'Forms' does not exist in the namespace system.windows
If you are writing Windows Forms code in a .Net Core app, then it's very probable that you run into this error:
Error CS0234 The type or namespace name 'Forms' does not exist in the namespace 'System.Windows' (are you missing an assembly reference?)
If you are using the Sdk style project file (which is recommended) your *.csproj file should be similar to this:
<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
<PropertyGroup>
<TargetFramework>netcoreapp3.1</TargetFramework>
<OutputType>WinExe</OutputType>
<UseWindowsForms>true</UseWindowsForms>
<RootNamespace>MyAppNamespace</RootNamespace>
<AssemblyName>MyAppName</AssemblyName>
<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.Windows.Compatibility" Version="3.0.0" />
</ItemGroup>
</Project>
Pay extra attention to these lines:
<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
<OutputType>WinExe</OutputType>
<UseWindowsForms>true</UseWindowsForms>
<PackageReference Include="Microsoft.Windows.Compatibility" Version="3.0.0" />
Note that if you are using WPF while referencing some WinForms libraries you should add <UseWPF>true</UseWPF>
as well.
Hint: Since .NET 5.0, Microsoft recommends to refer to SDK Microsoft.Net.Sdk
in lieu of Microsoft.Net.Sdk.WindowsDesktop
.
In case someone runs into this error when trying to reference Windows Forms components in a .NET Core 3+ WPF app (which is actually not uncommon), the solution is to go into the .csproj
file (double click it in VS2019) and add it to the property group node containing the target frameworks. Like this:
<PropertyGroup>
<TargetFramework>netcoreapp3.0</TargetFramework>
<UseWPF>true</UseWPF>
<UseWindowsForms>true</UseWindowsForms>
</PropertyGroup>
Expand the project in Solution Tree, Right-Click on References
, Add Reference
, Select System.Windows.Forms
on Framework
tab.
You need to add reference to some non-default assemblies sometimes.
From comments: for people looking for VS 2019+: Now adding project references is Right-Click on Dependencies
in Solution Explorer
.
For people looking for VS Code: How do I add assembly references in Visual Studio Code