CRUD Using Parameter SQL-Helper

<form id="form1" runat="server">
<div>
<h1>Registration Form</h1>
<asp:Label ID="lblId" runat="server" Visible="false" ></asp:Label>
<table style="font-size:18px">
<tr>
  <td> Name</td>
  <td><asp:TextBox ID="txtName" runat="server"></asp:TextBox></td>
</tr>
<tr>
  <td>Subject:</td>
  <td><asp:DropDownList ID="ddlSub" runat="server">
    <asp:ListItem>--Select Subjects-- </asp:ListItem>
    <asp:ListItem Value="1">Physics </asp:ListItem>
    <asp:ListItem Value="2">Chemistry </asp:ListItem>
    <asp:ListItem Value="3">Biology </asp:ListItem>
    <asp:ListItem Value="4">Maths </asp:ListItem>
    <asp:ListItem Value="5">English </asp:ListItem>
    <asp:ListItem Value="6">Computer </asp:ListItem>
    <asp:ListItem Value="7">History & Civics </asp:ListItem>
    </asp:DropDownList></td>
</tr>
<tr>
  <td>Email:</td>
  <td><asp:TextBox ID="txtEmail" runat="server"></asp:TextBox></td>
  </tr>
<tr>
  <td>Gender:</td>
  <td>
    <asp:RadioButtonList ID="rbtnListGender" runat="server" RepeatDirection="Horizontal">
        <asp:ListItem Text="Male" Value="Male"></asp:ListItem>
        <asp:ListItem Text="Female" Value="Female"></asp:ListItem>
    </asp:RadioButtonList>
  </td>
</tr>
<tr>
  <td></td>
  <td>
    <asp:Button ID="btnSave" runat="server" Text="Save" OnClick="btnSave_Click" />
    <asp:Button ID="btnUpdate" runat="server" Text="Update" OnClick="btnUpdate_Click"
        Visible="false" />
    <asp:Button ID="btnClear" runat="server" Text="Clear" OnClick="btnClear_Click" />
  </td>
</tr>
</table>
<h2>Registration Data</h2>
<table border="1" style="border-collapse:collapse">
<tr >
  <th>Name</th>
  <th>Email</th>
  <th>Gender</th>
  <th>Subject</th>
  <th colspan="2">Action</th>
</tr>
<asp:Repeater ID="rptStudents" runat="server" OnItemCommand="rptStudents_ItemCommand">
  <ItemTemplate>
   <tr>
    <td><%# Eval("Name") %></td>
    <td><%# Eval("Email") %></td>
    <td><%# Eval("Gender") %></td>
    <td><%# Eval("Subject") %></td>
    <td>
        <asp:LinkButton ID="lbtnEdit" runat="server" CommandName="Edit"
                CommandArgument='<%# Eval("Id") %>'>Edit</asp:LinkButton>
    </td>
    <td>
        <asp:LinkButton ID="lbtnDelete" runat="server" CommandName="Delete"
                    CommandArgument='<%# Eval("Id") %>'>Delete</asp:LinkButton>
    </td>
   </tr>
  </ItemTemplate>
</asp:Repeater>
</table>
</div>
</form>

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        bindStudents();
    }
}

protected void btnSave_Click(object sender, EventArgs e)
{ 
    SqlHelper obj = new SqlHelper();
    string[] cols = { "Name", "Gender", "Email", "SubjectId" };
    object[] vals = { txtName.Text, (rbtnListGender.SelectedValue), txtEmail.Text, ddlSub.SelectedValue };
    obj.insertValIntoTable("tblStudents", cols,vals);
    Response.Write("<script>alert('Data Saved Successfully')</script>");
    bindStudents();
    btnClear_Click(sender, e);
}

