Net Framework framework references are mostly added directly on the Internet
But this method of packaging is very unfriendly

After going through the Microsoft documentation, I found a direct reference to the System. Froms assembly. It is also hidden deep, at: link

Net Core version >= 3.0

In fact, just modify the project’s csproj file

The original file

<Project Sdk=”Microsoft.NET.
<PropertyGroup
<TargetFramework>netcoreapp3.1</TargetFramework
<OutputType>Library</OutputType>
</PropertyGroup>
</Project>.
Modify the SDK in the first line

<Project Sdk=”Microsoft.NET.Sdk.WindowsDesktop”>
<PropertyGroup
<TargetFramework>netcoreapp3.1</TargetFramework
<OutputType>Library</OutputType>
</PropertyGroup>
</Project>.
Note this extra WindowsDesktop

Just add this reference to find System.Windows.

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