AppendFormat

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;

                //get the values from the TextBoxes
                //then add it to the collections with a comma "," as the delimited values

                sc.Add(proj_id + "," + start_date + "," + end_date);

                rowIndex++;

            }

            //Call the method for executing inserts

            InsertRecords(sc);
            Response.Redirect(Request.Url.ToString());

            //r.Close();
            //conn.Close();

        }
    }
}

private void InsertRecords(StringCollection sc)
{

    SqlConnection conn = new SqlConnection(GetConnectionString());
    StringBuilder sb = new StringBuilder(string.Empty);

    string[] splitItems = null;

    foreach (string item in sc)
    {
        const string sqlStatement = "INSERT INTO Iterations (ProjectID, StartDate, EndDate) VALUES (@ProjectID, @StartDate, @EndDate)";

        if (item.Contains(","))
        {

            splitItems = item.Split(",".ToCharArray());
            sb.AppendFormat("{0}('{1}','{2}','{3}'); ", sqlStatement, splitItems[0], splitItems[1], splitItems[2]);

        }
    }

    try
    {

        conn.Open();
        SqlCommand cmd = new SqlCommand(sb.ToString(), conn);
        SqlCommand cmd2 = new SqlCommand(sql, conn);

        cmd.Parameters.Add("@ProjectID", SqlDbType.Int);
        cmd.Parameters.Add("@StartDate", SqlDbType.DateTime);
        cmd.Parameters.Add("@EndDate", SqlDbType.DateTime);

        System.Globalization.CultureInfo ci = new System.Globalization.CultureInfo("en-GB");

        cmd.Parameters["@ProjectID"].Value = proj_id;
        cmd.Parameters["@StartDate"].Value = Convert.ToDateTime(start_date, ci);
        cmd.Parameters["@EndDate"].Value = Convert.ToDateTime(end_date, ci);

        cmd.CommandType = CommandType.Text;
        cmd.ExecuteNonQuery();

        ...
}