I got this error recently while updating a project that I’m working on. It had been ported from WinForm to .Net Core 3.0, and worked fine – but suddenly I started getting errors where it claimed to not know what Form was. The output showed the error:

The “LegacyFrameworkPackages” parameter is not supported by the “ResolveFrameworkReferences” task

Unbeknownst to me, MS have changed the namespace for desktop apps; where previously your csproj might look like this:

<Project Sdk="Microsoft.NET.Sdk">
 
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <NullableReferenceTypes>true</NullableReferenceTypes>
    
    <AssetTargetFallback>uap10.0.18362</AssetTargetFallback>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>

It now needs to look reference the WindowsDesktop SDK:

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