What Is The Best Practice For Designing 3-tier Architecture?

Sep 21, 2007

I have created an application that I intended to be 3-tier, but I am not sure if I did it properly.  I constructed it like this:  I created a DLL that contains methods that validate the passed parameters, checks the data against business rules, and issues ADO.NET methods to access the data.  The ASP.NET presentation layer uses Object Data Sources to link to these methods.  With this architecture I consider the ASP.NET pages to be the presentation layer, the DLL to be the business layer, and the database itself to be the data layer.

Now I am wondering if the standard practice is to have a further division?  In this case, there would be a business layer DLL whose only purpose is to validate the parameters passed to it by the presentation layer, and to do business rules checking.  There would also be a data layer DLL whose purpose is to accept parameters from the business layer and issue ADO.NET methods to access the database.  In this scenario the database itself would be considered part of the data layer, or not considered to be a layer at all.

Either one will work, but I would like to implement the architecture that is most accepted, and allows the easiest maintenance.  What is the best practice for designing a 3-tier architecture?

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Handling Null Fields With Three-tier Architecture

Nov 28, 2006

I using Visual Web Designer Express (with Visual Basic), with a SQL Server 2000 database.   I have a prototype application running satisfactorily using code that builds queries, but I'm now trying to rebuild the application "properly" using three-tier architecture.  I have been following the principles of Scott Mitchell's tutorials.  I have created an database .xsd with a table adaptor, and replaced the rather grotty query-building code in the business layer with better code referencing the table adaptor.   Thus where the first version had code: -
        Dim queryString As String = "SELECT * FROM NZGDB_User WHERE USRid = '" & Userid & "'"        Dim message As String = ""        Dim Found As Boolean = False        Try            Using connection As New SqlConnection(GDB_AppSettings.Connection)                Dim command As New SqlCommand(queryString, connection)                connection.Open()
                Dim reader As SqlDataReader = command.ExecuteReader()
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the new version is much more elegant: -
        Dim taUser As New NZGDBTableAdapters.NZGDB_UserTableAdapter()
        Dim tbUser As NZGDB.NZGDB_UserDataTable = taUser.GetUserByUserid(userid)        If tbUser.Count <> 1 Then   '   Anything other than 0 or 1 should be impossible            Return "User not found"        End If
        Dim trUser As NZGDB.NZGDB_UserRow = tbUser(0)        _USRid = userid        _USRPassword = password        _USREmail = trUser.USREmail        _USRTitle = trUser.USRTitle        _USRName = trUser.USRName        _USRRole = trUser.USRRole        _USRAgreedConditions = trUser.USRAgreedConditions
However, there is a problem.  The database field USRAgreedConditions is a Datetime field that can be null.  The new version works perfectly when it is NOT null, but throws an exception: -
System.Data.StrongTypingException was unhandled by user code  Message="The value for column 'USRAgreedConditions' in table 'NZGDB_User' is DBNull."  Source="App_Code.wt2vzoc1"  ....
There is no point in writing: -        If Not IsDBNull(trUser.USRAgreedConditions) Then            _USRAgreedConditions = trUser.USRAgreedConditions        End Ifbecause the exception occurs within the automatically-created code in the data access layer.  I tried changing the Nullvalue property of the field USRAgreedConditions in the table adaptor, but the only valid option is (Throw Exception) unless the field is a String.  Of course USRAgreedConditions is a Datetime field, so I can't change the property.
It seems that my only options are: -1.   To stick with the old query-building code.   But this doesn't give me the advantages of a proper 3-tier architecture2.   To change the generated code in wt2vzoc.  This MUST be a bad idea - surely I should leave this code untouched.  Besides, what if the table adaptor has to be regenerated when I change the table design?3.   Code a Try block within the business layer: -    Try         _USRAgreedConditions = trUser.USRAgreedConditions    Catch ex As Exception         _USRAgreedConditions = Nothing    End Try
This seems to work OK, but seems less elegant than the original code in the old version: -       If IsDBNull(reader("USRAgreedConditions")) = False Then            _USRAgreedConditions = reader("USRAgreedConditions")       End IfIs there a better way?

