Retain previous data when adding new row to multiline TextBox using C# and VB.Net in ASP.Net

Hi nedash,

I have created sample code which full-fill your requirement.

HTML

<div>
    <asp:GridView ID="gvNumber" AutoGenerateColumns="false" runat="server">
        <Columns>
            <asp:TemplateField HeaderText="Number">
                <ItemTemplate>
                    <asp:Label ID="Lblmobile" Text='<%#Eval("Number")%>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Action">
                <ItemTemplate>
                    <asp:LinkButton ID="btnAdd" Text="Add" OnClick="LBaddNumber_Click" runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <br />
    <asp:Button Text="Add All" runat="server" class="btnClass" OnClick="OnAddAll" />
    <br />
    <br />
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <th>
                Number of count:<asp:Label ID="LblTshE" Text="" runat="server" />
            </th>
        </tr>
        <tr>
            <td>
                <asp:TextBox ID="TxtPhone" TextMode="MultiLine" Width="180px" Height="150px" runat="server" />
            </td>
        </tr>
    </table>
</div>
<div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
    <script type="text/javascript">
        $(function () {
            $('[id*=TxtPhone]').on("keyup", function () {
                var numbers = $(this).val().split(/\n/g);
                var count = 0;
                for (var i = 0; i < numbers.length; i++) {
                    if (numbers[i].trim() != '') {
                        count++;
                    }
                }

                $('[Id*=LblTshE]').html(count);
            });
        });
    </script>
</div>

 C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Number", typeof(int));
        dt.Rows.Add("25635863");
        dt.Rows.Add("254635863");
        dt.Rows.Add("256667863");
        dt.Rows.Add("256377563");
        gvNumber.DataSource = dt;
        gvNumber.DataBind();
    }
}

protected void LBaddNumber_Click(object sender, EventArgs e)
{
    GridViewRow row = (sender as LinkButton).NamingContainer as GridViewRow;
    string number = (row.FindControl("Lblmobile") as Label).Text.Trim();
    if (!string.IsNullOrEmpty(TxtPhone.Text))
    {
        TxtPhone.Text = number.Trim() + Environment.NewLine + TxtPhone.Text;
    }
    else
    {
        TxtPhone.Text = number.Trim();
    }
    LblTshE.Text = (TxtPhone.Text.Split('\r').Length).ToString();
}

protected void OnAddAll(object sender, EventArgs e)
{
    string numbers = string.Empty;
    if (!string.IsNullOrEmpty(TxtPhone.Text))
    {
        numbers = TxtPhone.Text + Environment.NewLine;
        foreach (GridViewRow row in gvNumber.Rows)
        {
            numbers += (row.FindControl("Lblmobile") as Label).Text.Trim() + Environment.NewLine;
        }

        TxtPhone.Text = numbers.Trim();
    }

    else
    {
        foreach (GridViewRow row in gvNumber.Rows)
        {
            numbers += (row.FindControl("Lblmobile") as Label).Text.Trim() + Environment.NewLine;
        }

        TxtPhone.Text = numbers.Trim();
    }

    CountMessage();
}

private void CountMessage()
{
    LblTshE.Text = (TxtPhone.Text.Split('\r').Length).ToString();
}

Vb.net

Protected Sub Page_Load(sender As Object, e As EventArgs)
	If Not IsPostBack Then
		Dim dt As New DataTable()
		dt.Columns.Add("Number", GetType(Integer))
		dt.Rows.Add("25635863")
		dt.Rows.Add("254635863")
		dt.Rows.Add("256667863")
		dt.Rows.Add("256377563")
		gvNumber.DataSource = dt
		gvNumber.DataBind()
	End If
End Sub

Protected Sub LBaddNumber_Click(sender As Object, e As EventArgs)
	Dim row As GridViewRow = TryCast(TryCast(sender, LinkButton).NamingContainer, GridViewRow)
	Dim number As String = TryCast(row.FindControl("Lblmobile"), Label).Text.Trim()
	If Not String.IsNullOrEmpty(TxtPhone.Text) Then
		TxtPhone.Text = number.Trim() + Environment.NewLine + TxtPhone.Text
	Else
		TxtPhone.Text = number.Trim()
	End If
	LblTshE.Text = (TxtPhone.Text.Split(ControlChars.Cr).Length).ToString()
End Sub

Protected Sub OnAddAll(sender As Object, e As EventArgs)
	Dim numbers As String = String.Empty
	If Not String.IsNullOrEmpty(TxtPhone.Text) Then
		numbers = TxtPhone.Text + Environment.NewLine
		For Each row As GridViewRow In gvNumber.Rows
			numbers += TryCast(row.FindControl("Lblmobile"), Label).Text.Trim() + Environment.NewLine
		Next

		TxtPhone.Text = numbers.Trim()
	Else

		For Each row As GridViewRow In gvNumber.Rows
			numbers += TryCast(row.FindControl("Lblmobile"), Label).Text.Trim() + Environment.NewLine
		Next

		TxtPhone.Text = numbers.Trim()
	End If

	CountMessage()
End Sub

Private Sub CountMessage()
	LblTshE.Text = (TxtPhone.Text.Split(ControlChars.Cr).Length).ToString()
End Sub

Screenshot