What are C# Generics - Part II

by agrace 11. April 2008 20:49

Generics I'm going to keep this as simple as possible. Your main goal is to gain an appreciation of why we would want to use generics in the first place. The typical candidate scenario for the use of generics is where we have a class that has a member variable, and the type of this variable should be defined by the client of the class, and not by the programmer. Likewise, for a method that has a parameter passed to it. In other words, the code in our classes and methods remains the same and the types of the data can change with each use. Think code reuse.

Up to now, this type of generality involved writing the same code over and over for each type you wanted to accomodate. And with collections, you would typically have to use the object class and then cast back. This would involve boxing and unboxing for value types, which would incur heavy performance hits, depending on the size of your collections. More importantly, when casting back, you could not be certain of the type in the collection. This could result in runtime errors after your code had shipped. More than anything else, the type safety afforded by generics is its biggest selling point. Type-safe code is the easiest code to maintain, plain and simple.

With generics, we declare type-parameterized code which we can instantiate with different types. We write the code with '<T>' placeholders for types and then plug in the actual types when we are creating an instance. We don't have to use the letter 'T', it's just convention.

// 1.1 Loosely-Typed Collection
Class Stack
  public object Pop();

Stack s = new Stack();

// Cast necessary
int i = (int) s.Pop();

// 2.0 Strongly-Typed Collection
Class Stack<T>
  public <T> Pop();

Stack<int> s = new Stack<T>();

// No cast necessary
int i = s.Pop();

You'll probably find the most ready use for generics when implementing collections. The generic collection classes are part of the C# 2.0 System.Collections.Generic namespace. Below is a list of the traditional 1.1 collections and their new 2.0 equivalents (from Krzysztof Cwalina):

Non-Generic Similar Generic Type
ArrayList List<T>
Hashtable Dictionary<TKey,TValue>
SortedList SortedList<TKey,TValue>
Queue Queue<T>
Stack Stack<T>
IEnumerable IEnumerable<T>
ICollection N/A (use IEnumerable<T> or anything that extends it)
N/A ICollection<T>
IList IList<T>
CollectionBase Collection<T>
ReadOnlyCollectionBase ReadOnlyCollectionBase<T>
DictionaryBase N/A (just implement IDictionary<TKey,TValue>)
N/A SortedDictionary<TKey,TValue>
N/A KeyedCollection<TKey,TItem>
N/A LinkedList<T>

In the next part, I'll try and cut through the jargon attached to generics and also discuss some practical uses for generics in everyday coding tasks. In the meantime, check out this excellent Overview of Generics in the .NET Framework.

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What are C# Generics - Part I

by agrace 9. April 2008 21:10

Generic Breakfast They say the best developers are lazy. I used to believe this and would cite code reuse as justification for my inertia! For a long time now, I have been intending to use generics and collections more in my code. I guess I meant well, but in the real world we tend to go for what we know works and a syntax that we feel comfortable with. There are so many new additions to the C# langauge now that it is tempting to want to jump in and play. However, I feel that a grasp of generics is an important skill to acquire first. So, I'm going to take a walk through the generics landscape and see how it all fits together...

Here's a loose definition from a Microsoft MSDN article: "Generics are the most powerful feature of C# 2.0. Generics allow you to define type-safe data structures, without committing to actual data types. This results in a significant performance boost and higher quality code, because you get to reuse data processing algorithms without duplicating type-specific code. In concept, generics are similar to C++ templates, but are drastically different in implementation and capabilities."

Generics Generics permit us to write code where the data types aren't hard-coded. If we have a lot of code that performs the same function but on different types, then we can get big savings in terms of performance and the amount of code that we actually have to create.

Let me say two things about generics right off the bat: first, the main motivation for generics is not one of performance. Performance is more of a side effect, if you will. It is more about what the name implies, that is, it gives a level of generality to our types. It helps us to factor out the behaviour of our classes from the data upon which they act. In addition to performance gains, we profit from increased code reuse and type safety. Second, although they share a similar motivation, generics and templates are fundamentally different animals: generics are created at runtime by the CLR and templates are created at compile time by the compiler.

The designers of generics define it as "a feature that permits classes, structures, interfaces, delegates, and methods to be parameterized by the types of data they store and manipulate." The goal of this series is to demystify generics and to encourage you incorporate it into your own code, even if only in a very basic way to begin.

In the next part, we'll take a look at some specifics including where we can use generics and and how to find our way through the new terminology associated with generics. We'll save 'Parametric Polymorphism' for a later installment ;-)

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Guest Book As promised, here's the final installment. You can find the download links for the final version of the code at the end of this article. Feel free to use this code and tweak it any way you wish. For demo purposes, the menu is on all the pages but you can easily factor this out into its own form. I would normally make this a separate control, but you will probably be integrating this application into your existing navigation anyway, so...

The membership system in ASP.NET 2.0 is pretty much plug-and-play right out of the box. When I worked with it first, I took some time to read up on the new membership controls before trying them out. For this particular topic, I cannot recommend the following two books highly enough:

Murach's ASP.NET 2.0 Upgrader's Guide (Lowe & Murach)
Pro ASP.NET 2.0 in C# 2005 (MacDonald & Szpuszta)

Just to recap, at this point you should have a working Guest Book application and database. The only thing left to do is to configure the membership system and add some administration forms to allow us to edit, update, delete and publish comments posted by users.


The next item on our list is to use the WSAT tool to add an admin user, create the admin role and add the new admin user to that role. By using a role, we can grant extra privileges to admin users in the future if needed. We can just do it once rather than having to grant them to each admin user individually. A picture is worth a thousand words, so please refer to the pics for guidance.



Access the WSAT by clicking on the icon at the top of Solution Explorer in VS 2008. Note that the administrator role is already set up in the config file and all you have to do is add the admin user to this role. I'm including a picture showing how to set up the access rule using the tool. The main problem people encounter here is the order of the entries in the authorization section of the config file. The administrator role is listed before 'users'. See the config file picture in part II of this series for clarification.

Admin Login

Guest Book Admin

We will add a new form called guestBookAdmin.aspx to the Admin folder which shows a list of the comments awaiting moderation in a GridView. Clicking on one of these brings up the commentDetails.aspx form. This is almost a replica of the addComment.aspx form which is populated with the data for this comment. The comment ID is passed to this new form when the 'select' link on the guestBookAdmin form is clicked. This is then used as a parameter to the stored procedure when fetching the data for this comment. Note also, that there is now a 'publish' check box. The administrator uses this to put a comment live after it has been edited and approved.

Edit Comment Details

Just a few words about the ObjectDataSource control here. This is an amazingly helpful control which we can use to create a declarative link between our front-end Web controls and our data access methods. Note that our data access class must have a default, parameterless constructor and none of the select or update methods can be static. This is just another reason why I prefer to inject a business layer between the front and back-ends. It gives us a nice comfort zone for future code customizations in the shape of new business rules and the like. Plus, we get to use a more friendly syntax. Ideally, each record should be a custom object but that's another story!! Bring on the new Entity Framework :-)

Be sure to check out the brand new security video tutorials from Scott Mitchell.

Download Code:

GuestBook.zip (99.61 kb)

GuestBook-DB.zip (1.01 kb)

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