Call .Net C# DLL method from Classic ASP

Question

I'm working on a classic asp project that needs to send a string to a DLL who will serialize it and send to a Zebra Thermal Printer.

I have already build my DLL and registered it using regasm followed by /codebase which make IIS recognizes it.

Althougt I'm able to set my object with Server.CreateObject("MyDLL") without errors, I'm having trouble trying to access the C# methods in it.

The C# code for the DLL was written as below:

using System;
using System.Runtime.InteropServices;
using System.IO;

//[assembly: ComVisible(true)]
//[assembly: Guid("6c87161d-1e02-40ef-8512-82f30bc1ae3e")]
namespace PrintZebra
{
    //[ClassInterface(ClassInterfaceType.None)]
    /// <summary>
    /// Classe 
    /// </summary>
    public class RawPrinterHelper

    {
        // Structure and API declarions:
        /// <summary>
        /// 
        /// </summary>
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
        public class DOCINFOA
        {
            [MarshalAs(UnmanagedType.LPStr)] public string pDocName;
            [MarshalAs(UnmanagedType.LPStr)] public string pOutputFile;
            [MarshalAs(UnmanagedType.LPStr)] public string pDataType;
        }
        [DllImport("winspool.Drv", EntryPoint = "OpenPrinterA", SetLastError = true, CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool OpenPrinter([MarshalAs(UnmanagedType.LPStr)] string szPrinter, out IntPtr hPrinter, IntPtr pd);

        [DllImport("winspool.Drv", EntryPoint = "ClosePrinter", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool ClosePrinter(IntPtr hPrinter);

        [DllImport("winspool.Drv", EntryPoint = "StartDocPrinterA", SetLastError = true, CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool StartDocPrinter(IntPtr hPrinter, Int32 level, [In, MarshalAs(UnmanagedType.LPStruct)] DOCINFOA di);

        [DllImport("winspool.Drv", EntryPoint = "EndDocPrinter", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool EndDocPrinter(IntPtr hPrinter);

        [DllImport("winspool.Drv", EntryPoint = "StartPagePrinter", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool StartPagePrinter(IntPtr hPrinter);

        [DllImport("winspool.Drv", EntryPoint = "EndPagePrinter", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool EndPagePrinter(IntPtr hPrinter);

        [DllImport("winspool.Drv", EntryPoint = "WritePrinter", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
        public static extern bool WritePrinter(IntPtr hPrinter, IntPtr pBytes, Int32 dwCount, out Int32 dwWritten);

        // SendBytesToPrinter()
        // When the function is given a printer name and an unmanaged array
        // of bytes, the function sends those bytes to the print queue.
        // Returns true on success, false on failure.
        public static bool SendBytesToPrinter(string szPrinterName, IntPtr pBytes, Int32 dwCount)
        {
            Int32 dwError = 0, dwWritten = 0;
            IntPtr hPrinter = new IntPtr(0);
            DOCINFOA di = new DOCINFOA();
            bool bSuccess = false; // Assume failure unless you specifically succeed.

            di.pDocName = ".NET RAW Document";
            di.pDataType = "RAW";

            // Open the printer.
            if (OpenPrinter(szPrinterName.Normalize(), out hPrinter, IntPtr.Zero))
            {
                // Start a document.
                if (StartDocPrinter(hPrinter, 1, di))
                {
                    // Start a page.
                    if (StartPagePrinter(hPrinter))
                    {
                        // Write your bytes.
                        bSuccess = WritePrinter(hPrinter, pBytes, dwCount, out dwWritten);
                        EndPagePrinter(hPrinter);
                    }
                    EndDocPrinter(hPrinter);
                }
                ClosePrinter(hPrinter);
            }
            // If you did not succeed, GetLastError may give more information
            // about why not.
            if (bSuccess == false)
            {
                dwError = Marshal.GetLastWin32Error();
            }
            return bSuccess;
        }