protected void rptStudents_ItemCommand(object source, RepeaterCommandEventArgs e)
{
  SqlHelper obj = new SqlHelper();
  if (e.CommandName == "Edit")
  {
    DataTable dt = obj.getDataTable("SELECT * FROM tblStudents WHERE Id = " + e.CommandArgument);
    lblId.Text = dt.Rows[0]["Id"].ToString();
    txtName.Text = dt.Rows[0]["Name"].ToString();
    txtEmail.Text = dt.Rows[0]["Email"].ToString();
    rbtnListGender.SelectedValue = dt.Rows[0]["Gender"].ToString();
    ddlSub.SelectedValue = dt.Rows[0]["SubjectId"].ToString();
    btnSave.Visible = false;
    btnUpdate.Visible = true;
  }
  else if (e.CommandName == "Delete")
  {
    obj.ExecuteNonQuery("DELETE FROM tblStudents WHERE Id = " + e.CommandArgument);
    Response.Write("<script>alert('Data Delete Successfully')</script");
                        bindStudents();
  }
}

protected void btnUpdate_Click(object sender, EventArgs e)
{
    SqlHelper obj = new SqlHelper();
    string[] cols = { "Name", "Gender", "Email", "SubjectId" };
    object[] vals = { txtName.Text, (rbtnListGender.SelectedValue), txtEmail.Text, ddlSub.SelectedValue};
    obj.updateValIntoTable("tblStudents", cols, vals, "Id", lblId.Text);
    Response.Write("<script>alert('Data Update Successfully')</script>");
    bindStudents();
    btnClear_Click(sender, e);
}

protected void btnClear_Click(object sender, EventArgs e)
{
    lblId.Text = ""; txtName.Text = ""; txtEmail.Text = ""; rbtnListGender.ClearSelection();
    ddlSub.SelectedIndex = 0; btnSave.Visible = true; btnUpdate.Visible = false;
}

protected void bindStudents()
{
    SqlHelper obj = new SqlHelper();
    rptStudents.DataSource = obj.getDataTable("SELECT st.Id, st.Name, st.Email, st.Gender, sub.Sub_Name AS 
            Subject" +" FROM tblStudents st left outer JOIN MasterSubjects sub ON sub.Id = st.SubjectId");

    rptStudents.DataBind();
}

public class SqlHelper
{
    SqlConnection sqlconn = new SqlConnection
          (@"Data Source= ; Initial Catalog= ; User ID= ; Password= ");

public void insertValIntoTable(string tablename, string[] columnNames, object[] value)
{ 
    int colsLength = columnNames.Length;
    string qry = "INSERT INTO "+ tablename+"(";
    for (int i = 0; i <= colsLength - 1; i++)
    {
        qry = qry + "" + columnNames[i].Trim()+",";
    }
    qry = qry.Substring(0, qry.Length-1) + ") VALUES(" ;
    for (int i = 0; i <= colsLength - 1; i++)
    {
        qry = qry + "@" + columnNames[i].Trim() + ",";
    }
    qry = qry.Substring(0, qry.Length - 1) + ")";
    SqlCommand cmd = new SqlCommand(qry, sqlconn);
    for (int i = 0; i <= colsLength - 1; i++)
    {
        cmd.Parameters.AddWithValue("@"+ columnNames[i].Trim(),
        (value[i] == null || value[i] == "") ? DBNull.Value : value[i]); 
    } 
    sqlconn.Open();
    cmd.ExecuteNonQuery();
    sqlconn.Close(); 
}

public void updateValIntoTable(string tablename, string[] columnNames,
                      object[] value,string condCol, object condValue)
{
    int colsLength = columnNames.Length;
    string qry = "update " + tablename + "  set ";
    for (int i = 0; i <= colsLength - 1; i++)
    {
        qry = qry + "" + columnNames[i].Trim() + "=@"+ columnNames[i].Trim() + ",";
    } 
    qry = qry.Substring(0, qry.Length - 1) + " where "+ condCol+"=@"+ condCol;

    SqlCommand cmd = new SqlCommand(qry, sqlconn);
    for (int i = 0; i <= colsLength - 1; i++)
    {
        cmd.Parameters.AddWithValue("@" + columnNames[i].Trim(),
        (value[i] == null || value[i] == "") ? DBNull.Value : value[i]);
    }
    cmd.Parameters.AddWithValue("@" + condCol, 
    (condValue == null || condValue == "") ? DBNull.Value : condValue);
    sqlconn.Open();
    cmd.ExecuteNonQuery();
    sqlconn.Close();
  }
}