How to return an array from a Visual C++ DLL or OLE server by using the ColeSafeArray class

MORE INFORMATION

You can receive, manipulate, and return Microsoft Visual Basic arrays in Microsoft Visual C++ with an understanding of safe arrays. A safe array is an array that contains information about the number of dimensions and the bounds of its dimensions. Because Microsoft Visual Basic natively uses these types of arrays, you have to construct safe arrays of the data you want to pass.

Normally, this is a difficult task, and hard to manage from Microsoft Visual C++. However, this task is easier if you use the ColeSafeArray class included with the Microsoft Foundation Classes. The following DLL routine demonstrates how to use this class to return a 10x10 two-dimensional array of doubles from a DLL.

NOTE: The same code and principles in this sample DLL also apply to an OLE server.

Steps to Create the Visual C++ DLL

1. Click New on the File menu and choose MFC AppWizard (dll).
2. Name the project "MyDll."
3. Add the following DLL routine to your main .cpp file (MyDll.cpp):

         VARIANT _stdcall retVariantArray(void) {
            COleSafeArray saRet;
            DWORD numElements[] = {10, 10}; // 10x10
   
            // Create the safe-array...
            saRet.Create(VT_R8, 2, numElements);
   
            // Initialize it with values...
            long index[2];
            for(index[0]=0; index[0]<10; index[0]++) {
               for(index[1]=0; index[1]<10; index[1]++) {
                  double val = index[0] + index[1]*10;
                  saRet.PutElement(index, &val);
               }
            }
   
            // Return the safe-array encapsulated in a VARIANT...
            return saRet.Detach();
         }
   					

4. Add the following line to the end of your .def file (MyDll.def):

         retVariantArray
   					

5. Build the DLL.

Steps to Create the Visual Basic Program That Uses the DLL

1. Start Visual Basic 5.0, and then start a new Standard EXE project.
2. Add a Command button to Form1.
3. Add the following code to Form1:

         Private Declare Function retVariantArray Lib "c:\mydll.dll" _
            () As Variant
   
         Private Sub Command1_Click()
            Dim myarr As Variant
   
            myarr = retVariantArray
   
            MsgBox "myArr(1,1) = " & myarr(1, 1)
         End Sub
   					

4. Run the program and click Command1. A message box displays the text "myArray(1,1) = 11."

Other Notes

Run-time error 5 "Invalid Procedure Call" or run-time error 11 "Divide by zero" may occur when performing this task.

Error 5 often occurs when calling into an OLE server whose method was not properly created. To correct this error, re-create this method using ClassWizard, choose the appropriate options, and then cut and paste the code into your new function.

If Error 11 occurs while attempting to access elements in your array, it may indicate that you did not construct the safe array correctly. It might not have been completely initialized or it could have been corrupted from logical errors in your code. To resolve this error, verify that you are not indexing the array past its bounds, or overwriting memory.