Validate Gridview Field

Please refer this

 HTML

<asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
    runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:TemplateField HeaderText="Select">
            <ItemTemplate>
                <asp:CheckBox ID="chkBxSelect" runat="server" AutoPostBack="true" OnCheckedChanged="CheckBox1_CheckedChanged" />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField DataField="Id" HeaderText="Id" ItemStyle-Width="30" />
        <asp:BoundField DataField="SubjetcName" HeaderText="Name" ItemStyle-Width="150" />
    </Columns>
</asp:GridView>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2] { new DataColumn("Id", typeof(int)),
                    new DataColumn("SubjetcName", typeof(string))});
        dt.Rows.Add(1, "Medical Record");
        dt.Rows.Add(2, "ProgrammingLang");
        dt.Rows.Add(3, "Nursing");
        dt.Rows.Add(4, "Nursing");
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}  

protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
{
    bool b = false;
    GridViewRow row = (sender as CheckBox).NamingContainer as GridViewRow;
    if ((row.FindControl("chkBxSelect") as CheckBox).Checked)
    {
        if (ViewState["PreviousValue"] != null && ViewState["PreviousRow"] != null)
        {

            if (ViewState["PreviousValue"].ToString() != row.Cells[2].Text)
            {
                foreach (GridViewRow gvRow in this.GridView1.Rows)
                {
                    if (ViewState["PreviousValue"].ToString() == gvRow.Cells[2].Text && Convert.ToInt32(ViewState["PreviousRow"]) != gvRow.RowIndex && !(gvRow.FindControl("chkBxSelect") as CheckBox).Checked)
                    {
                        b = true;
                    }
                }
                if (b)
                {
                    string message = "Please Select the same subject";
                    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + message + "')", true);
                }
                else
                {
                    // If you dont want to show this message you can hide it.
                    string message = "No more same subject found";
                    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + message + "')", true);
                }
            }
        }

        string checkedSubject = row.Cells[2].Text;
        ViewState["PreviousValue"] = checkedSubject;
        ViewState["PreviousRow"] = row.RowIndex;
    }
}