        public static bool SendFileToPrinter(string szPrinterName, string szFileName)
        {
            // Open the file.
            FileStream fs = new FileStream(szFileName, FileMode.Open);
            // Create a BinaryReader on the file.
            BinaryReader br = new BinaryReader(fs);
            // Dim an array of bytes big enough to hold the file's contents.
            Byte[] bytes = new Byte[fs.Length];
            bool bSuccess = false;
            // Your unmanaged pointer.
            IntPtr pUnmanagedBytes = new IntPtr(0);
            int nLength;

            nLength = Convert.ToInt32(fs.Length);
            // Read the contents of the file into the array.
            bytes = br.ReadBytes(nLength);
            // Allocate some unmanaged memory for those bytes.
            pUnmanagedBytes = Marshal.AllocCoTaskMem(nLength);
            // Copy the managed byte array into the unmanaged array.
            Marshal.Copy(bytes, 0, pUnmanagedBytes, nLength);
            // Send the unmanaged bytes to the printer.
            bSuccess = SendBytesToPrinter(szPrinterName, pUnmanagedBytes, nLength);
            // Free the unmanaged memory that you allocated earlier.
            Marshal.FreeCoTaskMem(pUnmanagedBytes);
            return bSuccess;
        }

        public static Int32 SendStringToPrinter(string szPrinterName, string szString)
        {
            IntPtr pBytes;
            Int32 dwCount;
            // How many characters are in the string?
            //dwCount = szString.Length;joao
            dwCount = (szString.Length + 1) * Marshal.SystemMaxDBCSCharSize;
            // Assume that the printer is expecting ANSI text, and then convert
            // the string to ANSI text.
            pBytes = Marshal.StringToCoTaskMemAnsi(szString);
            // Send the converted ANSI string to the printer.
            bool bSuccess = SendBytesToPrinter(szPrinterName, pBytes, dwCount);
            Int32 dwError = 0;
            if (bSuccess == false)
               {
                  dwError = Marshal.GetLastWin32Error();
               }
               Marshal.FreeCoTaskMem(pBytes);
            return dwError;
        }            

    }
}

In my ASP application I'm trying to access the SendStringToPrinter method as follows:

Dim objZebra    
set objZebra = Server.CreateObject("Opus127Etiquetas.RawPrinterHelper")

objZebra.SendStringToPrinter

But debugger says that objZebra does not support SendStringToPrinter property or method.

Is important to tell that all the permissions to access the DLL are granted and if I start a new VB project on Visual Studio 2015 adding my custom DLL as a reference it works like a charm.

The real question is: is it possible to do what I'm trying?

EDIT

After discussing with some members at the comments I realize that what I realy need to know is how to change the C# methods above from static to instance. As I said before, if I use this DLL on a Windows Application it works fine, but on ASP it didn't. I now know what the problem is but I don't know how to solve it.

Answer 1

After lots of research and nights awake, I realized what was wrong with my code.

First I want to thank @Lankymart and @SLaks for point me the way.

Let's go to the Code:

First I went to AssemblyInfo.cs and change the [assembly: ComVisible(false)], and before every class I wanted to be visible to COM I add [ComVisible(true)] which I find in this MSDN document: https://msdn.microsoft.com/en-us/library/ms182198.aspx

After that cames the most difficult part to me: Turn the static method into a instance method.

But, researching further and trying without give away, I changed my code from:

public static Int32 SendStringToPrinter(string szPrinterName, string szString)   
{
     ...
}

public class PrintHandler
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public Int32 SendStringToPrinter(string text, string printerName)
    {            
        return RawPrinterHelper.SendStringToPrinter(printerName, text);
    }
}

To:

[ComVisible(true)]
public Int32 SendStringToPrinter(string szPrinterName, string szString)
{
     ...
}

[ComVisible(true)]
public class PrintHandler
{
    RawPrinterHelper rph = new RawPrinterHelper();
    [MethodImpl(MethodImplOptions.NoInlining)]
    [ComVisible(true)]
    public Int32 SendStringToPrinter(string text, string printerName)
    {            
        return rph.SendStringToPrinter(printerName, text);
    }
}

After that all went well and I was able to call the method as follows:

set objZebra = Server.CreateObject("PrintZebra.RawPrinterHelper")   
objZebra.SendStringToPrinter strPrinterName, strText

That's it. Hope this could help any other with a nearly or same problem as mine.