How to port a Windows Forms desktop app to .NET Core

This article describes how to port your Windows Forms-based desktop app from .NET Framework to .NET Core 3.0 or later. The .NET Core 3.0 SDK includes support for Windows Forms applications. Windows Forms is still a Windows-only framework and only runs on Windows. This example uses the .NET Core SDK CLI to create and manage your project.

In this article, various names are used to identify types of files used for migration. When migrating your project, your files will be named differently, so mentally match them to the ones listed below:

File Description
MyApps.sln The name of the solution file.
MyForms.csproj The name of the .NET Framework Windows Forms project to port.
MyFormsCore.csproj The name of the new .NET Core project you create.
MyAppCore.exe The .NET Core Windows Forms app executable.

Prerequisites

  • Visual Studio 2019 for any designer work you want to do.

    Install the following Visual Studio workloads:

    • .NET desktop development
    • .NET Core cross-platform development
  • A working Windows Forms project in a solution that builds and runs without issue.

  • A project coded in C#.

  • .NET Core 3.0 or later.

Note

Visual Studio 2017 doesn't support .NET Core 3.0 projects. Visual Studio 2019 supports .NET Core 3.0 projects but doesn't yet support the visual designer for .NET Core 3.0 Windows Forms projects. To use the visual designer, you must have a .NET Windows Forms project in a solution that shares the forms files with the .NET Core project.

Consider

When porting a .NET Framework Windows Forms application, there are a few things you must consider.

  1. Check that your application is a good candidate for migration.

    Use the .NET Portability Analyzer to determine if your project will migrate to .NET Core 3.0. If your project has issues with .NET Core 3.0, the analyzer helps you identify those problems.

  2. You're using a different version of Windows Forms.

    When .NET Core 3.0 Preview 1 was released, Windows Forms went open source on GitHub. The code for .NET Core Windows Forms is a fork of the .NET Framework Windows Forms codebase. It's possible some differences exist and your app won't port.

  3. The Windows Compatibility Pack may help you migrate.

    Some APIs that are available in .NET Framework aren't available in .NET Core 3.0. The Windows Compatibility Pack adds many of these APIs and may help your Windows Forms app become compatible with .NET Core.

  4. Update the NuGet packages used by your project.

    It's always a good practice to use the latest versions of NuGet packages before any migration. If your application is referencing any NuGet packages, update them to the latest version. Ensure your application builds successfully. After upgrading, if there are any package errors, downgrade the package to the latest version that doesn't break your code.

  5. Visual Studio 2019 doesn't yet support the Forms Designer for .NET Core 3.0

    Currently, you need to keep your existing .NET Framework Windows Forms project file if you want to use the Forms Designer from Visual Studio.

Create a new SDK project

The new .NET Core 3.0 project you create must be in a different directory from your .NET Framework project. If they're both in the same directory, you may run into conflicts with the files that are generated in the obj directory. In this example, we'll create a directory named MyFormsAppCore in the SolutionFolder directory:

SolutionFolder
├───MyApps.sln
├───MyFormsApp
│   └───MyForms.csproj
└───MyFormsAppCore      <--- New folder for core project

Next, you need to create the MyFormsCore.csproj project in the MyFormsAppCore directory. You can create this file manually by using the text editor of choice. Paste in the following XML:

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

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>

</Project>

If you don't want to create the project file manually, you can use Visual Studio or the .NET Core SDK to generate the project. However, you must delete all other files generated by the project template except for the project file. To use the SDK, run the following command from the SolutionFolder directory:

dotnet new winforms -o MyFormsAppCore -n MyFormsCore

After you create the MyFormsCore.csproj, your directory structure should look like the following:

SolutionFolder
├───MyApps.sln
├───MyFormsApp
│   └───MyForms.csproj
└───MyFormsAppCore
    └───MyFormsCore.csproj

You'll want to add the MyFormsCore.csproj project to MyApps.sln with either Visual Studio or the .NET Core CLI from the SolutionFolder directory:

dotnet sln add .\MyFormsAppCore\MyFormsCore.csproj

Fix assembly info generation

Windows Forms projects that were created with .NET Framework include an AssemblyInfo.cs file, which contains assembly attributes such as the version of the assembly to be generated. SDK-style projects automatically generate this information for you based on the SDK project file. Having both types of "assembly info" creates a conflict. Resolve this problem by disabling automatic generation, which forces the project to use your existing AssemblyInfo.cs file.

