User Defined Type (UDT) as parameter in public Sub in class module (VB6)

Just define the sub as Friend scope. This compiles fine for me in a VB6 class.

Private Type testtype  x As StringEnd TypeFriend Sub testmethod(y As testtype)End Sub

From your error messages it appears your class is private. If you do want your class to be public - i.e. you are making an ActiveX exe or DLL and you want clients to be able to access the sub - then just make both the type and the sub Public.

So is there any way to have a public UDT used as a parameter in a public sub in a class?

In a word, no. The closest you can come with just Classic VB code would be to create a class that replicates the UDT and use that instead. There are definitely advantages there, but you're hosed if you need to pass that to, say, an API as well.

Another option is to define the UDT in a typelib. If you do that, it can be used as a parameter for a public method.

Ok, here's how to do it, if I can get my cat to leave me alone, that is.

In Form1 (with one command button on it):

Option Explicit'Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (ByVal dst As Long, ByVal src As Long, ByVal nBytes As Long)'Private Sub Command1_Click()' Okay, this is what won't work in VB6:'     Dim MyUdt1 As MyUdtType   ' Declare a variable with a publicly defined UDT (no problem).'     Form2.Show                ' We could have created some object with a class.  This was just easier for the demo.'           INSIDE OF FORM2:'               Public Sub MySub(MyUdt2 As MyUdtType)   ' It won't even let you compile this.'                   Msgbox MyUdt2.l'                   MyUdt2.l = 5'               End Sub'     Form2.MySub MyUdt1                                ' You'll never get this far.'     Unload Form2'     Msgbox MyUdt1.l'' The following is a way to get it done:'Dim MyUdt1 As MyUdtType         ' Declare a variable with a publicly defined UDT (no problem).Dim ReturnUdtPtr As Long        ' Declare a variable for a return pointer.MyUdt1.l = 3                    ' Give the variable of our UDT some value.Form2.Show                      ' Create our other object.'' Now we're ready to call our procedure in the object.' This is all we really wanted to do all along.' Notice that the VarPtr of the UDT is passed and not the actual UDT.' This allows us to circumvent the no passing of UDTs to objects.ReturnUdtPtr = Form2.MyFunction(VarPtr(MyUdt1))'' If we don't want anything back, we could have just used a SUB procedure.' However, I wanted to give an example of how to go both directions.' All of this would be exactly the same even if we had started out in a module (BAS).CopyMemory VarPtr(MyUdt1), ReturnUdtPtr, Len(MyUdt1)'' We can now kill our other object (Unload Form2).' We probably shouldn't kill it until we've copied our UDT data' because the lifetime of our UDT will be technically ended when we do.Unload Form2                    ' Kill the other object.  We're done with it.MsgBox MyUdt1.l                 ' Make sure we got the UDT data back.End Sub

In form2 (no controls needed). (This could have just as easily been an object created with a class.):

    Option Explicit'Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (ByVal dst As Long, ByVal src As Long, ByVal nBytes As Long)'Public Function MyFunction(ArgUdtPtr As Long) As Long' Ok, this is how we get it done.' There are a couple of things to notice right off the bat.' First, the POINTER to the UDT is passed (using VarPtr) rather than the actual UDT.' This way, we can circumvent the restriction of UDT not passed into objects.' Second, the following MyUdt2 is declared as STATIC.' This second point is important because the lifetime of MyUdt2 technically ends' when we return from this function if it is just DIMmed.' If we want to pass changes back to our caller, we will want to have a slightly longer lifetime.Static MyUdt2 As MyUdtType' Ok, we're here, so now we move the argument's UDT's data into our local UDT.CopyMemory VarPtr(MyUdt2), ArgUdtPtr, Len(MyUdt2)' Let's see if we got it.MsgBox MyUdt2.l' Now we might want to change it, and then pass back our changes.MyUdt2.l = 5' Once again, we pass back the pointer, because we can't get the actual UDT back.' This is where the MyUdt2 being declared as Static becomes important.MyFunction = VarPtr(MyUdt2)End Function

And Finally, this goes in a module (BAS) file.

    Option Explicit'' This is just the UDT that is used for the example.Public Type MyUdtType    l As LongEnd Type'