Sunday, December 26, 2010

Encapsulation: Local change - Local effect principle

One of the central principles of object oriented programming is Encapsulation. Encapsulation states that the implementation details of an object are hidden behind the methods that provide access to that data. But why is encapsulation a good idea? Why bother to do it in the first place? Just stating that it's "good OO design" isn't sufficient justification.

There is one primary justification of encapsulation. It's a principle I call "Local Change - Local Effect". If you change code in one spot, it should only require changes in a small neighborhood surrounding the original change. When used properly, encapsulation allows software to change gradually without requiring bulk changes throughout the system (Change of code in one place requires code change in many places is known as Domino effect).

Encapsulation helps follow this principle by allowing changes in the representation of an object's state. The methods for the object may be affected, but callers of those methods shouldn't be. The effects of the change are localized.

Polymorphism helps by allowing us to add new objects without changing existing code to know about them. You only need to add the new classes and new methods. You shouldn't need to change existing code.

Inheritance helps by providing one place to put common code for many similar objects. Changes to this code can be isolated to the superclass and may require no changes to subclasses in order to make them work.

There are many coding practices that tend to work against the local change/local effect principle. They include:
  • Copy and Paste Code- by making more copies of code, you have more things that need to be changed for any change in design.
  • Public instance variables - by making instance variables public, more people can use them directly and require more changes if you need to change the representation.
  • Manifest types - the type information for variables and parameters often causes domino effect changes. When you change the type that a method accepts, you may have to change its callers and their callers and so forth.
In any software system, the one thing you can count on is change. The local change/local effect principle makes change possible. Without it, as a system gets larger, it becomes more brittle and eventually becomes unmaintainable.
Think about your design principles. If they don't support local change/local effect, you may be building a system that will become too brittle to ever change again.


Monday, October 11, 2010

List of processes running on Remote/Local Computer using C#

ArrayList alist = new ArrayList();

// From remote machine
Process[] processes = Process.GetProcesses("RemoteComputerName"); 

// From local machine
Process[] processes = Process.GetProcesses();    

foreach (Process process in processes)

Friday, October 8, 2010

Limitations of COM Interop

Following is the list of some shortcomings:

  • Static/shared members: COM objects are fundamentally different from .Net types. One of the differences is lack of support for static/shared members.
  • Parameterized Constructors: COM types don't allow parameters to be passed into a constructor. This limits the control you have over initialization and the use of overloaded constructors.
  • Inheritance: One of the biggest issues is the limitations COM objects place on the inheritance chain. Members that shadow members in a base class aren't recognizable, and therefore, aren't callable or usable in real sense.
  • Portability: Operating Systems other than Windows don't have registry. Reliance on Windows registry limits the number of environments a .Net application can be ported to.

Why Visual Studio hangs

Every once in a while, VS seems to take forever to display a screen to the point that it seems to hang. Most of the time, it hangs, while accessing Fonts and Colors page in Tools/Options dialog. The issue is not that there is some weird code that executes very slowly. It happens that this page is implemented using .NET components. Now the majority of VS is built with native code and during most of its execution,, the CLR is never loaded. However, when the user accesses one of these features, the CLR must be loaded, before we can begin executing the relevant IL. It is this process that is time-consuming and annoying to the user. There are two problems for the users here: first, there is no feedback during loading of the CLR; second: the problem can occur multiple times within a single session of VS.

I am trying to figure out the reason for this second issue. Let me know, if any of you knows.

Optional Parameter issue with COM and C#/VB

As we all know, C# doesn't support optional parameters(till framework 3.5) whereas VB does.In the same way, COM components don't support parameter overloading, so for each value in a parameter list, we've got to pass in something, even if it does nothing. Moreover, COM parameters are always passes by reference, which means we can't pass NULL as a value.

In VB 2005, this is not really as issue because it supports optional parameters and we can just leave them out. But C# doesn't support this, so one have to create object variables and pass them in.

See following code sample:
using Microsoft.Office.Core;
using Microsoft.Office.Interop.Excel;  // Must have office installed
Application NewExcelApp = new Application;
NewExcelApp.Worksheets.Add();       // This will not compile

So, as a workaround, the Type.Missing field can be used and this field can be passed in with the C# code and the application will work as expected. 

