Creating a class library component (DLL) using Visual Studio .NET is pretty simple. In this tutorial, I will show you how to create a class library component and use it from a client application.

I have divided this tutorial in two parts. 

• Building a server class library.
• Building a consumer client application.
• Building a Server Class Library.

In this section, I will create a Math server class library with a couple of methods and properties in it. 

Create an Empty Class Library Project

Select File->New->Project->Visual Basic Projects->Class Library.

Select your project name and appropriate directory using Browse button and click OK. I give my component name "VBNetMathComponent" as you can see from Figure 1.



Figure 1.

Project and Its files

The Solution Explorer adds two Visual Basic classes to your project. First is AssemblyInfo.vb and second is Class1.vb.  We don't care about AssemblyInfo class. We will be concentrating on Class1.vb. See Figure 2.



Figure 2.

If you double click on Class1.vb, you will see the class declaration as following:

Public Class Class1
End Class

Here you can change your class name to whatever you want. We change our class name to McMathClass. Now my new class looks like the following:

Public Class McMathClass
End Class

Adding Methods and Properties to the Class

Now we will add one method and one property to the class. We define a public function called Add, which takes two input parameters and returns the sum of two. We also add a property called AllowAddition of type Boolean. The Add method works only when this property is true.

The source code for Add method and AllowAddition property is listed as following:

Public Class McMathClass
' Define a local variable to store the property value.
Private allowSum As Boolean
' Define Add function which adds two double values and retunes the
' sum of two
Public Function Add(ByVal val1 As Double, ByVal val2 As Double) As Double
If allowSum = True Then
' Return the sum of two input values
Return val1 + val2
Else
Return 0
End If
End Function
' Define AllowAddition property
Public Property AllowAddition() As String
Get
Return allowSum
End Get
Set(ByVal Value As String)
allowSum = Value
End Set
End Property
End Class

Build the DLL

Now build the DLL and see bin\debug directory of your project. You will see VBNetMathComponent.dll in this folder. This is your component.

Building a Class Library Consumer Client Application

Now we will create a client application that will call our class library and its members. We will create a console application that will consume our class library.

Create a Console Application

Again, we create the console application using Visual Studio .NET.

Select File->New->Project->Visual Basic Projects->Console Application.

I give my console application name MathLibraryConsumer as you can see from the Figure 3.



Figure 3.

Add Reference to the Library

Now next step is to add reference to the library. You can use Add Reference menu option to add a reference. Go to Project->Add reference and browse for VBNetMathComponent.dll as you can see from Figure 4.



Figure 4.

After completing this step, you will notice the component reference is added to your project references as you can see from Figure 5.



Figure 5.

Calling the Component

Now we create an instance of McMathClass, set its property and call Add method. As you can see from the following code, first I set AllowAddition property to false and call the Add method and send the output to the console. After that I set AllowAddition proeprty to true, call the Add method again, and send the output to the cosnole.

Module Module1
Sub Main()
' Define a private variable
Dim total As Double
' Create an instance of McMathClass
Dim myComp As New VBNetMathComponent.McMathClass()
' Set AllowAddition property to false
myComp.AllowAddition = False
Console.WriteLine("Adding two numbers with AllowAddition False:")
' Add two numbers
total = myComp.Add(3, 5)
' Send output to console
Console.WriteLine(total.ToString())
' Now set AllowAddition to true
myComp.AllowAddition = True
Console.WriteLine("Adding two numbers with AllowAddition True:")
' Add two numbers
total = myComp.Add(3, 5)
' Send output to console
Console.WriteLine(total.ToString())
Console.ReadLine()
End Sub
End Module

Now build and run the project. The output looks like Figure 6. As you can see from this output, the Add method returns 0 when the AllowAddition property is false, and returns the sum when the AllowAddition property is true.



Figure 6.

Summary

In this simple tutorial, you learned how to write a class library using VB.NET and consume it from a consumer client application. Using the same methodology, you can implement your component and extend the functionality.