Programming C#, Third Edition

Importing COM Components

Importing ActiveX controls turns out to be fairly straightforward. Many of the COM components that companies develop are not ActiveX controls, however: they are standard COM dynamic link library (DLL) files. To see how to use these with .NET, return to VB6 and create a COM business object that will act exactly as the component from the previous section did.

The first step is to create a new ActiveX DLL project. This is how VB6 creates standard COM DLLs. Name the class ComCalc and name the project ComCalculator. Save the file and project. Copy the methods from Example 22-4 into the code window.

Example 22-4. Implementing the methods for ComCalc

Public Function _
Add(left As Double, right As Double) _
As Double
    Add = left + right
End Function

Public Function _
Subtract(left As Double, right As Double) _
As Double
    Subtract = left - right
End Function

Public Function _
Multiply(left As Double, right As Double) _
As Double
    Multiply = left * right
End Function

Public Function _
Divide(left As Double, right As Double) _
As Double
    Divide = left / right
End Function

Build the DLL by using the menu sequence File, Make ComCalculator.dll. You can test this by returning to your earlier test program and removing the Calculator control from the form. Add the new DLL by opening the project reference window and navigating to the ComCalculator, as shown in Figure 22-11.

Figure 22-11. Adding ...