Display GridView rows value in multiline TextBox on Button click using C# in ASP.Net

Hi nedash,

Refer the below sample.

HTML

<div>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:TemplateField HeaderText="Number" ItemStyle-Width="200px" ItemStyle-VerticalAlign="Middle"
                ShowHeader="False">
                <ItemTemplate>
                    <asp:Label ID="LblName" runat="server" Text='<%#Eval("Number")%>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <asp:Button ID="Button7" runat="server" Text="Button" OnClick="Button7_Click" />
    <asp:TextBox ID="Txttest" runat="server" TextMode="MultiLine" Height="100px" Style="text-align: right"></asp:TextBox>
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[] { new DataColumn("Number", typeof(int)) });
        dt.Rows.Add(123654769);
        dt.Rows.Add(698547965);
        dt.Rows.Add(231564789);
        dt.Rows.Add(659874563);
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}

protected void Button7_Click(object sender, EventArgs e)
{
    string numbers = string.Empty;
    foreach (GridViewRow row in GridView1.Rows)
    {
        numbers += (row.FindControl("LblName") as Label).Text.Trim() + Environment.NewLine;
    }
    Txttest.Text = numbers.Trim();
}

Output

123654769 698547965 231564789 659874563