data table in ASP.NET 2.0(C#)

//Add a GridView control, one lable controls, four textbox controls and button control in the page .

DataTable myDt;
private DataTable CreateDataTable()
{
DataTable myDataTable = new DataTable();

DataColumn myDataColumn;

myDataColumn = new DataColumn();
myDataColumn.DataType = Type.GetType("System.String");
myDataColumn.ColumnName = "id";
myDataTable.Columns.Add(myDataColumn);

myDataColumn = new DataColumn();
myDataColumn.DataType = Type.GetType("System.String");
myDataColumn.ColumnName = "username";
myDataTable.Columns.Add(myDataColumn);

myDataColumn = new DataColumn();
myDataColumn.DataType = Type.GetType("System.String");
myDataColumn.ColumnName = "firstname";
myDataTable.Columns.Add(myDataColumn);

myDataColumn = new DataColumn();
myDataColumn.DataType = Type.GetType("System.String");
myDataColumn.ColumnName = "lastname";
myDataTable.Columns.Add(myDataColumn);

return myDataTable;
}
private void AddDataToTable(string id,string username, string firstname, string lastname, DataTable myTable)
{
DataRow row;

row = myTable.NewRow();

row["id"] = id;
row["username"] = username;
row["firstname"] = firstname;
row["lastname"] = lastname;

myTable.Rows.Add(row);
}
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
myDt = new DataTable();
myDt = CreateDataTable();
Session["myDatatable"] = myDt;

this.GridView1.DataSource = ((DataTable)Session["myDatatable"]).DefaultView;
this.GridView1.DataBind();
}
}

protected void btnAdd_Click(object sender, EventArgs e)
{
if (txtUserName.Text.Trim() == "")
{
this.lblTips.Text = "You must fill a username.";
return;
}
else
{
AddDataToTable(this.TextBox1.Text.Trim(),this.txtUserName.Text.Trim(), this.txtFirstName.Text.Trim(), this.txtLastName.Text.Trim(), (DataTable)Session["myDatatable"]);

GridView1.DataSource = ((DataTable)Session["myDatatable"]).DefaultView;
GridView1.DataBind();

this.txtFirstName.Text = "";
this.txtLastName.Text = "";
this.txtUserName.Text = "";
this.TextBox1.Text = "";
this.lblTips.Text = "";
}
}