HOW to appendtext (text) in Smart Device(MOBILE APPLICATION) in C#

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C#3.0

C#

i write code for reading data from serial port in smart device using C#(mobile application) in that i get the available COM Ports, i can also connecting and disconnecting the ports when sending data to com ports in that i can not transfer the data from sending text Box To Receiving text Box. write code for send button i am pasting here in that i can,t appending the data in smart device same code working C# window forms pl send code for append, text sending text Box To Receiving text Box*

in my code having
i used Two Labels in that
one Label for Available COM Ports
second Label for connect and disconnect show on this label
one combox
three buttons (connect,disconnect,send)
one Rich text box
one text box

        //THSI IS SMART DEVICE PROJECT USING c#
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        SerialPort port = new SerialPort();
        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                #region Display all available COM Ports
                string[] ports = SerialPort.GetPortNames();
                // Add all port names to the combo box:
                foreach (string port in ports)
                {
                    //this.cboPortName.Items.Add(port);
                    this.cbbCOMPorts.Items.Add(port);
                }
                #endregion
                this.btnDisconnect.Enabled = false;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        private void btnConnect_Click_1(object sender, EventArgs e)
        {
            if (port.IsOpen)
            {
                port.Close();
            }
            try
            {
                {
                    port.PortName = cbbCOMPorts.Text;
                    port.BaudRate = 96000;
                    port.Parity = Parity.None;
                    port.DataBits = 8;
                    port.StopBits = StopBits.One;
                }
                //.Encoding = System.Text.Encoding.Unicode 
                port.Open();
                lblMessage.Text = cbbCOMPorts.Text + " Connected. ";
                btnConnect.Enabled = false;
                btnDisconnect.Enabled = true;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            txtDataToSend.Focus();
        }
        private void btnDisconnect_Click_1(object sender, EventArgs e)
        {
            try
            {
                port.Close();
                lblMessage.Text = port.PortName + " disconnected.";
                btnConnect.Enabled = true;
                btnDisconnect.Enabled = false;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        private void btnSend_Click(object sender, EventArgs e)
        {
            try
            {
                port.Write(txtDataToSend.Text);
                //txtDataReceived->IS a data Received Text Box name 
               //txtDataToSend->is a data send Text Box name

                txtDataReceived.AppendText (txtDataToSend.Text ); // <<---- error here
                txtDataReceived.Text = txtDataReceived.Text + txtDataToSend.Text;//<--- i Tried but it not error but data cannot be appendtext in text box(txtDataReceived)
                txtDataReceived.ScrollToCaret();
               }
               txtDataToSend.Text = string.Empty;
           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.Message);
           }

       }

<-----------------------ERROR MESSAGE ----------------------->
-----WHEN I COMPILE OR RUN THE PROJECT IT SHOWING THE BELOW ERROR------
'System.Windows.Forms.TextBox' does not contain a definition for 'AppendText' and no extension method 'AppendText' accepting a first argument of type 'System.Windows.Forms.TextBox' could be found (are you missing a using directive or an assembly reference?)