Populate (Bind) TextBox from database on ASP.Net Button Click using C# and VB.Net

hi guys.

l'm try to popupate some data from database but always fail. I want if the btnAddCariAnggota click will show just nama_anggota not show all row in database.

this is the tbl_anggota

CREATE TABLE [dbo].[tbl_anggota](
	[id_anggota] [varchar](50) NOT NULL,
	[nama_anggota] [varchar](50) NULL,
	[alamat] [varchar](350) NULL,
	[no_telpon] [varchar](50) NULL,
	[saldo] [bigint] NULL,
 CONSTRAINT [PK_tbl_anggota] PRIMARY KEY CLUSTERED 
(
	[id_anggota] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

this is .aspx

<div class="col-md-6">
    <div class="form-group">
        <label for="id_anggota">
            ID Anggota</label>
        <div class="input-group">
            <asp:TextBox ID="txtid_anggota" runat="server" CssClass="form-control" placeholder="Enter ID Anggota"></asp:TextBox>
            <div class="input-group-append">
                <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                    <ContentTemplate>
                        <asp:Button ID="btnAddCariAnggota" CssClass="btn btn-warning" runat="server" Text="Cari Data Anggota" OnClick="btnAddCariAnggota_Click" />
                    </ContentTemplate>
                </asp:UpdatePanel>
            </div>
        </div>
    </div>
</div>
<div class="col-md-6">
    <div class="form-group">
        <label for="nama_anggota">
            Nama Anggota</label>
        <asp:TextBox ID="txtnama_anggota" runat="server" CssClass="form-control" placeholder="Enter Nama Anggota" ReadOnly="true"></asp:TextBox>
    </div>
</div>

this code behind

protected void btnCariAnggota_Click(object sender, EventArgs e)
{
    using (SqlConnection con = new SqlConnection(koneksi))
    {
        using (SqlCommand sqlcmd = new SqlCommand("Select * from tbl_anggota where id_anggota='" + txtid_anggota.Text.Trim() + "'",con))
        {
            con.Open();
            sqlcmd.Connection = con;
            using (SqlDataReader rdr = sqlcmd.ExecuteReader())
            {
                while (rdr.Read())
                {
                    txtnama_anggota.Text = rdr["nama_anggota"].ToString();
                }
            }
            con.Close();
        }
    }
}

any help could be appriciate.