How to use "GPIB-USB-HS" in C#

Hi there,

I have a mission is "use GPIB control several of instruments"

But I am not good at program and first use GPIB-USB-HS.

My boss only gave me previous project, but it's not work now.

It's show error code "CS0246" in program.

(Of course I have other error, but I think this is the most important)

I tried to add reference but program didn't work too.

(Project>Add reference>choose "National Instruments 488.2" and "NationalInstruments.Common")

using Ivi.Visa;
using NationalInstruments.Visa;

Above is the code with error.

I installed NI 488.2 first and I confirm the connection to instrument is successful (by Ni-VISA Interactive Control)

How to solve error code "CS0246"?

or where can I find sample code?

System: Win10

hardware: GPIB-USB-HS

Visual studio: 2019 community

Project: Windows Forms APP (.NET Framework)

Best regards,