There are three settings to add to the main <PropertyGroup> node.

  • GenerateAssemblyInfo
    When you set this property to false, it won't generate the assembly attributes. This avoids the conflict with the existing AssemblyInfo.cs file from the .NET Framework project.

  • AssemblyName
    The value of this property is the output binary created when you compile. The name doesn't need an extension added to it. For example, using MyCoreApp produces MyCoreApp.exe.

  • RootNamespace
    The default namespace used by your project. This should match the default namespace of the .NET Framework project.

Add these three elements to the <PropertyGroup> node in the MyFormsCore.csproj file:

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

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>

    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
    <AssemblyName>MyCoreApp</AssemblyName>
    <RootNamespace>WindowsFormsApp1</RootNamespace>
  </PropertyGroup>

</Project>

Add source code

Right now, the MyFormsCore.csproj project doesn't compile any code. By default, .NET Core projects automatically include all source code in the current directory and any child directories. You must configure the project to include code from the .NET Framework project using a relative path. If your .NET Framework project used .resx files for icons and resources for your forms, you'll need to include those too.

Add the following <ItemGroup> node to your project. Each statement includes a file glob pattern that includes child directories.

  <ItemGroup>
    <Compile Include="..\MyFormsApp\**\*.cs" />
    <EmbeddedResource Include="..\MyFormsApp\**\*.resx" />
  </ItemGroup>

Alternatively, you can create a <Compile> or <EmbeddedResource> entry for each file in your .NET Framework project.

Add NuGet packages

Add each NuGet package referenced by the .NET Framework project to the .NET Core project.

Most likely your .NET Framework Windows Forms app has a packages.config file that contains a list of all of the NuGet packages that are referenced by your project. You can look at this list to determine which NuGet packages to add to the .NET Core project. For example, if the .NET Framework project referenced the MetroFramework, MetroFramework.Design, and MetroFramework.Fonts NuGet packages, add each to the project with either Visual Studio or the .NET Core CLI from the SolutionFolder directory:

dotnet add .\MyFormsAppCore\MyFormsCore.csproj package MetroFramework
dotnet add .\MyFormsAppCore\MyFormsCore.csproj package MetroFramework.Design
dotnet add .\MyFormsAppCore\MyFormsCore.csproj package MetroFramework.Fonts

The previous commands would add the following NuGet references to the MyFormsCore.csproj project:

  <ItemGroup>
    <PackageReference Include="MetroFramework" Version="1.2.0.3" />
    <PackageReference Include="MetroFramework.Design" Version="1.2.0.3" />
    <PackageReference Include="MetroFramework.Fonts" Version="1.2.0.3" />
  </ItemGroup>

Port control libraries

If you have a Windows Forms Controls library project to port, the directions are the same as porting a .NET Framework Windows Forms app project, except for a few settings. And instead of compiling to an executable, you compile to a library. The difference between the executable project and the library project, besides paths for the file globs that include your source code, is minimal.

Using the previous step's example, lets expand what projects and files we're working with.

File Description
MyApps.sln The name of the solution file.
MyControls.csproj The name of the .NET Framework Windows Forms Controls project to port.
MyControlsCore.csproj The name of the new .NET Core library project you create.
MyCoreControls.dll The .NET Core Windows Forms Controls library.
SolutionFolder
├───MyApps.sln
├───MyFormsApp
│   └───MyForms.csproj
├───MyFormsAppCore
│   └───MyFormsCore.csproj
│
├───MyFormsControls
│   └───MyControls.csproj
└───MyFormsControlsCore
    └───MyControlsCore.csproj   <--- New project for core controls

Consider the differences between the MyControlsCore.csproj project and the previously created MyFormsCore.csproj project.

