How to use System.Windows.Forms in .NET Core class library

Question

I've created .NET Core class library and try to build it against net40 framework. I want to use Clipboard class from System.Windows.Forms assembly. How can I do this?

My project.json file:

{
    "version": "1.0.0-*",

    "dependencies": {
        "NETStandard.Library": "1.6.0"
    },

    "frameworks": {
        "netstandard1.6": {
            "imports": "dnxcore50",
            "buildOptions": {
                "define": [
                    "NETCORE"
                ]
            },
            "dependencies": {
                "System.Threading": "4.0.11",
                "System.Threading.Thread": "4.0.0",
                "System.Threading.Tasks":  "4.0.11"
                }
        },
        "net40": {
            "buildOptions": {
                "define": [
                    "NET40"
                    ]
                },
            "dependencies": {
                // dependency should be here but there is no such dll
            }
        }
    }
}

All my net40 specific code is under NET40 define. Any thoughts?

Answer 1

For VS2019 .NET Core 3.1:

1. right-mouse click on the project and select Unload Project
2. right-mouse click on the project and select "Edit foobar.csproj"
3. Example of using WPF and Winforms in .NET Core 3.1: where I added the UseWPF and UseWindowsForms tags. Also I changed Microsoft.NET.Sdk to Microsoft.NET.Sdk.WindowsDesktop to be able to use also wpf.

<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">

  <PropertyGroup Label="Globals">
    <SccProjectName>SAK</SccProjectName>
    <SccProvider>SAK</SccProvider>
    <SccAuxPath>SAK</SccAuxPath>
    <SccLocalPath>SAK</SccLocalPath>
  </PropertyGroup>

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <UseWPF>true</UseWPF>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>
...

4. save and right-mouse click on the project again and select Reload Project

Answer 2

for .Net 6 the .csproj file should contain:

<PropertyGroup>
     <TargetFramework>net6.0-windows</TargetFramework>
     <UseWPF>true</UseWPF>
     <UseWindowsForms>true</UseWindowsForms>
</PropertyGroup>

Note the "-windows" in the target framework.

UseWPF is optional.

Answer 3

What you need is "frameworkAssemblies", for example:

"frameworks": {
  "netstandard1.6": {
    "dependencies": {
      "NETStandard.Library": "1.6.0"
    }
  },
  "net40": {
    "frameworkAssemblies": {
      "System.Windows.Forms": {}
    }
  }
}

Working with Clipboard also requires setting the main thread as STA, so don't forget to add [STAThread] to Main() in your application.