Check it in below code snippet:

using Microsoft.Office.Core;
using Microsoft.Office.Interop.Excel;  // Must have office installed
private Object OptionalParamHandler = Type.Missing;
Application NewExcelApp = new Application;
NewExcelApp.Worksheets.Add(OptionalParamHandler ,OptionalParamHandler ,OptionalParamHandler ,OptionalParamHandler ); 

This approach allows your code to work in C# :)

Wednesday, September 15, 2010

Making assembly visible to a COM component

Following steps are necessary to make an assembly visible to a COM component:

  • Set the Register for COM option under the build configuration
  • Set the ComVisible attribute to true for each class you want exposed
  • Set the ComVisible attribute to false for any class members you want hidden
  • Set the ComVisible attribute to true for any class members that you want visible

Friday, September 10, 2010

Which is fastest While, Do-While, Foreach

Foreach will be faster as it usually maintain no explicit counter, like while and do-while. Foreach essentially say "do this to everything in this set", rather than "do this x times". This will potentially avoid off-by-one errors and make code simpler to read.

Using frequently build assembly in another applications (.Net)

Assume that you are creating a strong named MyAssembly that will be used in several applications. The assembly will be rebuilt frequently during the development cycle. But one must ensure that every time MyAssembly is rebuilt, it works properly with all applications that use it. So, in order to obtain this, we are required to configure the computer on which we develop the assembly such that each application uses the latest build of MyAssembly. To accomplish the above task, take the following action:

  • To point to the build output directory for the strong named assembly, create a DEVPATH environment variable
  • Add the following element to the machine configuration file: <developmentMode developerInstallation="true">. This element tells the CLR to use DEVPATH to locate assembly.

What is a serviced component?

A services component is a .Net component that uses component services of COM+, such as object pooling, transaction management etc. It executes within the managed execution environment of the .Net framework and shares its context with a COM+ application. It enables context sharing between COM+ and .Net framework classes. A serviced component is creates by defining  class that directly or indirectly derives from the ServicesComponent class. It utilizes COM+ services by using attributes of the System.EnterpriseServices namespace. A serviced component should be registered before it can access the component services of COM+. The following three types of registrations are used to register a serviced component:

  • Manual registration: The .Net Framework Service Installation (Regsvcs.exe) tool is used to manually register an assembly containing a serviced component. Manual registration is used for design-time testing to find out the error types that may occur at runtime.
  • Dynamic registration: In dynamic registration, as assembly with a serviced component is copied into the COM+ application's directory. Dynamic registration is used only when a serviced component is created by a managed client.
  • Programmatic registration: Programmatic registration is used to register as assembly containing a serviced component programmatically by creating as instance of the RegistrationHelper class.

Tuesday, September 7, 2010

Covariance and Contravariance feature of .NET 4.0

I'll consider below class hierarchy throughout this explanation:

class Account { }
class Savings : Account { }
class Current : Account { }
Now lets begin with Example 1:
class Program
delegate T MyDel<T>();
static void Main(string[] args)
MyDel<Savings> objSavings = () => new Savings();
MyDel<Account> objAccount = objSavings;
The code shown in Example 1 will not work in versions prior to 4.0. But if you change generic parameter T to OUT T in 4.0, then this assignment compatibility work via CoVariance. Now check this 4.0 code in Example 2:
class Program
delegate T MyDel<out T>();
static void Main(string[] args)
MyDel<Savings> objSavings = () => new Savings();
MyDel<Account> objAccount = objSavings;
The only change in Example 2 is additional OUT keyword.
Now lets move to ContraVariance:
ContraVariance supports the opposite direction of assignment with IN keyword as shown in Example 3:
class Program
delegate T MyDel<in T>(T t);
static void Main(string[] args)
MyDel<Account> objAccount = (account) => Console.WriteLine(account); MyDel<Savings> objSavings = objAccount;

These variance come into picture with Generics, Delegates, Interfaces and Collections.