This is redundant with get; and set; methods built-in, but this is a good way to see whats happening by doing it ourselves!

GetterSetterMethods.cs

using System;

/* Not all programming languages use Properties
 * 
 * Getter and Setter methods are used to encapsulate and protect fields
 * 
 */


public class Student
{
    private int _id;
    public string _name;
    public int _passMark;

    public void SetId(int Id)
    {
        if (Id <= 0)
        {
            throw new Exception("Student Id should be greater than zero");
        }
        this._id = Id;
    }

    public int GetId()
    {
        return _id;
    }
}

public class Program
{
    public static void Main()
    {
        Student S1 = new Student();
        S1.SetId(1221);
        Console.WriteLine("Student Id = {0}", S1.GetId());

        Console.ReadKey();

    }
}