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.NET Gotchas

Using a technology well

You might be wondering what this book is all about. As the author explains, the dictionary's definition of a gotcha is "an unexpected usually disconcerting challenge, revelation, or catch". Mr. Subramaniam defines the gotchas in his book as "those things that pop up unexpectedly when you're programming in .NET. … In this book I focus on the .NET framework and features that have consistently exhibited behavior that was not obvious to me." Mr. Subramaniam explains that the purpose of his book is not just to explain how to use a technology "but how to use it well and do things right". The book is intended for ".NET programmers in the trenches". He assumes that you are fairly familiar with .NET and all gotchas are presented in C# and VB.NET. The book is organized into 8 chapters focusing in on different areas of interest. Each topic in a chapter includes a discussion of the gotcha, code examples and possible solutions. There are also footnotes and a bibliography for further study of the topic.

The book contains fascinating discussions on a myriad of topics. Some of the topics discussed are Language API Gotchas, Visual Studio and Complier Gotchas as well as Inheritance and Polymorphism Gotchas If you do a lot of multithreading you will definitely want to read this chapter.

I would like to present a Gotcha that I have encountered that Mr. Subramaniam touches on in Gotcha #2: struct and classdiffer in behavior" and is discussed by Les Smith in his article located at www.knowdotnet.com/articles/boxedstructures.html. In .NET a class is treated as a reference type while a structure is treated as a value type. The gotcha occurs when you place a structure inside an arraylist. If you try to iterate through the arraylist to make changes to each structure, you will be surprised to find that the changes do not take effect. In my code below, I declare a structure and an arraylist. I add two entries to the arraylist and then iterate through the list to change the value of gamesPlayed.

Option Strict On

Public Structure Player
      Public firstName As String
      Public lastName As String
      Public gamesPlayed As Integer
End Structure

Public Sub ArrayList_Structure

    Dim myTeam As New ArrayList
    Dim myPlayer As Player
    Dim intCount As Integer
    Dim intEnd As Integer

    myPlayer.firstName = "Bob"
    myPlayer.lastName = "Smith"
    myPlayer.gamesPlayed = 0
    myTeam.Add(myPlayer)

    myPlayer.firstName = "Jane"
    myPlayer.lastName = "Doe"
    myPlayer.gamesPlayed = 0
    myTeam.Add(myPlayer)

    For Each myPlayer In myTeam

      myPlayer.gamesPlayed += 1

Next

   For Each myPlayer In myTeam

      Response.Write( _
        myPlayer.firstName _& " " _
        & myPlayer.lastName _& " " _
        & myPlayer.gamesPlayed.ToString _
          & "<br/>")

Next

The results of this code are:

Bob Smith 0
Jane Doe 0

This is definitely not what you expected but is due to the fact that structures and arraylist are different types. To solve this problem you could do the following:

intCount = 0
intEnd = myTeam.Count - 1

While intCount <= intEnd

    myPlayer = CType(myTeam(intCount), Player)
    myPlayer.gamesPlayed += 1
    myTeam(intCount) = myPlayer
    intCount += 1

End While

For Each myPlayer In myTeam

Response.Write( _
    myPlayer.firstName _& " " _
    & myPlayer.lastName _& " " _
    & myPlayer.gamesPlayed.ToString _
    & "<br/>")

Next

End Sub

By converting the selected entry back into a structure you are now able to change it and then you can put it back into the arraylist with the following results.

Bob Smith 1
Jane Doe 1

.Net Gotchas is a very interesting book that covers a whole slew of topics. It is well worth reading especially if you have encountered the same gotchas and could not discover a way to solve them.

More Stories By Steven Mandel

Steven Mandel has worked in the IT industry for over 15 years designing databases using Microsoft Access and SQL Server. He has developed Web and Windows applications using VB.NET and has written numerous articles and reviews about ASP.NET and VB.NET.

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Most Recent Comments
Nmixer 01/24/08 07:15:51 PM EST

Thank you for that, I was not sure exactly how an arraylist works and did not realize it can hold an array of objects like a structure. and reading your example made me relies that i could :) so thanks.
this is what i searched to get this page "declaring a structure as an arraylist" odd isn't it?

anyways thanks

Jackson123r 06/26/07 04:50:34 AM EDT

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.NET News Desk 05/10/06 04:31:53 PM EDT

You might be wondering what this book is all about. As the author explains, the dictionary's definition of a gotcha is 'an unexpected usually disconcerting challenge, revelation, or catch'. Mr. Subramaniam defines the gotchas in his book as 'those things that pop up unexpectedly when you're programming in .NET. ? In this book I focus on the .NET framework and features that have consistently exhibited behavior that was not obvious to me.' Mr. Subramaniam explains that the purpose of his book is not just to explain how to use a technology 'but how to use it well and do things right'. The book is intended for '.NET programmers in the trenches'. He assumes that you are fairly familiar with .NET and all gotchas are presented in C# and VB.NET.

Matt J. 05/04/06 01:52:48 PM EDT

Having not seen the list of gotchas discussed, here's a simple one that got me: In VB, even though I set the default to Compare Text, functions like String.IndexOf() are still case sensitive! I understand why. I just don't like it!