AUTHOR S NOTE in Visual C#

Maker Code 3/9 in Visual C# AUTHOR S NOTE

Asynchronous event handling is not supported in the first version of PowerShell. In fact, if you try to use it, the PowerShell runtime will detect it and generate a fatal exception, killing your shell session. Yes, this is extreme. The rationale is that it s better to fail early and in an obvious way than allow the program to continue in a corrupt state. We tried simply generating a message, but people would miss it, and wonder why their script wasn t working. People don t miss killing the session. They aren t very happy about it, but they notice. So why prevent asynchronous events PowerShell has no mechanism for synchronizing access to objects. This means that there is no way to do reliable multithreaded programming. We expect to address this in a future version of the interpreter. In the meantime, there are lots of things that use synchronous events, and fortunately WinForms is one of them. We ll look at how this works in WinForms later on.

While polling, first we check to see whether there is a key available J and stop if there is. We want to do it this way instead of using Ctrl-C to interrupt the script, because we want to be able to properly close the listener. On each poll, we check to see whether there are any pending requests and service them if there are 1). We get the data from the request into the buffer we created earlier 1!. Since we want to work with strings instead of bytes, we ll convert the contents of the buffer into a string. Now get the HTTP request out of the text we received. This will look something like GET /3*5 HTTP/1.1 . The first part is the type of operation requested, which is called

the HTTP method. The next part is the URL, which is what we want to work with, and finally the version of the HTTP protocol. We want to extract this text, but first we need to split the request string into lines. In the HTTP protocol, lines are separated with the <carriage return><newline> sequence. We ll use the [regex]::Split() 1@ method to do this. If we got a valid request then extract the expression out of the URL with regular expressions 1#. Notice that we re very carefully validating the expression 1$. This is because we re going to pass it to Invoke-Expression. And since Invoke-Expression can do anything (like format your hard drive), we need to be really, really, really careful about validating data from an untrusted source such as a web browser. Really careful! Now let s do something a bit tricky 1$. We ll call Invoke-Expression to do the evaluation, but we re doing it in a scriptblock. In that scriptblock, we re defining a trap handler. If there is a problem with the expression (such as division by zero) the trap statement will catch the exception, record the error, and return a default value. This useful pattern is a simple way to guarantee a valid value is returned. We ll format the HTML table that used to display the result by using string expansion 1% in a here-string. This makes constructing the HTML fairly simple and direct. And likewise in the invalid expression case, we ll format a table to display the hint and the example link 1^. Notice that we re hard-coding the example host to be localhost. We re assuming that you re running the server and the client on the same computer. Finally, send the response header and content 1&. Most of the default values for SendHeader are fine, so the call is pretty simple. When we re done servicing this request, we close the connection to this client 1*. Something to keep in mind is that we re polling for activity. If we poll as fast as we can, we ll waste a lot of CPU and interfere with other activities on this computer. To avoid this, we ll sleep for 100 milliseconds on each pooling loop 1(. Eventually we ll exit the service loop because the user pressed a key requesting that the server be stopped. We ll stop the listener, and we re done 2). That s the end of our web server script. We ve written a complete web server that serves dynamically created pages in less than 200 lines of PowerShell script. And, while it s not by any means a production-worthy application, it illustrates that PowerShell can be used in scenarios that are normally considered outside the range of a shell language. This is largely possible because of the power of the .NET framework.