.NET (Core and Framework)

var app = new Microsoft.Office.Interop.Excel.Application();
var wb = app.Workbooks.Open(sourceFilePath);
wb.SaveAs(Filename: destinationFilePath, FileFormat: Microsoft.Office.Interop.Excel.XlFileFormat.xlOpenXMLWorkbook);
wb.Close();
app.Quit();

Dim parts() As String = strReturn.Split(" "c)
Dim buffer() As Integer = Array.ConvertAll(parts, Function(p) Convert.ToInt32(p.Substring(2), 16))

Dim Number1 As String = "2812"
Dim Str As String = "ATDT" & Number1 & "R" & Chr(&HD)
Dim buffer() As Byte = System.Text.Encoding.ASCII.GetBytes(Str)
frmMain.SerialPort1.Write(buffer, 0, buffer.Length)

Richard Deeming wrote:

Convert.ToInt32 does not allow you to use a Hex prefix. Neither the VB prefix (&H) nor the C# prefix (0x) will work.

Convert.ToInt32("2A", 16)   ' Works
Convert.ToInt32("0x2A", 16) ' Works
Convert.ToInt32("&H2A", 16) ' FormatException: Could not find any recognizable digits
Convert.ToInt32("0x2A")     ' FormatException: Input string was not in a correct format
Convert.ToInt32("&H2A")     ' FormatException: Input string was not in a correct format

public class MyRunAs
    {
        [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
        internal static extern bool CreateProcessWithTokenW(
            IntPtr dupeTokenHandle,
            LogonFlags dwLogonFlags,  
            string applicationName,
            string commandLine,
            CreationFlags dwCreationFlags, 
            IntPtr environment,
            string currentDirectory,
            ref STARTUPINFO sui,
            out PROCESS_INFORMATION processInfo);

        [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
        internal static extern bool DuplicateTokenEx(IntPtr tokenHandle, int
        dwDesiredAccess,
        ref SECURITY_ATTRIBUTES lpTokenAttributes, int
        SECURITY_IMPERSONATION_LEVEL,
        int TOKEN_TYPE, ref IntPtr dupeTokenHandle);

        [DllImport("userenv.dll", SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern bool CreateEnvironmentBlock(
         ref IntPtr lpEnvironment,
         IntPtr hToken,
         bool bInherit);

        [DllImport("advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        static extern bool CreateProcessAsUser(
             IntPtr Token,
             [MarshalAs(UnmanagedType.LPTStr)] string ApplicationName,
             [MarshalAs(UnmanagedType.LPTStr)] string CommandLine,
             ref SECURITY_ATTRIBUTES ProcessAttributes,
             ref SECURITY_ATTRIBUTES ThreadAttributes,
             bool InheritHandles,
             CreationFlags dwCreationFlags,
             IntPtr Environment,
             [MarshalAs(UnmanagedType.LPTStr)] string CurrentDirectory,
             ref STARTUPINFO StartupInfo,
             out PROCESS_INFORMATION ProcessInformation);


        [StructLayout(LayoutKind.Sequential)]
        internal struct SECURITY_ATTRIBUTES
        {
            internal int nLength;
            internal int lpSecurityDescriptor;
            internal bool bInheritHandle;

        }

        public enum CreationFlags
        {
            DefaultErrorMode = 0x04000000,
            NewConsole = 0x00000010,
            NewProcessGroup = 0x00000200,
            SeparateWOWVDM = 0x00000800,
            Suspended = 0x00000004,
            UnicodeEnvironment = 0x00000400,
            ExtendedStartupInfoPresent = 0x00080000
        }

        public enum LogonFlags
        {
            WithProfile = 1,
            NetCredentialsOnly
        }

        [StructLayout(LayoutKind.Sequential)]
        internal struct STARTUPINFO
        {
            internal int cb;
            [MarshalAs(UnmanagedType.LPTStr)]
            internal string lpReserved;
            [MarshalAs(UnmanagedType.LPTStr)]
            internal string lpDesktop;
            [MarshalAs(UnmanagedType.LPTStr)]
            internal string lpTitle;
            internal int dwX;
            internal int dwY;
            internal int dwXSize;
            internal int dwYSize;
            internal int dwXCountChars;
            internal int dwYCountChars;
            internal int dwFillAttribute;
            internal int dwFlags;
            internal short wShowWindow;
            internal short cbReserved2;
            internal IntPtr lpReserved2;
            internal IntPtr hStdInput;
            internal IntPtr hStdOutput;
            internal IntPtr hStdError;
        }

        [StructLayout(LayoutKind.Sequential)]
        internal struct PROCESS_INFORMATION
        {
            internal IntPtr hProcess;
            internal IntPtr hThread;
            internal int dwProcessId;
            internal int dwThreadId;
        }

        // SECURITY_IMPERSONATION_LEVEL
        const int SecurityAnonymous = 0;
        const int SecurityIdentification = 1;
        const int SecurityImpersonation = 2;
        const int SecurityDelegation = 3;

        // TOKEN_TYPE
        const int TokenPrimary = 1;
        const int TokenImpersonation = 2;

