When a property is used as an argument to a method, VB6 passes the property’s value using by-value semantics, regardless of whether the receiving parameter is declared with the ByRef keyword. Under the same circumstances, however, VB.NET passes the property’s value using by-reference semantics. This detail can cause a runtime exception when the property is one of those exposed by the Connection, Recordset, and other ADODB objects. Consider the following VB6 code:

    Sub Test(fld As ADODB.Field)
        ByRefProc fld.DefinedSize
    End Sub

    Sub ByRefProc(n As Short)
        ' any code here
    End Sub

Even if the ByRefProc method doesn’t modify the incoming value, when the method exits VB.NET attempt to assign the (unmodified) value back to the DefinedSize property of the ADODB.Field object. This attempt cases a COMException error with the following message:

    Operation is not allowed when the object is open

This problem occurs only because a few ADODB properties become nonwritable when the Connection or Recordset object is open. It never occurs if the property is always read-only, because in that case VB.NET doesn’t attempt the assignment on exiting the method.

The obvious fix for this problem is using ByVal in the definition of the method’s parameter, if possible. If it isn’t possible, at least ensure that the VB6 code passes a copy of the property’s value:

    Sub Test(fld As ADODB.Field)
        ByRefProc (fld.DefinedSize)     ' use parenthesis to force by-value semantics
    End Sub