C# 4 and .NET 4

366 ❘ ChaPTer 15 errOrs And exceptiOns
The ProcessNextPerson() method also contains an inner try block:
public void ProcessNextPerson()
{
if (isDisposed)
{
throw new ObjectDisposedException("peopleToRing");
}
}
if (!isOpen)
{
throw new UnexpectedException(
"Attempted to access coldcall file that is not open");
}
try
{
string name;
name = sr.ReadLine();
if (name == null)
throw new ColdCallFileFormatException("Not enough names");
if (name[0] == 'B')
{
throw new SalesSpyFoundException(name);
}
Console.WriteLine(name);
}
catch(SalesSpyFoundException ex)
{
Console.WriteLine(ex.Message);
}
finally
{
}
Two possible problems exist with the file here (assuming that there actually is an open file connection;
the ProcessNextPerson() method checks this first). First, you might read in the next name and discover
that it is a sales spy. If that condition occurs, the exception is trapped by the first of the catch blocks in
this method. Because that exception has been caught here, inside the loop, it means that execution can
subsequently continue in the Main() method of the program and the subsequent names in the file will
continue to be processed.
A problem might also occur if you try to read the next name and discover that you have already reached
the end of the file. The way that the StreamReader object’s ReadLine() method works is if it has gone
past the end of the file, it doesn’t throw an exception, but simply returns null. Therefore, if you find a
null string, you know that the format of the file was incorrect because the number in the first line of the
file indicated a larger number of names than were actually present in the file. If that happens, you throw a
ColdCallFileFormatException, which will be caught by the outer exception handler (which will cause the
execution to terminate).
Once again, you don’t need a finally block here because there is no cleanup to do; however, this time an
empty finally block is included, just to show that you can do so, if you want.
The example is nearly finished. You have just two more members of ColdCallFileReader to look at: the
NPeopleToRing property, which returns the number of people supposed to be in the file, and the Dispose()
method, which closes an open file. Notice that the Dispose() method returns only if it has already been
called — this is the recommended way of implementing it. It also checks that there actually is a file stream to
close before closing it. This example is shown here to illustrate defensive coding techniques, so that’s what
you are doing!
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