Question1.cs

using System;
using System.Collections.Generic;

namespace TestApp {
	class Program {
		static Dictionary<string, string> users = new Dictionary<string, string>();

        static void Main(string[] args) {

			Input20Users();
			ShowAllUsers();
			SearchAndDelete();
			SearchAndDisplay();
        }

		static void Input20Users() {
			for(byte i = 0; i<4; i++) {
				Console.Write("Enter mobile #: ");
				string mob = Console.ReadLine();

				Console.Write("Enter name: ");
				string name = Console.ReadLine();

				if(users.ContainsKey(mob)) {
					Console.WriteLine("This mobile no already exists.");
					i--;
					continue;
                }

				users.Add(mob, name);
            }
        }

		static void ShowAllUsers() {
			foreach(var user in users) {
				Console.WriteLine("{0}:{1} ", user.Key, user.Value);
            }
        }

		static void SearchAndDelete() {
			Console.Write("Enter mobile no to delete: ");
			string mob = Console.ReadLine();

			if(users.ContainsKey(mob)) {
				users.Remove(mob);
				Console.WriteLine("Record {0} deleted.", mob);
				return;
            }

			Console.WriteLine("Record {0} Not found.", mob);
        }

		static void SearchAndDisplay() {
			Console.Write("Enter name to display: ");
			string name = Console.ReadLine();

			foreach(var user in users) {
				if(user.Value == name) {
					Console.WriteLine("Mobile #: {0}, Name: {1}", user.Key, user.Value);
					return;
				}
			}

			Console.WriteLine("Record {0} not found.", name);
		}
	}
}

Question2.cs

using System;
using System.Collections;

namespace TestApp {
    class MyArrayList : ArrayList {
        public override int Add(object value) {
            base.Insert(0, value);
            return 0;
        }

        public override void Remove(object obj) {
            while(base.Contains(obj)) {
                base.Remove(obj);
            }
        }

        public override void Insert(int index, object value) {
            if(index > base.Count || index < 0) index = 0;

            base.Insert(index, value);
        }
    }
}

Question3.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp {
	class Program {
        static void Main(string[] args) {
            List<int> vals = new List<int>() {
                1, 2, 3, 2, 4, 6, 5, 7, 9, 8, 8
            };

            var descList = vals.OrderByDescending(v => v);

            foreach(int v in descList) {
                Console.WriteLine(v);
            }

            var evenList = vals.Where(v => v % 2 == 0);

            foreach(int v in evenList) {
                Console.WriteLine(v);
            }

            Console.WriteLine("Min: {0}", vals.Min());
            Console.WriteLine("Max: {0}", vals.Max());
            Console.WriteLine("Avg: {0}", vals.Average());
            Console.WriteLine("Sum: {0}", vals.Sum());
            Console.WriteLine("Count: {0}", vals.Count());
        }
	}
}

Question4.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp {
    class Country {
        public string Name;
        public string Capital;
        public long Population;

        public override string ToString() {
            return string.Format("Name: {0} \nCapital: {1} \nPopulation: {2} \n", Name, Capital, Population);
        }
    }

    class Program {
        static void Main(string[] args) {
            List<Country> countries = new List<Country>() {
                new Country(){ Name = "Pakistan", Capital = "Islamabad", Population=220000000},
                new Country(){ Name = "India", Capital = "New Delhi", Population=1000000000},
                new Country(){ Name = "Afghanistan", Capital = "Kabul", Population=35000000},
                new Country(){ Name = "Iran", Capital = "Tehran", Population=81000000}
            };

            var q1 = countries.Where(v => v.Population >= 100 && v.Population <= 200);

            foreach(Country country in q1) {
                Console.Write(country);
            }

            var q2 = countries.Where(v => v.Name.ToLower().StartsWith("p"));

            foreach(Country country in q2) {
                Console.Write(country);
            }

            var q3 = countries.OrderBy(v => v.Name);

            foreach(Country country in q3) {
                Console.Write(country);
            }
        }
    }
}

Question5.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp {
    class Student {
        public string Name;
        public char Gender;
        public float Cgpa;
        public string Discipline;
        public DateTime Dob;

        public override string ToString() {
            return string.Format("Name: {0} \nGender: {1} \nCgpa: {2} \nDiscipline: {3} \nDob: {4} \n", Name, Gender, Cgpa, Discipline, Dob);
        }
    }

    class Program {
        static void Main(string[] args) {
            List<Student> students = new List<Student>() {
                new Student(){ Name="Javed", Gender='M', Cgpa=3.3f, Discipline="BSCS", Dob=new DateTime()},
                new Student(){ Name="Ali", Gender='M', Cgpa=3.0f, Discipline="BSIT", Dob=new DateTime()},
                new Student(){ Name="Zara", Gender='F', Cgpa=3.4f, Discipline="MSCS", Dob=new DateTime()},
                new Student(){ Name="Hamza", Gender='M', Cgpa=2.3f, Discipline="BSCS", Dob=new DateTime()}
            };


            var q1 = students.Select(v => new { v.Name, v.Gender });

            foreach(var s in q1) {
                Console.WriteLine("Name: {0}, Gender: {1}", s.Name, s.Gender);
            }

            var q2 = students.OrderBy(v => v.Discipline).ThenByDescending(v => v.Cgpa);

            foreach(var s in q2) {
                Console.Write(s);
            }

            var q3 = students.Where(v => v.Cgpa > 3.0f).OrderBy(v => v.Name);

            foreach(var s in q3) {
                Console.Write(s);
            }
        }
    }
}

Question6.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp { 
    class Employee {
        public string Name;
        public string Gender;
        public int DeptId;
        public int Salary;
    }

    class Department {
        public int Id;
        public string Name;
        public string City;
    }

    class Program {
        static void Main(string[] args) {
            var employees = new List<Employee>();
            var departments = new List<Department>();

            var q1 = employees.Join(
                departments, 
                e => e.DeptId, 
                d => d.Id, 
                (e, d) => new { e, d }
            );

            foreach(var o in q1) {
                Console.Write(o);
            }

            var q2 = employees
                .Join(departments,
                    e => e.DeptId,
                    d => d.Id,
                    (e, d) => new { e, d })
                .Where(v => v.d.City.ToLower() == "lahore")
                .Select(v => new { v.e.Name, v.e.Gender, v.e.Salary, DeptName = v.d.Name });

            foreach(var o in q2) {
                Console.Write(string.Format("{0},{1},{2},{3}", o.Name, o.Gender, o.Salary, o.DeptName));
            }

            var q3 = employees.OrderBy(v => v.Salary);

            foreach(var o in q3) {
                Console.Write(o); 
            }
        }
    }
}