender: Male, Count: 2
  John
  Moin
Branch: CSE, Gender: Female, Count: 1
  Sara
Branch: ECE, Gender: Male, Count: 1
  Ram
Branch: ECE, Gender: Female, Count: 1
  Sania
Branch: EEE, Gender: Male, Count: 1
  Ravi
Branch: EEE, Gender: Female, Count: 1
  Anjali

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نصائح:

• استخدم ToLookup لما تحتاج تجمع وتبحث بسرعة حسب مفتاح.
• لو محتاج الأداء، ToLookup أفضل من GroupBy في بعض السيناريوهات لأنه بيكون جاهز بمجرد إنشاؤه.
• يمكنك الوصول للمجموعة مباشرة: studentLookup["CSE"]