How to add Button Text inside GridView to TextBox with line break on click using C# in ASP.Net

Hi nedash,

I have created sample code which full-fill your requirement. Please modify as per your requirement.

HTML

<form id="form1" runat="server">
<div>
    <asp:GridView ID="gvNumber" AutoGenerateColumns="false" runat="server">
        <Columns>
            <asp:TemplateField HeaderText="Number">
                <ItemTemplate>
                    <asp:Label ID="LblNumber" Text='<%#Eval("Number")%>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Action">
                <ItemTemplate>
                    <asp:Button ID="btnAdd" Text="Add" OnClick="LBaddNumber_Click" runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <br />
    <br />
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <th>
                Number of count:<asp:Label ID="lblCount" Text="" runat="server" />
            </th>
        </tr>
        <tr>
            <td>
                <asp:TextBox ID="txtNumber" TextMode="MultiLine" Width="180px" Height="150px" runat="server" />
            </td>
        </tr>
    </table>
</div>
</form>

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 Button).NamingContainer as GridViewRow;
    string number = (row.FindControl("LblNumber") as Label).Text.Trim();
    txtNumber.Text = number.Trim() + Environment.NewLine + txtNumber.Text;
    lblCount.Text = (txtNumber.Text.Split('\r').Length - 1).ToString();
}

I hope works for you.