Display GridView Row details in TextBox when CheckBox is checked (OnCheckChanged) in ASP.Net

Hi AliYilmaz,

I have created one sample that full-fill your requirement.

HTML

<div>
    <table>
        <tr>
            <td>
                Id
            </td>
            <td>
                <asp:Label ID="lblId" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Name
            </td>
            <td>
                <asp:TextBox ID="txtName" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                Country
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlCountry">
                     <asp:ListItem Text="-Select-" Value="0" />
                     <asp:ListItem Text="India" Value="1" />
                     <asp:ListItem Text="Turkey" Value="2" />
                     <asp:ListItem Text="France" Value="3" />
                        <asp:ListItem Text="Russia" Value="4" />
                 </asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td colspan="2">
                <asp:Button Text="Update" runat="server" OnClick="UpdateRecord" />
            </td>
        </tr>
    </table>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:CheckBox ID="chkcsec" runat="server" OnCheckedChanged="GetSelectedRecord" AutoPostBack="true" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:BoundField DataField="Id" HeaderText="Id" ItemStyle-Width="30" />
            <asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="150" />
            <asp:BoundField DataField="Country" HeaderText="Country" ItemStyle-Width="150" />
        </Columns>
    </asp:GridView>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id", typeof(int)),
            new DataColumn("Name", typeof(string)),
            new DataColumn("Country",typeof(string)) });
        dt.Rows.Add(1, "John Hammond", "Turkey");
        dt.Rows.Add(2, "Mudassar Khan", "India");
        dt.Rows.Add(3, "Suzanne Mathews", "France");
        dt.Rows.Add(4, "Robert Schidner", "Russia");
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}
protected void GetSelectedRecord(object sender, EventArgs e)
{
    CheckBox chk = sender as CheckBox;
    GridViewRow row = (GridViewRow)(chk.NamingContainer);
    int index = row.RowIndex;
    for (int i = 0; i < GridView1.Rows.Count; i++)
    {
        if (i != index)
            ((CheckBox)GridView1.Rows[i].FindControl("chkcsec")).Checked = false;
    }
    if (chk.Checked)
    {
        lblId.Text = row.Cells[1].Text;
        txtName.Text = row.Cells[2].Text;
        ddlCountry.ClearSelection();
        ddlCountry.Items.FindByText(row.Cells[3].Text).Selected = true;
    }
    else
    {
        lblId.Text = string.Empty;
        txtName.Text = string.Empty;
        ddlCountry.SelectedIndex = 0;
    }
}
protected void UpdateRecord(object sender, EventArgs e)
{
    //Update code goes here.
}

Screenshot