C# - Use A String Collection To Pass In The Output From The Textboxes To A Parameterized Query

I have a dynamic gridview which can be used to add new rows of two columns to specify a start date and an end date. I am trying to use a string collection to pass in the output from the textboxes to a parameterized query as shows. I am getting an incorrect syntax near 9 error. How can I make this work? I need to use the stringbuilder.

protected void btnSaveIterations_Click(object sender, EventArgs e)
{
int rowIndex = 0;
StringCollection sc = new StringCollection();
if (ViewState["CurrentTable"] != null)
{
DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
if (dtCurrentTable.Rows.Count > 0)
{
for (int i = 1; i <= dtCurrentTable.Rows.Count; i++)
{
//extract the TextBox values
TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("start_iteration");
TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[2].FindControl("end_iteration");
start_date = box1.Text;
end_date = box2.Text;...............