Using Roslyn and .NET Core : how do I resolve "type or namespace name "Forms" does not exist in the namespace 'System.Windows'

Question

I am developing a system that uses Roslyn to do runtime compilation of C# code.

I am adding assemblies using this code, passing the fully qualified assembly file name to the method.

public bool AddAssembly(string assemblyDll)
{
     if (string.IsNullOrEmpty(assemblyDll)) return false;

     var file = Path.GetFullPath(assemblyDll);

     if (!File.Exists(file))
     {
         // check framework or dedicated runtime app folder
         var path = Path.GetDirectoryName(typeof(object).Assembly.Location);
         file = Path.Combine(path, assemblyDll);

         if (!File.Exists(file))
             return false;
     }

     if (References.Any(r => r.FilePath == file)) 
         return true;

     try
     {
         var reference = MetadataReference.CreateFromFile(file);
         References.Add(reference);
     }
     catch
     {
         return false;
     }

     return true;
}

All worked well until I wanted to build a Winforms form in the code. Despite adding System.Windows.Forms.dll to the assembly references I still get the error

Type or namespace name "Forms" does not exist in the namespace 'System.Windows'

Reading other articles on this error message the solutions all involve Visual Studio and editing the .csproj file to add

<UseWindowsForms>true</UseWindowsForms>

So, how do I get Roslyn to recognize that I want to use Winforms forms?

Answer 1

I solved my problem and thought it might be useful for someone else.

To summarise. This was part of an R&D exercise we are doing.

Summary of the spec was - Write a program to a read C# source code file, compile it and write the dll to a SQL database. Then write another program to retrieve the dll from SQL and run a method in the dll.

To do this I was using Roslyn APIs (.Net Compiler Platform SDK) and .Net Core.

To get this to work you have to reference any assembly that the code will need when Roslyn compiles it. That is what the code in the question was doing.

In my case these were in

C:\Program Files\dotnet\shared\Microsoft.NETCore.App\8.0.3\

All went well until I picked some code that generated a Winforms Form. Then I got the error above. Eventually I created a "Windows Forms App" project to see what VS 2022 referenced and found that it had another framework in its Dependencies

Microsoft.WindowsDesktop.App.WindowsForms

This refered to files in the following directory

C:\Program Files\dotnet\packs\Microsoft.WindowsDesktop.App.Ref\8.0.3\ref\net8.0\

Once I referenced

System.Windows.Forms.dll and System.Windows.Forms.Primitives.dll

Roslyn compiled perfectly.