        //dwLogonFlags Specifies the logon option
        const int LOGON_WITH_PROFILE = 1;
        const int LOGON_NETCREDENTIALS_ONLY = 2;

        // Access Token constants
        private const int TOKEN_QUERY = 0x0008;
        private const int TOKEN_DUPLICATE = 0x0002;
        private const int TOKEN_ASSIGN_PRIMARY = 0x0001;
        private const int STARTF_USESHOWWINDOW = 0x00000001;
        private const int STARTF_FORCEONFEEDBACK = 0x00000040;
        private const int CREATE_UNICODE_ENVIRONMENT = 0x00000400;
        private const int TOKEN_IMPERSONATE = 0x0004;
        private const int TOKEN_QUERY_SOURCE = 0x0010;
        private const int TOKEN_ADJUST_PRIVILEGES = 0x0020;
        private const int TOKEN_ADJUST_GROUPS = 0x0040;
        private const int TOKEN_ADJUST_DEFAULT = 0x0080;
        private const int TOKEN_ADJUST_SESSIONID = 0x0100;
        private const int STANDARD_RIGHTS_REQUIRED = 0x000F0000;
        private const int TOKEN_ALL_ACCESS =
            STANDARD_RIGHTS_REQUIRED |
            TOKEN_ASSIGN_PRIMARY |
            TOKEN_DUPLICATE |
            TOKEN_IMPERSONATE |
            TOKEN_QUERY |
            TOKEN_QUERY_SOURCE |
            TOKEN_ADJUST_PRIVILEGES |
            TOKEN_ADJUST_GROUPS |
            TOKEN_ADJUST_DEFAULT |
            TOKEN_ADJUST_SESSIONID;

        public static bool CreateTokenChild()
        {
                    
            try {
                
                
                STARTUPINFO startInfo = new STARTUPINFO();
                startInfo.cb = Marshal.SizeOf(startInfo);
                // not needed ?!
                // startInfo.lpDesktop = @"winsta0\default"; 

                // Create Process with token
                IntPtr dupeTokenHandle = IntPtr.Zero;
                // IntPtr tokenHandle = WindowsIdentity.GetCurrent().Token;

                // get impersonation token
                // we can do that since the owner of this thread 
                // has "act as part of operating system" priviledge
                // use UPN of target user
                WindowsIdentity id = new WindowsIdentity("user@domain");
                IntPtr tokenHandle = id.Token;
                
                // Setting security attributes
                SECURITY_ATTRIBUTES lpTokenAttributes = new SECURITY_ATTRIBUTES();
                lpTokenAttributes.nLength = Marshal.SizeOf(lpTokenAttributes);

                // SECURITY_ATTRIBUTES threadAttributes = new SECURITY_ATTRIBUTES();
                // threadAttributes.nLength = Marshal.SizeOf(lpTokenAttributes);

                // get primary token for user to be impersonated to
                bool retVal = DuplicateTokenEx(
                    tokenHandle,
                    TOKEN_ALL_ACCESS, 
                    ref lpTokenAttributes, 
                    SecurityImpersonation, 
                    TokenPrimary, 
                    ref dupeTokenHandle);

                if (!retVal)
                {
                    int winError = Marshal.GetLastWin32Error();
                    File.AppendAllText("C:\\tmp\\out.log", DateTime.Now.ToLongTimeString() + " " + winError + Environment.NewLine);

                    return false;
                }

                // app to be started
                string app = @"c:\Windows\System32\notepad.exe";
                // command line arguments
                string cmd = null;
                // startup path
                string spath = @"C:\";

                // create environment for user to be impersonated to
                IntPtr env = GetEnvironmentBlock(dupeTokenHandle);

                // the process to be created
                PROCESS_INFORMATION processInfo;

                bool ret = CreateProcessWithTokenW(
                    dupeTokenHandle,
                    LogonFlags.WithProfile,
                    app,
                    cmd,
                    CreationFlags.UnicodeEnvironment,
                    env,
                    spath,
                    ref startInfo, 
                    out processInfo);
                
                /*bool ret = CreateProcessAsUser(dupeTokenHandle, spot, cmd, ref lpTokenAttributes, ref threadAttributes, true,
                    CreationFlags.UnicodeEnvironment, env, "C:\\", ref startInfo, out processInfo);
                    */

                if (!ret)
                {
                    int winError = Marshal.GetLastWin32Error();
                    File.AppendAllText("C:\\tmp\\out.log", DateTime.Now.ToLongTimeString() + " error: " + winError + Environment.NewLine);

                    return false;
                }                
                else
                {
                    File.AppendAllText("C:\\tmp\\out.log", DateTime.Now.ToLongTimeString() + " success " + Environment.NewLine);
                }               

            }

            catch (Exception e)
            {
                return false ;
            }
            finally
            {
                // TODO
                // close handles
            }
            return true;
                        
        }

        private static IntPtr GetEnvironmentBlock(IntPtr token)
        {
            var envBlock = IntPtr.Zero;
            if (!CreateEnvironmentBlock(ref envBlock, token, false))
            {
                throw new Win32Exception(Marshal.GetLastWin32Error(), "CreateEnvironmentBlock failed");
            }
            return envBlock;
        }

    }