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 using System;

namespace BaResearch.Data.msSql
{
interface ISqlDataObject
{
System.Data.SqlClient.SqlConnection Connection { get; }
string ConnectionString { get; set; }
}
}

 ___________________________________using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

namespace BaResearch.Data.msSql
{

public class SqlDataObject : ISqlDataObject
{

private string _ConnectionString;

public SqlConnection Connection
{
get { return new SqlConnection(_ConnectionString); }
}

public string ConnectionString
{
get
{
return _ConnectionString;
}
set
{
_ConnectionString = value;
}
}

}

}

 __________________________________using System;

namespace BaResearch.Data
{
interface IBid
{
string Client { get; set; }
string Contact { get; set; }
string Sponsor { get; set; }
string Priority { get; set; }
string BidStatus { get; set; }
}
}

 _____________________________________________using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

namespace BaResearch.Data.msSql
{

public class Bid : SqlDataObject, IBid
{

#region private member & variables

private int _BidID;
private string _DateCreated;
private string _CreatedBy;

private string _Client;
private string _Contact;
private string _Sponsor;
private string _Priority;
private string _BidStatus;

private SqlDataAdapter sqlda = new SqlDataAdapter();
private SqlCommand _command;
private SqlParameter[] _parameters =
{
new SqlParameter("@client",SqlDbType.NVarChar),
new SqlParameter("@contact",SqlDbType.NVarChar),
new SqlParameter("@sponsor",SqlDbType.NVarChar),
new SqlParameter("@priority",SqlDbType.NVarChar),
new SqlParameter("@bidstatus",SqlDbType.NVarChar),
};

#endregion

#region public member variables - stored procedures

//private string sp_Create = "bids_sp_insert";
//private string sp_Update_ByID = "bids_sp_updatebyid";
//private string sp_Delete_ByID = "bids_sp_deletebyid";
//private string sp_Select_All = "bids_sp_selectall";
//private string sp_Select_ByID = "bids_sp_selectbyid";
//private string sp_Select_ByValue = "bids_sp_selectbyvalue";

#endregion

#region constructors and destructors

public Bid()
{
if (this.ConnectionString == null)
{
this.ConnectionString = ConfigurationManager.ConnectionStrings["SQL.ConnectionString"].ConnectionString;
}
else
{
new Bid(this.ConnectionString);
}
}

public Bid(string ConnectionString)
{
this.ConnectionString = ConnectionString;
}

#endregion

#region methods & events

/// <summary>
/// Attach parameters to an SqlCommand
/// </summary>
/// <param name="sqlcmd">SqlCommand</param>
private void attachparameters(SqlCommand sqlcmd)
{
sqlcmd.Parameters.Clear();
foreach (SqlParameter param in _parameters)
{
sqlcmd.Parameters.Add(param);
}
}

/// <summary>
/// Assign value to the parameters
/// </summary>
/// <param name="sqlcmd">SqlCommand</param>
/// <param name="value">Client object</param>
private void assignparametervalues(SqlCommand sqlcmd, Bid value)
{
this.attachparameters(sqlcmd);
// todo: assign parameter values here
sqlcmd.Parameters[0].Value = value.Client;
sqlcmd.Parameters[1].Value = value.Contact;
sqlcmd.Parameters[2].Value = value.Sponsor;
sqlcmd.Parameters[3].Value = value.Priority;
sqlcmd.Parameters[4].Value = value.BidStatus;
}

public virtual void Create(Bid value)
{
_command = new SqlCommand();
_command.Connection = this.Connection;
_command.CommandText = @"INSERT INTO bids (client, contact, sponsor, priority, bidstatus, createdby)
VALUES (@client, @contact, @sponsor, @priority, @bidstatus, @createdby);";
_command.CommandType = CommandType.Text;

