C# 0 The Complete Reference

NullReferenceException

:

// Use the NullReferenceException.

using System;

class X {

  int x;

  public X(int a) {

    x = a;

  }

  public int Add(X o) {

    return x + o.x;

  }

}

// Demonstrate NullReferenceException.

class NREDemo {

  static void Main() {

    X p = new X(10);

    X q = null; // q is explicitly assigned null

    int val;

Exception

Meaning

ArrayTypeMismatchException

Type of value being stored is incompatible with the type of 

the array.

DivideByZeroException

Division by zero attempted.

IndexOutOfRangeException

Array index is out of bounds.

InvalidCastException

A runtime cast is invalid.

OutOfMemor yException

Insufficient free memor y exists to continue program 

execution. For example, this exception will be thrown if 

there is not sufficient free memor y to create an object via 

new

.

Over flowException

An arithmetic over flow occurred.

NullReferenceException

An attempt was made to operate on a null reference—that 

is, a reference that does not refer to an object.

StackOver flowException

The stack was overrun.

T

ABLE

 13-1 

Commonly Used Exceptions Defi ned Within the 

System

 Namespace

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    try {

      val = p.Add(q); // this will lead to an exception

    } catch (NullReferenceException) {

      Console.WriteLine("NullReferenceException!");

      Console.WriteLine("fixing...\n");

      // Now, fix it.

      q = new X(9);

      val = p.Add(q);

    }

    Console.WriteLine("val is {0}", val);

  }

}

The output from the program is shown here:

NullReferenceException!

fixing...

val is 19

The program creates a class called