C# – How to configure NHIbernate event listeners for Update and Save

cnhibernate

Following on from my previous question How to implement LastUpdate in NHibernate Entities?.

I have two columns on my audited tables in my database:

created datetime default getdate() not null (the creation date of this row)
lastUpdate datetime null (the datetime this row was last updated)

I want to create a listener for updates only in NHibernate, because the database engine takes care of new records with the default constraint. I tried to create a Custom listener with this code:

public class CustomUpdateEventListener : DefaultSaveOrUpdateEventListener
{
    protected override object PerformSaveOrUpdate(SaveOrUpdateEvent evt)
    {
        var entity = evt.Entity as IAuditableEntity;
        if (entity != null)
        {
            ProcessEntityBeforeUpdate(entity);
        }
        return base.PerformSaveOrUpdate(evt);
    }

    internal virtual void ProcessEntityBeforeUpdate(IAuditableEntity entity)
    {
        entity.UpdateDate = DateTime.Now;
    }
}

and it works great for updates, but it also gets run for Save events (when I add new rows to the database). I don't want it to fire for new rows. I tried to change the code to listen to Update events only but I can't work it out.

I tried to change the class to inherit from DefaultUpdateEventListener but there's no UpdateEvent (only SaveOrUpdate events or PreUpdate or PostUpdate)

I'm wondering if I should use the PreUpdate event and making my listener inherit from DefaultUpdateEventListener but then I'm not sure which method I need to override. There is no Update method to override.

Best Answer

You could make your class implement the IPreUpdateEventListener interface (and its OnPreUpdate method). Then add your class to the session event listerners through the config:

NHibernate.Cfg.Configuration cfg = new NHibernate.Cfg.Configuration();
cfg.EventListeners.PreUpdateEventListeners = 
    new IPreUpdateEventListener[] {new YourEventListener()};
sessionFactory = cfg.BuildSessionFactory();

Related Solutions

C# – How to create an Excel (.XLS and .XLSX) file in C# without installing Microsoft Office

You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:

ExcelLibrary

This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.

It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.

ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.

You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.

There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.

Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)

Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License

Here some example code for ExcelLibrary:

Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:

//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");

//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;

//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();

//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();

adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();

//Add the table to the data set
ds.Tables.Add(dt);

//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);

Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.

C# – How to convert a byte array to a hexadecimal string, and vice versa

You can use Convert.ToHexString starting with .NET 5.
There's also a method for the reverse operation: Convert.FromHexString.

For older versions of .NET you can either use:

public static string ByteArrayToString(byte[] ba)
{
  StringBuilder hex = new StringBuilder(ba.Length * 2);
  foreach (byte b in ba)
    hex.AppendFormat("{0:x2}", b);
  return hex.ToString();
}

or:

public static string ByteArrayToString(byte[] ba)
{
  return BitConverter.ToString(ba).Replace("-","");
}

There are even more variants of doing it, for example here.

The reverse conversion would go like this:

public static byte[] StringToByteArray(String hex)
{
  int NumberChars = hex.Length;
  byte[] bytes = new byte[NumberChars / 2];
  for (int i = 0; i < NumberChars; i += 2)
    bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
  return bytes;
}

Using Substring is the best option in combination with Convert.ToByte. See this answer for more information. If you need better performance, you must avoid Convert.ToByte before you can drop SubString.

Best Answer

Related Solutions

C# – How to create an Excel (.XLS and .XLSX) file in C# without installing Microsoft Office

C# – How to convert a byte array to a hexadecimal string, and vice versa

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