using System;

using System.Linq;

using System.Collections.Generic;

// https://stackoverflow.com/questions/60359312/store-linq-function-into-variable-define-on-the-fly?noredirect=1#comment106773551_60359312

public static class Program

{

    public static void Main()

    {

        List<StudentInfo> StudentsList = new List<StudentInfo>();

​

        // Group 1

        StudentsList.Add( new StudentInfo { Student="Gungun", University="cdf", GroupID = 1, IsQualified=false});

        StudentsList.Add( new StudentInfo { Student="John", University="abc", GroupID = 1, IsQualified=false});

        StudentsList.Add( new StudentInfo { Student="Jack", University="def", GroupID = 1, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Peter", University="abc", GroupID = 1, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Lemity", University="abc", GroupID = 1, IsQualified=false} );

​

        // Group 2

        StudentsList.Add( new StudentInfo { Student="Donald", University="def", GroupID = 2, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Olivia", University="ggh", GroupID = 2, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Emity", University="OPQ", GroupID = 2, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Emma", University="OPQ", GroupID = 2, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Alan", University="OPQ", GroupID = 2, IsQualified=false} );

​

        // Group 3

        StudentsList.Add( new StudentInfo { Student="Adam", University="yub", GroupID = 3, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Jacob", University="OPQ", GroupID = 3, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Matthew", University="OPQ", GroupID = 3, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Saint", University="abc", GroupID = 3, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Joshua", University="qer", GroupID = 3, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Aubrey", University="fef", GroupID = 3, IsQualified=false} );

​

        // Group 4

        StudentsList.Add( new StudentInfo { Student="Caleb", University="DEF", GroupID = 4, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Anna", University="ABC", GroupID = 4, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Chill", University="GHI", GroupID = 4, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Alexis", University="JKL", GroupID = 4, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="Zoe", University="MNO", GroupID = 4, IsQualified=false} );

​

        // Group 5

        StudentsList.Add( new StudentInfo { Student="Dylan", University="PQR", GroupID = 5, IsQualified=false} );

        StudentsList.Add( new StudentInfo { Student="White", University="OPQ", GroupID = 5, IsQualified=false} );

​

        Console.WriteLine(SomeFunction(StudentsList, p => p.IsQualified == true).Count());

        Console.WriteLine(SomeFunction(StudentsList, p => p.Student == "John").Count());

        Console.WriteLine(SomeFunction(StudentsList, p => p.GroupID == 1).Count());

        /// Use Extension Method. 

        Console.WriteLine("Use extionsion method");

        Console.WriteLine(StudentsList.ExtensionMethod_SomeFunction(p => p.IsQualified == true).Count());

        Console.WriteLine(StudentsList.ExtensionMethod_SomeFunction(p => p.Student == "John").Count());

        Console.WriteLine(StudentsList.ExtensionMethod_SomeFunction(p => p.GroupID == 1).Count());

    }

    private static IEnumerable<IGrouping<int, StudentInfo>>  SomeFunction(List<StudentInfo> list, Func<StudentInfo, bool> selector)

    {

        return list.GroupBy(x => x.GroupID)

                                  .Where(g => g.Any(selector) )

                                  .Select(g => g);

    }

    public static IEnumerable<IGrouping<int, StudentInfo>> ExtensionMethod_SomeFunction(this IEnumerable<StudentInfo> list, Func<StudentInfo, bool> selector) 

    {

        return list.GroupBy(x => x.GroupID)

                                  .Where(g => g.Any(selector) )

                                  .Select(g => g);

    }

    public class StudentInfo

    {

      public string Student { get; set; }

      public string University { get; set; }

      public int GroupID { get; set; }

      public bool IsQualified {get; set;}

​

    }

}