FindOutputFilePath.cs

public string FindOutputFilePath(IVsTextManager2 textManager, IVsEditorAdaptersFactoryService adapters)
{
    if(textManager.GetActiveView2(1, null, (uint)_VIEWFRAMETYPE.vftCodeWindow, out IVsTextView activeView) == VSConstants.S_OK)
    {
        var textView = adapters.GetWpfTextView(activeView);
        var doc = textView.TextBuffer.CurrentSnapshot.GetOpenDocumentInCurrentContextWithChanges();
        return doc.Project.OutputFilePath;
    }

    return null;
}