Can you help us with an oop problem in c#?

May be this alternate helps you

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

namespace ConsoleApplication1
{

    class Vehicle
    {
        public static int Count = 0;
        public Vehicle()
        {
            Count++;
        }
    }

    class Car :Vehicle
    {
        public static int Count = 0;
        public Car()
        {
            Count++;
        }
    }

    class Truck : Vehicle
    {
        public static int Count = 0;
         public Truck()
        {
            Count++;
        }
    }

    class Class3
    {
        public static void Main()
        {
            Vehicle V1 = new Car();
            Vehicle V2 = new Car();

            Vehicle V3 = new Truck();

            Console.WriteLine("No of Cars =" + Car.Count);
            Console.WriteLine("No of Truck =" + Truck.Count);
            Console.WriteLine("No of Vehicle=" + Vehicle.Count);

        }
    }
}

i could it! and i want to share the code with you =)

using System.Collections.Generic;
using System;
namespace Visitor {
    public interface IVisitor {
        void Visit(Car c);
        void Visit(Truck t);
    }
    public class PaintVisitor : IVisitor {
        public void Visit(Car car) {
            car.Paint("red");
        }
        public void Visit(Truck truck) {
            truck.Paint("blue");
        }
    }
    public abstract class Vehicle {
        private string _color = "Black";
        public void Paint(string color) {
            this._color = color;
        }
        public abstract void Accept(IVisitor v);
        public override string ToString() {
            return string.Format("{0} - {1}", this.GetType(), this._color);
        }
    }
    public class Car : Vehicle {
        public override void Accept(IVisitor v) {
            v.Visit(this);
        }
    }
    public class Truck : Vehicle {
        public override void Accept(IVisitor v) {
            v.Visit(this);
        }
    }
    public class Program {
        public static void Main(string[] args) {
            List<Vehicle> vehicles = new List<Vehicle>();
            vehicles.Add(new Car());
            vehicles.Add(new Truck());
            vehicles.Add(new Car());
            vehicles.ForEach(x => Console.WriteLine(x));
            PaintVisitor pVisitor = new PaintVisitor();
            vehicles.ForEach(x => x.Accept(pVisitor));
            vehicles.ForEach(x => Console.WriteLine(x));
            Console.Read();
        }
    }
}

thanks everyone

Khaniya thanks!

Now if i wanna paint all cars but no the trucks?

IList<Vehicles> vehicles = ......

foreach(Vehicle v in vehicles) {
    if(v is Car){
        v.Paint("Red");
    }
}

the point isn't ask for the type of instance, i can't solve it!!

my teacher told me, that the solution for this problem is "Double Dispatching" (Visitor) but i don't know how implement this pattern, any idea?

When you are counting Car or Truck, it is fine with your solution. "is" operator is quite capable of determining the type of an object.

For more simplistic an On demand logic, you can also use LINQ to filter out the element from the list.

int cars = vehicle.Where(item => item is Car).Count;

Here Where allow you to filter elements based on its type as well.

Another implementation could be use of ObservableCollection.

ObservableCollection notifies when a new object is added or removed. You can handle CollectionChanged event to get your counter updated when a new object is added.