C# 2008 Programmer's Reference

system as a whole. However, that really isn't feasible, because you have to wait for everyone to submit his or her source code. A much better way is to get each programmer to build his or her part of the project as a standalone library (DLL). You can then get the latest version of each library and test the application as a whole. This approach has an added benefit — when a deployed application needs updating, you only need to update the particular library that needs updating. This is extremely useful if the project is large. In addition, organizing your project into multiple assemblies ensures that only the needed libraries (DLLs) are loaded during runtime.

To see the benefit of creating multi-file assemblies, let's create a new Class Library project, using Visual Studio 2008, and name it MathUtil. In the default Class1.cs, populate it with the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace MathUtil {
 public class Utils {
  public int Fibonacci(int num) {
   if (num <= 1) return 2; //---should return 1; error on purpose---
   return Fibonacci(num - 1) + Fibonacci(num - 2);
  }
 }
}

This Utils class contains a method called Fibonacci(), which returns the n^th number in the Fibonacci sequence (note that I have purposely injected an error into the code so that I can later show you how the application can be easily updated by replacing the DLL). Figure 15-5 shows the first 20 numbers in the correct Fibonacci sequence.

Figure 15-5

Build the Class Library project (right-click on the project's name in Solution Explorer, and select Build) so that it will compile into a DLL — MathUtil.dll.

Add a Windows Application project to the current solution, and name it WindowsApp-Util. This application will use the Fibonacci() method defined in MathUtil.dll. Because the MathUtil.dll assembly is created in the same solution as the Windows project, you can find it in the Projects tab of the Add Reference dialog (see Figure 15-6). Select the assembly, and click OK.

Figure 15-6

The MathUtil.dll assembly will now be added to the project. Observe that the Copy Local property for the MathUtil.dll assembly is set to True (see Figure 15 -7). This means that a copy of the assembly will be placed in the project's output directory (that is, the binDebug folder).

Figure 15-7

When you add a reference to one of the classes in the .NET class library, the Copy Local property for the added assembly will be set to False. That's because the .NET assembly is in the Global Assembly Cache (GAC), and all computers with the .NET Framework installed have the GAC. The GAC is discussed later in this chapter.

Switch to the code-behind of the default Form1 and code the following statements:

namespace WindowsApp_Util {
 public partial class Form1 : Form {
  public Form1() {
   InitializeComponent();
  }
  private void Form1_Load(object sender, EventArgs e) {
   CallUtil();
  }
  private void CallUtil() {
   MathUtil.Utils util = new MathUtil.Utils();
   MessageBox.Show(util.Fibonacci(7).ToString());
  }
 }
}

Set a breakpoint at the CallMathUtil() method (see Figure 15-8).

Figure 15-8 

Right-click on the WindowsApp-Util project name in Solution Explorer, and select Start as Startup Project. Press F5 to debug the application. When the application stops at the breakpoint, view the modules loaded into memory by selecting Debug→Windows→Modules (see Figure 15-9).

Figure 15-9

Observe that MathUtil.dll library has not been loaded yet. Press F11 to step into the CallMathUtil() function (see Figure 15-10). The MathUtil.dll library is now loaded into memory.

Figure 15-10

Press F5 to continue the execution. You should see a message box displaying the value 42. In the bin Debug folder of the Windows application project, you will find the EXE assembly as well as the DLL assembly (see Figure 15-11).

Figure 15-11

Updating the DLL

The Fibonacci() method defined in the MathUtil project contains a bug. When num is less than or equal to 1, the method should return 1 and not 2. In the real world, the application and the DLL may already been deployed to the end user's computer. To fix this bug, you simply need to modify the Utils class, recompile it, and then update the user's computer with the new DLL:

namespace MathUtil {
 public class Utils {
  public int Fibonacci(int num) {
   if (num <= 1) return 1; //---fixed!---
   return Fibonacci(num - 1) + Fibonacci(num - 2);
  }
 }
}

Copy the recompiled MathUtil.dll from the binDebug folder of the MathUtil project, and overwrite the original MathUtil.dll located in the binDebug folder of the Windows project. When the application runs again, it will display the correct value, 21 (previously it displayed 42).