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

   <PropertyGroup>
-    <OutputType>WinExe</OutputType>
     <TargetFramework>netcoreapp3.0</TargetFramework>
     <UseWindowsForms>true</UseWindowsForms>

     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
-    <AssemblyName>MyCoreApp</AssemblyName>
-    <RootNamespace>WindowsFormsApp1</RootNamespace>
+    <AssemblyName>MyControlsCore</AssemblyName>
+    <RootNamespace>WindowsFormsControlLibrary1</RootNamespace>
   </PropertyGroup>

   <ItemGroup>
-    <Compile Include="..\MyFormsApp\**\*.cs" />
-    <EmbeddedResource Include="..\MyFormsApp\**\*.resx" />
+    <Compile Include="..\MyFormsControls\**\*.cs" />
+    <EmbeddedResource Include="..\MyFormsControls\**\*.resx" />
   </ItemGroup>

 </Project>

Here is an example of what the .NET Core Windows Forms Controls library project file would look like:

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

  <PropertyGroup>

    <TargetFramework>netcoreapp3.0</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>

    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
    <AssemblyName>MyCoreControls</AssemblyName>
    <RootNamespace>WindowsFormsControlLibrary1</RootNamespace>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="..\MyFormsControls\**\*.cs" />
    <EmbeddedResource Include="..\MyFormsControls\**\*.resx" />
  </ItemGroup>

</Project>

As you can see, the <OutputType> node was removed, which defaults the compiler to produce a library instead of an executable. The <AssemblyName> and <RootNamespace> were changed. Specifically the <RootNamespace> should match the namespace of the Windows Forms Controls library you are porting. And finally, the <Compile> and <EmbeddedResource> nodes were adjusted to point to the folder of the Windows Forms Controls library you are porting.

Next, in the main .NET Core MyFormsCore.csproj project add reference to the new .NET Core Windows Forms Control library. Add a reference with either Visual Studio or the .NET Core CLI from the SolutionFolder directory:

dotnet add .\MyFormsAppCore\MyFormsCore.csproj reference .\MyFormsControlsCore\MyControlsCore.csproj

The previous command adds the following to the MyFormsCore.csproj project:

  <ItemGroup>
    <ProjectReference Include="..\MyFormsControlsCore\MyControlsCore.csproj" />
  </ItemGroup>

Compilation problems

If you have problems compiling your projects, you may be using some Windows-only APIs that are available in .NET Framework but not available in .NET Core. You can try adding the Windows Compatibility Pack NuGet package to your project. This package only runs on Windows and adds about 20,000 Windows APIs to .NET Core and .NET Standard projects.

dotnet add .\MyFormsAppCore\MyFormsCore.csproj package Microsoft.Windows.Compatibility

The previous command adds the following to the MyFormsCore.csproj project:

  <ItemGroup>
    <PackageReference Include="Microsoft.Windows.Compatibility" Version="2.0.1" />
  </ItemGroup>

Windows Forms Designer

As detailed in this article, Visual Studio 2019 only supports the Forms Designer in .NET Framework projects. By creating a side-by-side .NET Core project, you can test your project with .NET Core while you use the .NET Framework project to design forms. Your solution file includes both the .NET Framework and .NET Core projects. Add and design your forms and controls in the .NET Framework project, and based on the file glob patterns we added to the .NET Core projects, any new or changed files will automatically be included in the .NET Core projects.

Once Visual Studio 2019 supports the Windows Forms Designer, you can copy/paste the content of your .NET Core project file into the .NET Framework project file. Then delete the file glob patterns added with the <Source> and <EmbeddedResource> items. Fix the paths to any project reference used by your app. This effectively upgrades the .NET Framework project to a .NET Core project.

Next steps