Calling Native Code from VB Managed Code

In this activity, the VB project from 17.3 will be modified to call the TimDLL.dll as developed in the Native Code chapter to get the local time.

What is required:

• As per C# version (17.5 Calling Native Code from C# Managed Code) except (apart from VB-C# normal syntax differences):

  • In the  internal static class  PInvoke

    • The signature is:
      

          < DllImport("TimeDLL.dll") > _
          Public Shared Sub GetTime(ByRef timeStr As StringBuilder)
          End Sub

• Also need Import reference: Imports System.Runtime.InteropServices in both of the PInvoke class code.

Implementation

1. Again use the DLL from 14.8. This time start with the VB application from 17.3 
   
   Next add a class which will encapsulate the native code in the VB application:
   
2. Add a class to the application, call it PInvoke
   App project context (Right-click on the app in Solution Explorer)—>Add Class
   Code it as follows:
   

   Imports System.Runtime.InteropServices
   Imports System.Text
   Public Class PInvoke
   End Class

3. Add the following to the PInvoke class:
   

       < DllImport("TimeDLL.dll") > _
       Public Shared Sub GetTime(ByRef timeStr As StringBuilder)
       End Sub

   Note the line extender at the end of the first line.

4. Add the following reference at the top of the app code:
   

   Imports System.Text

5. Modify the TimeButton event handler to:
   

   'Create a StringBuilder object with capacity for 100 characters  
   Dim  timeStr as StrinlgBuilder = new StringBuilder(100)
   'Call the native code 
   PInvoke.GetTime(timeStr)
   'Display the string 
   textBox1.Text = timeStr.ToString( )
   

6. Build and test run the application.
   
   A runtime error will be reported that the GetTime entry point couldn't be found in the DLL, if exercise 17.5 hasn't been completed. Refer to 17.5 Step 7 to resolve this. The modified app should now function correctly. Again, the VB app did not need recompilation.

   
   Figure 17.x The Managed Code VB app accessing the native code to get the local time string

NEXT:  17.7 C# Managed Code DLL

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