Passing an array from VC++ DLL to VB

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Introduction

This article shows how to pass an array from a VC++ DLL to a VB application

The steps

• Create an ATL project named ArrayTry; Select DLL; Click Finish.
• Use ATL Object Wizard to add a simple object.
• Give the short name as myarray, click OK.
• The next step is to add a method which takes an array as well as returns it after filling it. You can always add a method using the wizard but VC6 throws an error in this case as we are using SAFEARRAY. So we will do it manually which I think is more logical.
  
• So go to your .idl file and add the following below the UUID declaration of your interface

interface Imyarray : IDispatch
{
    [id(1), helpstring("method LongArray")] 
        HRESULT LongArray([in,out] SAFEARRAY(long)* pLarr);
};

• Go to your class's .h file and add the following at the very end,

// Imyarray
public:
    STDMETHOD(LongArray)(/*[in,out]*/ SAFEARRAY** pLarr);

• Go to your class' .cpp file and add the following

STDMETHODIMP Cmyarray::LongArray(SAFEARRAY** pLarr)
{
    
    long lElements; // number of elements in the array
    long iCount;
    HRESULT lResult; // return code for OLE functions

    // pointer to the elements of the array
    long *pArrayElements; 

    long Larray[3] = {4,2,3};//existing array

    //checking if it is an one-dimensional array
    if ( (*pLarr)->cDims != 1 ) return(1);

    // checking if it is an array of longs
    if ( (*pLarr)->cbElements != 4 ) return(2);


    //Getting the number of elements of the array
    lElements = (*pLarr)->rgsabound[0].cElements;

    // locking the array before using its elements
    lResult=SafeArrayLock(*pLarr);
    if(lResult)return(lResult);

    // using the array
    pArrayElements=(long*) (*pLarr)->pvData;
    for (iCount=0; iCount<lElements; iCount++)
        pArrayElements[iCount] = Larray[iCount];

    // releasing the array
    lResult=SafeArrayUnlock(*pLarr);
    if (lResult) return(lResult);


    return S_OK;
}

• In the above function we take an array , which will be passed ( by a VB application ) & fill it with the values from an existing array.
• Press F7 to build the DLL , go to Tools and register your DLL. Your DLL is now ready for use.
• Now Create a VB application as an EXE; Add a command button, double click to add the following code

Private Sub Command1_Click()
Dim x As New TRYARRLib.larray

    Dim ArrayOfLongs(3) As Long 'Declare the array
      
    x.LongArray ArrayOfLongs  'Get the array
      
    For Counter = 0 To 2
        'print the array contents
        Debug.Print ArrayOfLongs(Counter) 
    Next
      
End Sub

• Don't forget to add a reference to your DLL ( Go to projects/references and point to your .tlb file )

That's It

Watch the results in the immediate window. Please write to me for any explanations/doubts , better way to do the same thing.

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