PRB: Error When You Access an Array Field of a .NET Structure from COM (316936)

SYMPTOMS

When you access an Array field of a structure that is defined in .NET from inside a COM DLL, you may receive the following error message:

"An unhandled exception of type 'System.ArgumentException' occurred in ProjectName"

Additional information: Wrong number of arguments or invalid property assignment.

RESOLUTION

Before you invoke the Array field of the structure, assign the array to another Variant variable. The following sample resembles the modified code:

temp = Arg1.Ary
MsgBox temp(arg2, arg3)

NOTE: If you use Option Explicit, you must declare the variable temp. Refer to the "More Information" section of this article for the complete code list.

MORE INFORMATION

Steps to Reproduce the Behavior

Create a new Visual Basic 6.0 ActiveX DLL. Class1 is added by default.

Paste the following sample code in Class1:

Public Sub Goo(ByVal Arg1 As Variant, ByVal arg2 As Variant, ByVal arg3 As Variant)
    MsgBox TypeName(Arg1.Ary)        'This returns Decimal() as expected.
    MsgBox Arg1.Ary(arg2, arg3)	'This array access returns an unexpected exception.
End Sub

Compile the ActiveX DLL to register the Project1.dll.
Create a new Visual Basic .NET console application.
Add a reference to Project1.dll.

Replace the existing code in Module1.vb with the following sample code:

Public Module Module1
    Public Structure S1
        Public x As Integer
        Dim Ary(,) As Decimal
    End Structure

    Dim s As S1

    Sub Main()
        Dim Obj As Project1.Class1 = New Project1.Class1()
        ReDim s.Ary(100, 0)
        s.Ary(0, 0) = 10
        Obj.Goo(s, 0, 0)	'This call causes the exception.
    End Sub
End Module

Press F5 to compile and then run the program.