Here is an updated version of sankar answer (thanks for that one, really helped me), for the ActivateForFile method. I Changed the itemArray to an IShellItemArray instead of an IntPtr and added the interfaces and methods from shell32.
public enum ActivateOptions
{
    None = 0x00000000,  // No flags set
    DesignMode = 0x00000001,  // The application is being activated for design mode, and thus will not be able to
    // to create an immersive window. Window creation must be done by design tools which
    // load the necessary components by communicating with a designer-specified service on
    // the site chain established on the activation manager.  The splash screen normally
    // shown when an application is activated will also not appear.  Most activations
    // will not use this flag.
    NoErrorUI = 0x00000002,  // Do not show an error dialog if the app fails to activate.                                
    NoSplashScreen = 0x00000004,  // Do not show the splash screen when activating the app.
}
[ComImport, Guid("2e941141-7f97-4756-ba1d-9decde894a3d"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IApplicationActivationManager
{
    // Activates the specified immersive application for the "Launch" contract, passing the provided arguments
    // string into the application.  Callers can obtain the process Id of the application instance fulfilling this contract.
    IntPtr ActivateApplication([In] String appUserModelId, [In] String arguments, [In] ActivateOptions options, [Out] out UInt32 processId);
    IntPtr ActivateForFile([In] String appUserModelId, [In] [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 2)] /*IShellItemArray* */ IShellItemArray itemArray, [In] String verb, [Out] out UInt32 processId);
    IntPtr ActivateForProtocol([In] String appUserModelId, [In] IntPtr /* IShellItemArray* */itemArray, [Out] out UInt32 processId);
}
[ComImport, Guid("45BA127D-10A8-46EA-8AB7-56EA9078943C")]//Application Activation Manager
class ApplicationActivationManager : IApplicationActivationManager
{
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)/*, PreserveSig*/]
    public extern IntPtr ActivateApplication([In] String appUserModelId, [In] String arguments, [In] ActivateOptions options, [Out] out UInt32 processId);
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
    public extern IntPtr ActivateForFile([In] String appUserModelId, [In] [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 2)]  /*IShellItemArray* */ IShellItemArray itemArray, [In] String verb, [Out] out UInt32 processId);
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
    public extern IntPtr ActivateForProtocol([In] String appUserModelId, [In] IntPtr /* IShellItemArray* */itemArray, [Out] out UInt32 processId);
}
[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe")]
interface IShellItem
{
}
[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("b63ea76d-1f85-456f-a19c-48159efa858b")]
interface IShellItemArray
{
}
internal class Program2
{
    [DllImport("shell32.dll", CharSet = CharSet.Unicode, PreserveSig = false)]
    private static extern void SHCreateItemFromParsingName(
        [In] [MarshalAs(UnmanagedType.LPWStr)] string pszPath,
        [In] IntPtr pbc,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid iIdIShellItem,
        [Out] [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 2)] out IShellItem iShellItem);
    [DllImport("shell32.dll", CharSet = CharSet.Unicode, PreserveSig = false)]
    private static extern void SHCreateShellItemArrayFromShellItem(
        [In] [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 2)] IShellItem psi,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid iIdIShellItem,
        [Out] [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 2)] out IShellItemArray iShellItemArray);
    private static void Main(string[] args)
    {
        ApplicationActivationManager appActiveManager = new ApplicationActivationManager();//Class not registered
        uint pid;
        appActiveManager.ActivateApplication("2c123c17-8b21-4eb8-8b7f-fdc35c8b7718_n2533ggrncqjt!App", null, ActivateOptions.None, out pid);
        Console.WriteLine("Activated");
        Console.ReadLine();
        IShellItemArray array = GetShellItemArray(@"C:\temp\somefile.xyz");
        appActiveManager.ActivateForFile("2c123c17-8b21-4eb8-8b7f-fdc35c8b7718_n2533ggrncqjt!App", array, "Open",
            out pid);
        Console.WriteLine("Activated for file");
        Console.ReadLine();
    }
    private static IShellItemArray GetShellItemArray(string sourceFile)
    {
        IShellItem item = GetShellItem(sourceFile);
        IShellItemArray array = GetShellItemArray(item);
        return array;
    }
    private static IShellItem GetShellItem(string sourceFile)
    {
        IShellItem iShellItem = null;
        Guid iIdIShellItem = new Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe");
        SHCreateItemFromParsingName(sourceFile, IntPtr.Zero, iIdIShellItem, out iShellItem);
        return iShellItem;
    }
    private static IShellItemArray GetShellItemArray(IShellItem shellItem)
    {
        IShellItemArray iShellItemArray = null;
        Guid iIdIShellItemArray = new Guid("b63ea76d-1f85-456f-a19c-48159efa858b");
        SHCreateShellItemArrayFromShellItem(shellItem, iIdIShellItemArray, out iShellItemArray);
        return iShellItemArray;
    }