Throwing Exceptions

Throwing Exceptions

You can make your code explicitly raise an exception by using the throw statement. The syntax for the throw statement is the following:

throw [Exception Object];

For example, following code will throw exception when supplied file name and display file content when the file exist is not exist:

class Program
{
 static void readFile(string fileName)
 {
  if(File.Exists(fileName))
  {
   FileStream myFile = File.OpenRead(fileName);
   int data;
   do
   {
    data=myFile.ReadByte();
    if (data == -1)
     return;
    else
     Console.Write((char)data);
   }while(true);
  }
  else
  {
   throw new ArgumentNullException();
  }
 }

 static void Main(string[] args)
 {
  try
  {
   readFile("a.txt");
  }
  catch(ArgumentNullException)
  {
   Console.WriteLine("File not exist");
  }
 }
} 

Throwing Without an Exception Object

The throw statement can also be used without an exception object, inside a catch block with following restriction:

• This form rethrows the current exception, and the system continues its search for additional handlers for it.
• This form can be used only inside a catch statement.

For example, the following code rethrows the exception from inside the first catch clause:

class Program 
{
 public static void PrintArg(string arg)
 {
  try
  {
   try
   {
    if (arg == null)
    {
     ArgumentNullException myEx = new ArgumentNullException("arg");
     throw myEx;
    }
    Console.WriteLine(arg);
   }
   catch (ArgumentNullException e)
   {
    Console.WriteLine("Inner Catch: {0}", e.Message);
    throw;
   }
  } 
  catch
  {
   Console.WriteLine("Outer Catch: Handling an Exception.");
  }
 }

 static void Main() 
 {
  string s = null;
  PrintArg(s);
  Console.Read();
 }
}  

This code produces the following output:

Inner Catch: Value cannot be null.
Parameter name: arg
Outer Catch: Handling an Exception.

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