How to programatically add an existing project to a solution folder in Visual Studio

Question

I have a solution that I want to programatically add an existing project to.

Opening the solution using envdte automation is relatively easy

        var envDteType = Type.GetTypeFromProgID("VisualStudio.DTE.15.0");

        var envDte = Activator.CreateInstance(envDteType, true);
        var dte2 = (DTE2)envDte;

        var solution = (Solution4)dte2.Solution;
        solution.Open(filename);

But adding an existing project to a solution folder is not.

Note this isn't using a template which is what most google results give

Using

var referencesFolder = solution.AddSolutionFolder("References");
referencesFolder.ProjectItems.AddFromFile(fullPathToCsProj);

Adds the project file as a plain old file, not a CSharp project.

The use case is we are in nuget update hell with our library management and will dereference nuget and add a direct project reference instead. There are around 55 projects that require hundreds of dereferences hence the automation.

Answer 1

The trick is to add cast the returned project to a SolutionFolder (note the .Object)

var referencesFolder = (SolutionFolder) solution
                                  .AddSolutionFolder("References")
                                  .Object;

referencesFolder.AddFromFile(fullPathToCsProj);

This gives access to AddFromFile on the root entity which adds the project properly