How do I import and call unmanaged C dll with ANSI C string “char *” pointer string from VB.NET?

Question

I have written my own function, which in C would be declared like this, using standard Win32 calling conventions:

int Thing( char * command, char * buffer, i         


        

Answer 1

You cannot convert a StringBuilder instance to a string instance, instead, use the 'ToString' method to convert it back to the string type...here's the portion of the code in the dllCall function...

retCode = Thing(Command, Buffer, bufsz)
Results = Buffer.ToString();

Answer 2

It depends on how you use the buffer argument in your C code. If you only pass a string from your VB.NET code to your C code then declaring it ByVal String is good enough. If however you let the C code return a string in the buffer then you have to declare it ByVal StringBuilder and initialize it properly before the call. For example:

Public Class dllInvoker
    Declare Ansi Function Thing Lib "Thing1.dll" (ByVal Command As String, ByVal Buffer As StringBuilder, ByRef BufferLength As Integer) As Integer

    Shared Function dllCall(ByVal Command As String, ByRef Results As String) As Integer
        Dim Buffer As StringBuilder = New StringBuilder(65536)
        Dim Length As Integer = Buffer.Capacity
        Dim retCode As Integer = Thing(Command, Buffer, Length)
        Results = Buffer.ToString()
        Debug.Assert(Results.Length = Length)
        Return retCode
    End Function
End Class

Note the ambiguity in the returned Length value.