C# Object Class

<-- C# BaseC# Sealed -->
  • Object Class is the ultimate base class of all kinds of classes or class hierarchies in the .NET framework.
  • All the methods in Object Class can be used in any classes and the virtual methods of Object Class can be overridden in any classes.
  • If a class is defined without inheriting from any other class explicitly, the class will inherit from Object Class implicitly.

For example:

public class A
{
...
}

It is equivalent to the following.

public class A : Object
{
...
}
Methods in Object Class

Example 01-58-01

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using System;

public class Rectangle
{
    private int length;
    private int width;

    public Rectangle(int length, int width)
    {
        this.length = length;
        this.width = width;
    }

    public int area()
    {
        return length * width;
    }
}

public class TestRectangle
{
    public static void Main()
    {
        Rectangle a = new Rectangle(5, 6);
        Rectangle b = new Rectangle(5, 6);

        Console.WriteLine(a.Equals(b) ? "a equals b." : "a doesn't equal b.");
        Console.WriteLine(a.ToString());
        Console.WriteLine("Hash Code = {0}", a.GetHashCode());
        Console.Read();
    }
}

Output

a doesn't equal b.
Rectangle
Hash Code = 46104728

Explanation

  • Line 3-18: Define public class Rectangle which implicitly inherited from Object class.
  • Line 8-12: Instructor with 2 parameters.
  • Line 20-32: Define a class TestRectangle implicitly inherited from Object class.
  • Line 22: Main method starts here.
  • Line 24-25: Create instances of the class Rectangle with the same initial values.
  • Line 27: Equals() method was defined in Object class and is inherited in Rectangle class. Because a and b are all references. We'll use the method to test for reference equality.
  • Line 28: ToString() is also a method defined in Object class and inherited in Rectangle.
  • Line 29: GetHashCode() was defined in Object class as well and return the hash code of the reference.
Overriding Methods of Object Class

Example 01-58-02

We'll add 3 override methods for the class Rectangle in the example 01-58-01

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using System;

public class Rectangle
{
    private int length;
    private int width;

    public Rectangle(int length, int width)
    {
        this.length = length;
        this.width = width;
    }

    public int area()
    {
        return length * width;
    }

    public override bool Equals(object obj)
    {
        if (this.GetType() == obj.GetType())
        {
            Rectangle that = (Rectangle)obj;
            return this.length == that.length && this.width == that.width;
        }
        else
        {
            return false;
        }
    }

    public override int GetHashCode()
    {
        return area();
    }

    public override string ToString()
    {
        return "Rectangle: Length=" + this.length.ToString() + ", Width=" + this.width.ToString() + ", Area=" + area().ToString();
    }
}

public class TestRectangle
{
    public static void Main()
    {
        Rectangle a = new Rectangle(5, 6);
        Rectangle b = new Rectangle(5, 6);

        Console.WriteLine(a.Equals(b) ? "a equals b." : "a doesn't equal b.");
        Console.WriteLine(a.ToString());
        Console.WriteLine("Hash Code = {0}", a.GetHashCode());
        Console.Read();
    }
}

Output

a equals b.
Rectangle: Length=5, Width=6, Area=30
Hash Code = 30

Explanation

  • Line 19-30: Define a method Equals() to override the same virtual method in Object class.
  • Line 21: GetType() is defined in Object class to return the type of the reference.
  • Line 23: Explicitly cast obj to Rectangle type.
  • Line 24: Return true if the field length and width of the 2 references are equal at the same time.
  • Line 50: a.Equals(b) returns true because the override method is called.
  • Line 32-35: Define the override method GetHashCode() to return the area of the Rectangle.
  • Line 37-40: Define the override method ToString().
  • Line 51-52: The override methods are called instead of the original methods in Object class.
  • The result is different from that in the example 01-58-01 because the override methods are called.
<-- C# BaseC# Sealed -->