Thus for used base class for some commom reusable methods in every page of my application...
public class BaseClass:System.Web.UI.Page
{
   public string Get         
        Base is used when you override a method in a derived class but just want to add additional functionality on top of the original functionality
For example:
  // Calling the Area base method:
  public override void Foo() 
  {
     base.Foo(); //Executes the code in the base class
     RunAdditionalProcess(); //Executes additional code
  }
The base keyword is used to refer to the base class when chaining constructors or when you want to access a member (method, property, anything) in the base class that has been overridden or hidden in the current class. For example,
class A {
    protected virtual void Foo() {
        Console.WriteLine("I'm A");
    }
}
class B : A {
    protected override void Foo() {
        Console.WriteLine("I'm B");
    }
    public void Bar() {
        Foo();
        base.Foo();
    }
}
With these definitions,
new B().Bar();
would output
I'm B
I'm A
You can use base to fill values in the constructor of an object's base class.
Example:
public class Class1
{
    public int ID { get; set; }
    public string Name { get; set; }
    public DateTime Birthday { get; set; }
    public Class1(int id, string name, DateTime birthday)
    {
        ID = id;
        Name = name;
        Birthday = birthday;
    }
}
public class Class2 : Class1
{
    public string Building { get; set; }
    public int SpotNumber { get; set; }
    public Class2(string building, int spotNumber, int id, 
        string name, DateTime birthday) : base(id, name, birthday)
    {
        Building = building;
        SpotNumber = spotNumber;
    }
}
public class Class3
{
    public Class3()
    {
        Class2 c = new Class2("Main", 2, 1090, "Mike Jones", DateTime.Today);
    }
}