VB6 and .NET Standard Class Library Interop

This sample shows how to call methods in a .NET Standard 2.0 class library from a Visual Basic 6 application. You can also use Full .NET Framework (4.7.2 or other versions) for your class libraries and it will make no difference.

Step 1 - Create .NET Standard Class Library

Create an interface for the methods you need to use from VB6 and make it visible to COM

[Guid("45f838c9-d8cd-4198-b2b3-f4e8c2a5b588")]
[ComVisible(true)]
public interface IMathUtils
{
    double CalculateComplexMethod();
    void CommandWithParameters(int number1, int number2);
}

Guid must be unique for this interface

Implement the interface in a class and also make it visible to COM. Don't use the Guid as the interface

[Guid("293bcd3a-e771-45d5-8a53-5413997c2de8")]
[ComVisible(true)]
[ProgId("NetStandardLibrary.MathUtils")]
public class MathUtils : IMathUtils
{
    // For COM interop we need parameterless constructor
    public MathUtils() { }

    public double CalculateComplexMethod() => 2 * 2;

    public void CommandWithParameters(int number1, int number2)
    {
        // Do something in here like saving to a JSON file, POST to some REST API or anything
        // without needing to code in prehistoric VB6
    }
}

Compile library, generate tlb from assembly and register tlb and dll in development. Use Developer Command Prompt for Visual Studio as Administrator

regasm.exe /tlb /codebase NetStandardLibrary.dll

Microsoft .NET Framework Assembly Registration Utility version 4.7.3190.0 for Microsoft .NET Framework version 4.7.3190.0 Copyright (C) Microsoft Corporation. All rights reserved.

RegAsm : warning RA0000 : Registering an unsigned assembly with /codebase can cause your assembly to interfere with other applications that may be installed on the same computer. The /codebase switch is intended to be used only with signed assemblies. Please give your assembly a strong name and re-register it. Types registered successfully Assembly exported to '[Path]\NetStandardLibrary\bin\Release\netstandard2.0\NetStandardLibrary.tlb', and the type library was registered successfully

Step 2 - Add reference to library tlb from VB6

Open your VB6 application and browse NetStandardLibrary.tlb from Project -> References

This will add the reference to project .vbp file
Reference=*\G{A1A932E1-98D8-38DE-81F5-6B67BC4AF161}#1.0#0#..\NetStandardLibrary\bin\Release\netstandard2.0\NetStandardLibrary.tlb#NetStandardLibrary

Step 3 - Run methods from library

Dim oMathMethods As IMathUtils
Set oMathMethods = New NetStandardLibrary.MathUtils

MsgBox ("Result from complex method is: " & oMathMethods.CalculateComplexMethod())

Step 4 - Success

You can now extend your old VB6 application and keep your customer's and yourself happy writing as little as possible code in VB6 and avoiding Visual Studio 6 IDE

Step 5 - Deployment in client computers

Add the NetStandard dll file of your library to your installer
No need to deploy tlb file
This dll will not register with regsvr32.exe your will need to use regasm.exe. Use 32 or 64bit version depending on your needs.
Register with /codebase argument
Unregister with /unregister argument
To locate regasm.exe in client computer you can find the related registry keys in HKLM\Software\Microsoft.NetFramework. For example: C:\Windows\Microsoft.NET\Framework64\v4.0.30319 for 64bits or C:\Windows\Microsoft.NET\Framework\v4.0.30319 for 32bits

Extras

VB6 in Github

In order to use Github for your VB6 source code add this extensions to your Visual Studio .gitignore file. Like in this project .gitignore file for VB6

*.csi
*.exp
*.lib
*.lvw
*.dca

VB6 running on Windows 10 x64bits

Here are a few links to help you setup VB6 in Windows 10.

http://blog.danbrust.net/2015/09/14/installing-visual-basic-studio-6-on-windows-10

https://blogs.msdn.microsoft.com/luisdem/2018/12/03/how-to-install-visual-basic-6-0-on-windows-10/

VB6 deployment in client computers using NSIS

If you use NSIS for your deployment this macros and functions will help you.

; Given a .NET version number, this function returns that .NET framework's
; install directory. Returns "" if the given .NET version is not installed.
; Params: [version] (eg. "v2.0")
; Return: [dir] (eg. "C:\WINNT\Microsoft.NET\Framework\v2.0.50727")
Function GetDotNetDir
	Exch $R0 ; Set R0 to .net version major
	Push $R1
	Push $R2
 
	; set R1 to minor version number of the installed .NET runtime
	ClearErrors
	EnumRegValue $R1 HKLM "Software\Microsoft\.NetFramework\policy\$R0" 0
	IfErrors getdotnetdir_err
 
	; set R2 to .NET install dir root
	ReadRegStr $R2 HKLM "Software\Microsoft\.NetFramework" "InstallRoot"
	IfErrors getdotnetdir_err
 
	; set R0 to the .NET install dir full
	StrCpy $R0 "$R2$R0.$R1"
 
getdotnetdir_end:
	Pop $R2
	Pop $R1
	Exch $R0 ; return .net install dir full
	Return
 
getdotnetdir_err:
	StrCpy $R0 ""
	Goto getdotnetdir_end
FunctionEnd

!macro RegAsm DllFileName
; get directory of .NET framework installation
	Push "v4.0"
	Call GetDotNetDir
	Pop $R0

	; Perform our install
	; e.g. use the .Net path in $R0 to call RegAsm.exe
	;TODO: User nsExec to avoid black window
	ExecWait '"$R0\RegAsm.exe" /codebase "${DllFileName}"'
!macroend

gerardo-lijs / VB6-NetStandard-Interop