C#

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

  <PropertyGroup>
    <TargetFramework>netstandard1.4</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="System.Collections.Immutable" Version="1.4.0" />
    <PackageReference Include="System.Net.Sockets" Version="4.3.0" />
    <PackageReference Include="System.ValueTuple" Version="4.4.0" />
    <PackageReference Include="System.Interactive.Async" Version="3.1.1" />
    <PackageReference Include="System.Reactive.Core" Version="3.1.1" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\..\Galador.Reflection\Galador.Reflection.PCL\Galador.Reflection.PCL.csproj" />
  </ItemGroup>

</Project>

Super Lloyd wrote:

And also updated them to lastest .csproj format

static class Program
{
    [STAThread]
    static void Main(string[] args)
    {
        bool withoutActivation = false;
        if (args.Length > 0 && !String.IsNullOrWhiteSpace(args[0]))
            withoutActivation = args[0] == "1";

        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1(withoutActivation));
    }
}

public partial class Form1 : Form
{
    private bool WithoutActivation;

    public Form1(bool withoutActivation)
    {
        WithoutActivation = withoutActivation;

        InitializeComponent();

        // I placed a lone label on the form:
        label1.Text = WithoutActivation ? "without activation" : "with activation";
    }

    protected override bool ShowWithoutActivation
    {
        get
        {
            return WithoutActivation;
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        bool withoutActivation = false;
        while (!Console.KeyAvailable)
        {
            Thread.Sleep(3000);
            Process.Start("[path to forms-app.exe goes here]", withoutActivation ? "1" : "0");
            withoutActivation = !withoutActivation;
        }
    }
}

e.BringToForeground = false;

using System;
using System.Windows.Forms;
using System.Linq;
using Microsoft.VisualBasic.ApplicationServices;

namespace FileArranger
{
	/// <summary>
	/// Class with program entry point.
	/// </summary>
	
	internal sealed class Program
	{
		/// <summary>
		/// Program entry point.
		/// </summary>
		/// 

		[STAThread]
		static void Main()
		{
			Application.EnableVisualStyles();
			Application.SetCompatibleTextRenderingDefault(false);
			
			string[] args = Environment.GetCommandLineArgs();
			var controller = new SingleInstanceController();
			controller.Run(args);
		}
	}
	
	public class SingleInstanceController : WindowsFormsApplicationBase
	{
		public SingleInstanceController()
		{
			IsSingleInstance = true;
			StartupNextInstance += StartedAgainInstance;
		}
		
		void StartedAgainInstance(object sender, StartupNextInstanceEventArgs e)
		{
			var form = MainForm as MainForm;
			e.BringToForeground = false;
			form.ReadArguments(e.CommandLine.ToArray());
		}
		
		protected override void OnCreateMainForm()
		{
			MainForm = new MainForm();
		}
	}
}

protected override bool ShowWithoutActivation
{
    get { return true; }
}

using System;
using System.Windows.Forms;
using SA.Universal.SingleInstance;

namespace FileArranger
{
	internal sealed class Program
	{
		[STAThread]
		static void Main(string[] commandLine)
		{
            if (SingleInstanceManager.IsSecondInstance)
            {
                SingleInstanceManager.HandleRemoteCommandLine(commandLine);
                SingleInstanceManager.ActivateFirstInstance();
                return;
            }

            Application.EnableVisualStyles();
			Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
		}
	}
}

using SA.Universal.SingleInstance;

SingleInstanceManager.FilesLoading += (sender, eventArgs) => {
    Invoke(new System.Action(() => ReadArguments(eventArgs.CommandLine)));
};

if (SingleInstanceManager.IsSecondInstance)
{
    SingleInstanceManager.HandleRemoteCommandLine(commandLine);
    SingleInstanceManager.ActivateFirstInstance();
    return;
}

alin1 wrote:

I suspect the issue is here:

if (SingleInstanceManager.IsSecondInstance)
{
    SingleInstanceManager.HandleRemoteCommandLine(commandLine);
    SingleInstanceManager.ActivateFirstInstance();
    return;
}

The second instance should pass the command line arguments to the first instance. But how to do that?