try
{
sqlda.InsertCommand = _command;
this.assignparametervalues(sqlda.InsertCommand, value);
sqlda.InsertCommand.Parameters.AddWithValue("@createdby", value.CreatedBy);
sqlda.InsertCommand.Parameters.Add("@bidid", SqlDbType.Int);
sqlda.InsertCommand.Parameters["@bidid"].Direction = ParameterDirection.ReturnValue;
sqlda.InsertCommand.Connection.Open();
sqlda.InsertCommand.ExecuteNonQuery();
sqlda.InsertCommand.Connection.Close();
sqlda.InsertCommand.Connection.Dispose();
}
catch (Exception e)
{
throw new Exception(e.Message);
}
}

public virtual int Update(Bid value)
{
int result;

_command = new SqlCommand();
_command.Connection = this.Connection;
_command.CommandText = @"UPDATE bids SET client=@client, contact=@contact, sponsor=@sponsor,
priority=@priority, bidstatus=@bidstatus
WHERE bidid=@bidid";
_command.CommandType = CommandType.Text;

try
{
sqlda.UpdateCommand = _command;
using (SqlCommand _cmd = sqlda.UpdateCommand)
{
this.assignparametervalues(_cmd, value);
_cmd.Parameters.AddWithValue("@bidid", value.BidID);
_cmd.Connection.Open();
result = _cmd.ExecuteNonQuery();
_cmd.Connection.Close();
_cmd.Connection.Dispose();
}
}
catch (Exception e)
{
throw new Exception(e.Message);
}

return result;
}

public virtual void Delete(Bid value)
{
_command = new SqlCommand();
_command.Connection = this.Connection;
_command.CommandText = @"DELETE bids WHERE bidid=@bidid;";
_command.CommandType = CommandType.Text;

try
{
sqlda.DeleteCommand = _command;
using (SqlCommand _cmd = sqlda.DeleteCommand)
{
_cmd.Parameters.AddWithValue("@bidid", value.BidID);
_cmd.Connection.Open();
_cmd.ExecuteNonQuery();
_cmd.Connection.Close();
_cmd.Connection.Dispose();
}
}
catch (Exception e)
{
throw new Exception(e.Message);
}
}

public virtual DataSet List(string filter)
{
DataSet result = new DataSet();

_command = new SqlCommand();
_command.Connection = this.Connection;
_command.CommandText = @"SELECT * FROM bids_view_listall
WHERE (clientname like @value) or (contactname like @value)
or (sponsor like @value) ORDER BY datecreated DESC";
_command.CommandType = CommandType.Text;

try
{
if (filter != null)
{
filter = filter.Replace(" ", "%");
}
sqlda.SelectCommand = _command;
sqlda.SelectCommand.Parameters.AddWithValue("@value", "%" + filter + "%");
sqlda.Fill(result);
}
catch (Exception e)
{
throw new Exception(e.Message);
}

return (DataSet)result;
}

#endregion

#region IBid Members

public string Client
{
get
{
return _Client;
}
set
{
_Client = value;
}
}

public string Contact
{
get
{
return _Contact;
}
set
{
_Contact = value;
}
}

public string Sponsor
{
get
{
return _Sponsor;
}
set
{
_Sponsor = value;
}
}

public string Priority
{
get
{
return _Priority;
}
set
{
_Priority = value;
}
}

public string BidStatus
{
get
{
return _BidStatus;
}
set
{
_BidStatus = value;
}
}

#endregion

#region Properties

public int BidID
{
get
{ return _BidID; }
set
{ _BidID = value; }
}

public string DateCreated
{
get
{
return _DateCreated;
}
set
{
_DateCreated = value;
}
}

public string CreatedBy
{
get
{
return _CreatedBy;
}
set
{
_CreatedBy = value;
}
}

#endregion

}

}

 _____________________________________________
 
Am I on a right path? I will greatly appreciate in any comments or suggestions for a design. I am creating an n-tier application and i don't know if my design is right. I dont have the proper schooling for creating this kind of applications and I am still on layer of data access and business logic.

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......

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