ServiceHelper.cs

using System.ComponentModel;
using System.Runtime.InteropServices;

namespace System.ServiceProcess
{
	internal static class ServiceControllerExtensions
	{
		[DllImport("advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
		[return: MarshalAs(UnmanagedType.Bool)]
		private static extern Boolean ChangeServiceConfig(IntPtr hService, UInt32 nServiceType, UInt32 nStartType, UInt32 nErrorControl, String lpBinaryPathName, String lpLoadOrderGroup, IntPtr lpdwTagId, [In] char[] lpDependencies, String lpServiceStartName, String lpPassword, String lpDisplayName);

		private const uint SERVICE_NO_CHANGE = 0xFFFFFFFF;

		public static void SetStartType(this ServiceController svc, ServiceStartMode mode)
		{
			using (var serviceHandle = svc.ServiceHandle)
				if (!ChangeServiceConfig(serviceHandle.DangerousGetHandle(), SERVICE_NO_CHANGE, (uint)mode, SERVICE_NO_CHANGE, null, null, IntPtr.Zero, null, null, null, null))
					throw new ExternalException("Could not change service start type.", new Win32Exception());
		}
	}
}