OleDbConnection connection = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + WorkDir + "Реестр.xls" + ";Extended Properties=\"Excel 8.0;HDR=Yes;\"");
                connection.Open();

                OleDbCommand comd = new OleDbCommand("SELECT * FROM [Лист1$] WHERE (No='" + TextBox6.Text + "')", connection);               
                if (comd.ExecuteNonQuery() > 0)
                {
                    ErrLabel.Text = "Документ с указанным номером существует";
                    flag = false;
                }
                else
                {
                    OleDbCommand command = new OleDbCommand("INSERT INTO [Лист1$] VALUES ('" + TextBox6.Text + "','" + Date + "','" + TextBox2.Text + "','" + TextBox3.Text + "','" + TextBox4.Text + "','" + TextBox5.Text + "','" + TextBox7.Text + "');", connection);
                    command.ExecuteNonQuery();   
                }

OleDbConnection connection = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + WorkDir + "Реестр.xls" + ";Extended Properties=\"Excel  8 . 0 ;HDR=Yes;\"");
                connection.Open();

                OleDbDataAdapter comd = new OleDbDataAdapter("SELECT * FROM [Лист1$] WHERE (No='" + TextBox6.Text + "')", connection);
                DataSet dataset = new DataSet();
                comd.Fill(dataset);
                if (dataset.Tables[ 0 ].Rows.Count >  0 )
                {
                    ErrLabel.Text = "Документ с указанным номером существует";
                    flag = false;
                }
                else
                {
                    OleDbCommand command = new OleDbCommand("INSERT INTO [Лист1$] VALUES ('" + TextBox6.Text + "','" + Date + "','" + TextBox2.Text + "','" + TextBox3.Text + "','" + TextBox4.Text + "','" + TextBox5.Text + "','" + TextBox7.Text + "');", connection);
                    command.ExecuteNonQuery();   
                }