SqlException unhandled : Incorrect syntax near '12'

Dear users,

I am having problem with inserting date into SQL server. My code is as following:

        protected void btnSave_Click(object sender, EventArgs e)

        {
            using (SqlConnection con = new SqlConnection("Data Source=MEHDI-PC\\SQLEX;Initial Catalog=PIMS;Integrated Security=True"))
            {
                using (SqlCommand cmd = new SqlCommand())
                {

                    string DatePrinted = "";

                    if (txtDatePrinted.Text != "")
                        DatePrinted = DateTime.ParseExact(txtDateReceived.Text, "G", CultureInfo.GetCultureInfo("en-US")).ToString("yyyy-MM-dd");
                    else
                        DatePrinted = "NULL";

                    string DateReceived = "";

                    if (txtDateReceived.Text != "")
                        DateReceived = DateTime.ParseExact(txtDateReceived.Text, "G", CultureInfo.GetCultureInfo("en-US")).ToString("yyyy-MM-dd");
                    else
                        DateReceived = "NULL";

                    string DueDate = "";

                    if (txtDueDate.Text != "")
                        DueDate = DateTime.ParseExact(txtDateReceived.Text, "G", CultureInfo.GetCultureInfo("en-US")).ToString("yyyy-MM-dd");
                    else
                        DueDate = "NULL";

                    string ActualDate = "";

                    if (txtActualDate.Text != "")
                        ActualDate = DateTime.ParseExact(txtDateReceived.Text, "G", CultureInfo.GetCultureInfo("en-US")).ToString("yyyy-MM-dd");
                    else
                        ActualDate = "NULL";

                    String sql = "Insert into dbo.Documents(Ref,Subject,Src,Dst,Medium,Date_Prin
ted,Date_Received,Document_Type,Action_Required,Du
e_Date,Actual_Date,[Content],Tag,Issue_No,Attachme
nt,Notes,Assigned_To,Reply_Ref,Priority,Status,Res
ponse) values ('" + txtRef.Text + "','" + txtSubject.Text + "','" + ddlSource.Text + "' ,'" + ddlDestination.Text + "','" + ddlMedium.Text + "','" + DatePrinted + "','" + DateReceived + "','" + ddlDocumentType.Text + "','" + cbxAction.Checked + "','" + DueDate + "','" + ActualDate + "','" + txtContent.Text + "','" + txtTag.Text + "','" + txtIssue.Text + "','" + txtAttachment.Text + "','" + txtNotes.Text + "','" + ddlAssignedTo.Text + "','" + txtReplyRef.Text + "','" + ddlPriority.Text + "','" + ddlStatus.Text + "','" + ddlResponse.Text + "')";
  
                    cmd.Connection = con;
                    cmd.CommandText = sql;

                    con.Open();
                    //dataset object to get all select statement results
                    //DataSet ds = new DataSet();

                    //sql dataadoptor to fill dataset
                    cmd.ExecuteNonQuery();
                }
                if (con.State == ConnectionState.Open)
                {
                    con.Close();
                }

                MultiView1.SetActiveView(vRecord);
                txtRef.Text = string.Empty;
                txtSubject.Text = string.Empty;
                ddlSource.Text = string.Empty;
                ddlDestination.Text = string.Empty;
                ddlMedium.Text = string.Empty;
                txtDatePrinted.Text = string.Empty;
                txtDateReceived.Text = string.Empty;
                ddlDocumentType.Text = string.Empty;
                cbxAction.Checked = false;
                txtDueDate.Text = string.Empty;
                txtActualDate.Text = string.Empty;
                txtContent.Text = string.Empty;
                txtTag.Text = string.Empty;
                txtIssue.Text = string.Empty;
                txtAttachment.Text = string.Empty;
                txtNotes.Text = string.Empty;
                ddlAssignedTo.Text = string.Empty;
                txtReplyRef.Text = string.Empty;
                ddlPriority.Text = string.Empty;
                ddlStatus.Text = string.Empty;
                ddlResponse.Text = string.Empty;
            }

            }

After I run this code, and enter any date value in the datefield, it throws this message:

SqlException unhandled : Incorrect syntax near '12'


I am failed to see anything wrong with my code. Please help me identify the problem.

You help will be much appreciated.

Regards