Thanks for all the responses.

I was able to use old dll's by following both the practices mentioned above.

To Continue to use old dll's Alias is first point which i have added to make it ansi. Secondly I have used another function to convert code to unicode(2 bytes).

Example: 

Originally I had something like this as below:

Function long MyFunction(string szArgument) Library "MyDll.dll"

Updated to the below so as to use it with Migrated code.

Function long MyFunction(string szArgument) Library "MyDll.dll" Alias For "MyFunction;Ansi" progma_pack(1)

Thanks once again.

Besides what Roland suggested, you need to take the C++ source code and re-compile it using Visual Studio 2017 or newer.  We have discovered some incompatibilities when customers use really old versions of VC because we are using newer version of Visual Studio to compile the C++ code of PowerBuilder 2017 and 2019.

Starting with PB 10, all string and character variables are Unicode (2 bytes) as opposed to Ansi (1 byte).

To continue to use your older Ansi dll, you need to use the Alias clause with ;Ansi at the end. This tells PB to convert the string.

Function long MyFunction(string szArgument) Library "MyDll.dll" Alias For "MyFunction;Ansi"