Error CS0234 the type or namespace name does not exist ...

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C#6.0

I created a Class-Library project in .net6, and then imported some existing codes in Framework 4.0. For the 2 lines below

using System.Windows.Forms;
using Excel = Microsoft.Office.Interop.Excel;

I got

Error CS0234 the type or namespace name does not exist ..

Then I revised the .csproj and added

<PropertyGroup>
  <UseWindowsForms>true</UseWindowsForms>
  ...
</PropertyGroup>

<itemgroup>
<comreference include="Microsoft.Office.Excel.dll">
<guid>00020813-0000-0000-c000-000000000046
<versionmajor>1
<versionminor>9
<wrappertool>tlbimp
<lcid>0
<isolated>false


The error for the "Excel" one is gone, but the one for "Windows.Forms" still exists. How to resolve this problem. Thanks if you can help.

What I have tried:

Error CS0234 the type or namespace name does not exist ... for using System.Windows.Forms;