IApplicationActivationManager::ActivateApplication in C#?

Try with the below code snippet which works for me.

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] IntPtr /*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] IntPtr /*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);
}


class Program
{
    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();
        Console.ReadLine();
    }
}

Hope you might already aware how to get the Application name to activate. However, wanted to share that information here for other who might search the same. We can get the AppUserModelID from the below registry location:

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;
    }