Alias in Function Declaration overloaded?

Question

I have some VB6 code that I am converting to VB.net and came across this section

Declare Function TmSendByLen Lib "tmctl.dll"  Alias "TmSendByLength"(ByVal id As Integer, ByRef msg As Any, ByVal blen As Integer) As Integer
'snip'

Function TmSendByLength(ByVal id As Integer, ByVal msg As String, ByVal blen As Integer) As Integer
 TmSendByLength = TmSendByLen(id, msg, blen)
End Function

I have not come across the Alias term before but I can guess what it does. What I am unsure of is the reasoning behind overloading the alias. If that is what is happening.

I need to create overloads for the TmSendByLen function as the 'As Any' is not supported in VB.net so I am not sure if I should just remove the alias or if I should leave it in place.

Answer 1

The Alias doesn't specify that the function is overloaded exactly, but that the name specified is really named something else in the called dll.

Since your example is a bit confusing (because of the repeated names), I'll use a slightly modified version to explain:

Declare Function TmSendByLen Lib "tmctl.dll" Alias "TmSendByLength" (ByVal id As Integer, ByRef msg As Any, ByVal blen As Integer) As Integer)

Function InternalVersion(ByVal id As Integer, ByVal msg As String, ByVal blen As Integer) As Integer
    InternalVersion = TmSendByLen(id, msg, blen)
End Function

So in this modified version, the TmSendByLength name is the one that the referenced function's entrypoint is really called in tmctl.dll. TmSendByLen is what we're referring to it as in our code, and InternalVersion is the name of the wrapper function.

I would imagine that this is so that InternalVersion can be called across modules/classes, while the TmSendByLen version was intended to be private.

To answer the second part of your question, Alias is still available in VB.NET, although As Any isn't. (You can find information about it here.) Whether you want to remove the Alias or not is completely up to you, but either way, I suspect you're going to need to use As IntPtr (or SafeHandle) instead of As Any.

Answer 2

The "Alias" keyword in VB6 is probably doing what you think it does, however, it's the function name in quotes after the keyword "alias" that is the actual function name in the DLL (i.e. TmSendByLength). The function name after the "Declare Function" part (i.e. TmSendByLen) is effectively the alias that the VB6 code will use.

As you correctly point out, VB6 will not allow the "As Any" parameter type, so from the original VB6 code you posted, the developer has declared a VB6 function which incidentally has the exact same name as the "real" function in the DLL, and altered the parameters to this function to accept only a string type for the "msg" parameter.

The VB6 code as-is isn't actually overloading any function, but rather it's wrapping the DLL function with a VB6 specific one that constrains the "msg" parameter type.

In VB.NET, since you can't specify "as any", I believe you can replace this with "as object", although that may not be very helpful since other VB.NET calling code could pass virtually anything to this parameter. What you're more likely going to want to do is to create real overloaded functions in VB.NET where the "msg" parameter differs by the type that you want to accept. This way, you can allow multiple different types, but still maintain some constraints on which types can be passed to the function.

Here's a couple of links that may well help:

VB6 "As Any" in VB.Net

PInvoke