NETSDK1136: The target framework must be Windows

If UseWindowsForms or UseWPF is true, .NET assumes that your project is a Windows app, and so the platform has to be set to Windows. This error can happen if you have a project-to-project reference where one is set to Windows and the other is not. The full error message is similar to the following example:

The target platform must be set to Windows (usually by including -windows in the TargetFramework property) when using Windows Forms or WPF, or referencing projects or packages that do so.

For example, set TargetFramework to net6.0-windows, as shown in this project file:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>net6.0-windows</TargetFramework>
  </PropertyGroup>
</Project>