Passing a string arg to a DLL function

Hello,

I've been trying to pass a string reference to a DLL function (created by myself). I always get the only 4 bytes in the return string 'DataOut':


private Sub CommandRead_Click()

Dim DataOut as string
Dim BlkNo as Integer
Dim BlkLen as Integer

DataOut = 0

'Read Data
BlkNo = 0
BlkLen = 32
Status = ReadBlock(BlkNo, BlkLen, DataOut)
Text1.Text = DataOut

End Sub

public Declare Function ReadBlock Lib "TestMemCard.dll" (byval BlkNo as Integer, byval BlkLen as Integer, byval DataOut as string) as Long






During creation of "TestMemCard.dll" , the function 'ReadBlock' was defined as

extern "C" long CALLDLL WINAPI ReadBlock(BYTE BlkNo,BYTE BlkLen,char *DataOut)

...where 'BYTE' is unsigned char.

I tried both 'ByVal' and 'ByRef' for 'DataOut', and get always 4 bytes for 'ByVal' while 'RyRef' gives illegal operation.

I appreciate if anyone could help. Thanks.