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• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.
• In UpdateInvoice method, instead of passing multiple properties, pass data model itself and use properties which are required inside 'Update1' method.
• You will need to put the cleanup of the connection in a Dispose() after making the data access class IDisposable.

• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.
• In UpdateInvoice method, instead of passing multiple properties, pass data model itself and use properties which are required inside Update1 method.
• You will need to put the cleanup of the connection in a Dispose() after making the data access class IDisposable. As pointed out by ratchet freak in another answer.

• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.
• In UpdateInvoice method, instead of passing multiple properties, pass data model itself and use properties which are required inside 'Update1' method.

    using CompanyX;
    using CompanyX.DataAccess.CompanyXTableAdapters;

    namespace CompanyX.DataAccess
    {
        public class CompanyXDataAccess
        {
            InvoiceTableAdapter ta = new InvoiceTableAdapter();
            SqlConnection readerConnection = new SqlConnection(DB.GetReaderConnectionString());
            SqlConnection writerConnection = new SqlConnection(DB.GetWriterConnectionString());
        
            public InvoiceDataTable GetInvoiceByID(int ID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.GetByID(ID);
                }
            }

            public InvoiceDataTable GetInvoice(string userSID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.Get(userSID);
                }
            }

            public void DeleteInvoice(int ID)
            {
                using (ta.Connection = writerConnection)
                {
                    ta.Delete1(ID);
                }
            }

            public void UpdateInvoice(Invoice m)
            {
                using (ta.Connection = writerConnection)
                {  
                     ta.Update1(m);
                }
            }
        }
    }

• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.
• In UpdateInvoice method, instead of passing multiple properties, pass data model itself and use properties which are required inside 'Update1' method.
• You will need to put the cleanup of the connection in a Dispose() after making the data access class IDisposable.

    using CompanyX;
    using CompanyX.DataAccess.CompanyXTableAdapters;

    namespace CompanyX.DataAccess
    {
        public class CompanyXDataAccess : IDisposable
        {
            InvoiceTableAdapter ta = new InvoiceTableAdapter();
            SqlConnection readerConnection = new SqlConnection(DB.GetReaderConnectionString());
            SqlConnection writerConnection = new SqlConnection(DB.GetWriterConnectionString());
        
            public InvoiceDataTable GetInvoiceByID(int ID)
            {
                ta.Connection = readerConnection;
                return ta.GetByID(ID);
            }

            public InvoiceDataTable GetInvoice(string userSID)
            {
                ta.Connection = readerConnection;
                return ta.Get(userSID);
            }

            public void DeleteInvoice(int ID)
            {
                ta.Connection = writerConnection;
                ta.Delete1(ID);
            }

            public void UpdateInvoice(Invoice m)
            {
                ta.Connection = writerConnection                                     
                ta.Update1(m);
            }

            public void Dispose()
            {
                // Do clean up
            }
        }
    }

• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.

    using CompanyX;
    using CompanyX.DataAccess.CompanyXTableAdapters;

    namespace CompanyX.DataAccess
    {
        public class CompanyXDataAccess
        {
            InvoiceTableAdapter ta = new InvoiceTableAdapter();
            SqlConnection readerConnection = new SqlConnection(DB.GetReaderConnectionString());
            SqlConnection writerConnection = new SqlConnection(DB.GetWriterConnectionString());
        
            public InvoiceDataTable GetInvoiceByID(int ID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.GetByID(ID);
                }
            }

            public InvoiceDataTable GetInvoice(string userSID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.Get(userSID);
                }
            }

            public void DeleteInvoice(int ID)
            {
                using (ta.Connection = writerConnection)
                {
                    ta.Delete1(ID);
                }
            }

            public void UpdateInvoice(Invoice m)
            {
                using (ta.Connection = writerConnection)
                {  
                    ta.Update1(m.ID, ... n numbers of params ..., m.ModifiedBy);
                }
            }
        }
    }

• Use namespace CompanyX instead of using in every method's return type.
• Create single instance of InvoiceTableAdapter and SqlConnection and reuse it.
• In UpdateInvoice method, instead of passing multiple properties, pass data model itself and use properties which are required inside 'Update1' method.

    using CompanyX;
    using CompanyX.DataAccess.CompanyXTableAdapters;

    namespace CompanyX.DataAccess
    {
        public class CompanyXDataAccess
        {
            InvoiceTableAdapter ta = new InvoiceTableAdapter();
            SqlConnection readerConnection = new SqlConnection(DB.GetReaderConnectionString());
            SqlConnection writerConnection = new SqlConnection(DB.GetWriterConnectionString());
        
            public InvoiceDataTable GetInvoiceByID(int ID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.GetByID(ID);
                }
            }

            public InvoiceDataTable GetInvoice(string userSID)
            {
                using (ta.Connection = readerConnection)
                {
                    return ta.Get(userSID);
                }
            }

            public void DeleteInvoice(int ID)
            {
                using (ta.Connection = writerConnection)
                {
                    ta.Delete1(ID);
                }
            }

            public void UpdateInvoice(Invoice m)
            {
                using (ta.Connection = writerConnection)
                {  
                    ta.Update1(m);
                }
            }
        }
    }