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I developed a Windows Forms Control, using Framework 4.7.2, and successfully created the DLL.
Next, when I create a Windows app and add the new control into the Visual Basic Toolbox,
the control will be added but I'm told the new toolbox control is not enabled
for the current Windows application (developed using Core 3.1 or Core 5.0) because
the Framework used to create the Forms Control (4.7.2) is not compatible with
Core 3.1 or Core 5.0.
How can I create a forms control that will be enabled in a Windows application toolbox using Core 3.1 or Core 5.0? Both Cores seem to be incompatible with Framework 4.0 to 4.8.
A set of .NET Framework managed libraries for developing graphical user interfaces.
Answer accepted by question author
I appreciate your answer but I don't know how to switch to the SDK project format. Would you explain further and give me the steps to complete the change? Thanks for your time.
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You need to multi target as discussed here.
That involves switching to the SDK project format as you are probably not using it yet. Then you can use the TargetFrameworks element to specify both net472 and net5.0. If you need NET Core 3.1 then include netcoreapp3.1. You could also use .NET standard if you wanted but that probably doesn't make sense in WInforms.
Commenting here so it is tied to the answer I gave.
To convert to SDK format requires a few changes to the project file but may or may not work properly. Firstly it is important to note that it may break existing projects and therefore shouldn't be done on things like ASP.NET web projects. Secondly, to do this you have to be using NuGet package references and not a packages.config. Thirdly, it will break AssemblyInfo.cs so if you have any customizations you need to do extra work. The full instructions are available here. I'll give the highlights. Note in the docs that there is a conversion tool you can use to automate this in some cases. You should try it first.
Convert the project file - this is the easy part but ensure VS is closed while you do this.
1) Create a new dummy Windows Forms App project that targets .NET 5. Open this project in a text editor. This will be your new project file. This is the target project T.
2) Open your existing project file in a text editor. This is the source project S.
3) For each PropertyGroup in S copy it to the PropertyGroup in T if it contains a value that isn't the default. In most cases there won't be any so you don't need to do anything. It is probably easier to wait until you're done and then use the UI to change these.
4) Add the following elements to the PropertyGroup and set according: Description,Product,Version.
5) Save and close the target project file.
5) Rename the existing source project file as a backup.
7) Copy and rename the target project file into the existing project directory with the original project file name.
8) Open the solution in Visual Studio.
By default all your source files should auto-magically show up. If there are any missing or their build actions aren't set properly then add them manually.
I'm going to assume you are not using Nuget and therefore had no packages.config file. If you did then move this file out of the project and open it in a text editor. Then using Manage NuGet Packages on the project node in Solution Explorer add them back one by one.
Finally the AssemblyInfo.cs file is now part of the project file. If you have any customizations in this file you can keep it around but the build will auto-generate the bulk of the file contents. Therefore remove any Product,Version*,Description like attributes in the file. They are now in the PropertyGroup of the project file. If the file ends up being empty then delete the file as it isn't needed anymore.
Last step, multi-target. Open the project file (you can do that by double clicking it in Solution Explorer). One of the first elements is TargetFramework. Change that to TargetFrameworks. Then change the existing value from net5.0 to net5.0;net472. Save the file and VS will potentially generate an error when trying to reload the project. Restart VS if needed. You are now multi-targeting and using the SDK format. An example file might look like this.
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>WinExe</OutputType>
<TargetFrameworks>net5.0-windows;net472</TargetFrameworks>
<UseWindowsForms>true</UseWindowsForms>
</PropertyGroup>
</Project>
You don't need so many separate comments to enrich an answer. Just go and edit the original answer please. That's the right way to put everything together in a nice format.