StrangeTools/MessageBox/MessageBox.csproj

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

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFrameworks>net20;net45;net5.0-windows</TargetFrameworks>
    <UseWindowsForms>true</UseWindowsForms>
    <RootNamespace>MBox</RootNamespace>
  </PropertyGroup>

  <ItemGroup>
    <Reference Include="System.Windows.Forms" />
  </ItemGroup>

</Project>