What is DiffGram ?

The DiffGram is one of the two XML formats that you can use to render DataSet object contents to XML. A good use is reading database data to an XML file to be sent to a Web Service. ADO.NET introduced the DataSet class to support the disconnected, distributed data-access scenarios. With DataSet, the data retrieved from the database is cached in-memory, in addition to the constraints and relationships among the tables. When the ADO.NET DataSet is serialized as XML (for example, when returning a DataSet from an ASP.NET XML Web service method), the XML format used for DataSet serialization is known as DiffGram. Like Updategrams, DiffGrams also contains the tags that specify the original and new state of data. SQLXML and .NET Managed classes can be used to execute DiffGrams to perform the database updates, however there are many things that are supported by Updategrams and not by DiffGrams (ability to pass parameters being one example).

Now let's see an example of DiffGrams. The code is to reads data from Employees tables and write in an XML document in DiffGram format.
Dim connectionString As String = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=c:\Northwind.mdb"
Dim sql As String = "SELECT EmployeeID, FirstName, LastName, Title FROM Employees"
Dim conn As OleDbConnection = Nothing
Dim ds As DataSet = Nothing
' Create and open connection
conn = New OleDbConnection(connectionString)
If conn.State <> ConnectionState.Open Then
conn.Open()
End If 'Create a data adapter
Dim adapter As New OleDbDataAdapter(sql, conn)
' Create and fill a DataSet
ds = New DataSet("TempDtSet")
adapter.Fill(ds, "DtSet")
' Write XML in DiffGram format
ds.WriteXml("DiffGramFile.xml", XmlWriteMode.DiffGram)
' Close connection
If conn.State = ConnectionState.Open Then
conn.Close()
End If

Now if you update data, you'll see new additions to the XML file with and tags and if there are any errors occur during the updatio, the entries will go to the section.
You can use the ReadXml method to read XML documents in DiffGram format. The first parameter of ReadXml is the XML document name and second parameter should be XmlReadMode.DiffGram. See the following code snippet:

' Create a DataSet Object
Dim ds As New DataSet

' Fill with the data
ds.ReadXml("DiffGramFile.xml", XmlReadMode.DiffGram)

DataRowState members
Added
Add added rows to a DataRowCollection of a DataSet and AcceptChanges has not been called. Deleted
All the deleted rows. Detached Rows were created but not added to the row collection. Either waiting for the addition or have removed from the collection.
Modified
Modified rows and AcceptChanges has not been called.
Unchanged
Unchanged rows since last AcceptChanges was called.

The following code copies only modified rows of ds to new DataSet tempDs.
Dim tempDs As DataSet
tempDs = ds.GetChanges(DataRowState.Modified)

What are the Session State Modes/ Session States

Storage location
InProc - session kept as live objects in web server (aspnet_wp.exe)
StateServer - session serialized and stored in memory in a separate process aspnet_state.exe). State Server can run on another machine
SQLServer - session serialized and stored in SQL server

Performance
InProc - Fastest, but the more session data, the more memory is consumed on the web server, and that can affect performance.
StateServer - When storing data of basic types (e.g. string, integer, etc), in one test environment it's 15% slower than InProc. However, the cost of serialization/deserialization can affect performance if you're storing lots of objects. You have to do performance testing for your own scenario.
SQLServer - When storing data of basic types (e.g. string, integer, etc), in one test environment it's 25% slower than InProc. Same warning about serialization as in StateServer.

Performance tips for Out-of-Proc (OOP) modes
If you're using OOP modes (State Server or SQL Server), one of your major cost is the serialization/deserialization of objects in your session state. ASP.NET performs the serialization/deserialization of certain "basic" types using an optimized internal method. ("Basic" types include numeric types of all sizes (e.g. Int, Byte, Decimal, String, DateTime, TimeSpan, Guid, IntPtr and UIntPtr, etc) .
If you have a session variable (e.g. an ArrayList object) that is not one of the "basic" types, ASP.NET will serialize/deserialize it using the BinaryFormatter, which is relatively slower.

So for performance sake it is better to store all session state data using one of the "basic" types listed above. For example, if you want to store two things, Name and Address, in session state, you can either (a) store them using two String session variables, or (b) create a class with two String members, and store that class object in a session variable. Performance wise, you should go with option (a).

Robustness
InProc - Session state will be lost if the worker process (aspnet_wp.exe) recycles, or if the appdomain restarts. It's because session state is stored in the memory space of an appdomain. The restart can be caused by the modification of certain config files such as web.config and machine.config, or any change in the \bin directory (such as new DLL after you've recompiled the application using VS) For details, see KB324772. In v1, there is also a bug that will cause worker process to restart. It's fixed in SP2 and in v1.1. See KB321792.
If you're using IIS 6.0, you may want to go to IIS Manager, go to Application Pools/DefaultAppPool, and see if any of the parameters on the Recycling and Performance tabs are causing the IIS worker process (w3svc.exe) to shutdown.
StateServer - Solve the session state loss problem in InProc mode. Allows a webfarm to store session on a central server. Single point of failure at the State Server.
SQLServer - Similar to StateServer. Moreover, session state data can survive a SQL server restart, and you can also take advantage of SQL server failover cluster, after you've followed instructions in KB 311209.

Caveats
InProc - It won't work in web garden mode, because in that mode multiple aspnet_wp.exe will be running on the same machine. Switch to StateServer or SQLServer when using web garden. Also Session_End event is supported only in InProc mode.
StateServer - In a web farm, make sure you have the same in all your web servers. See KB 313091 on how to do it. - Also, make sure your objects are serializable. See KB 312112 for details. - For session state to be maintained across different web servers in the web farm, the Application Path of the website (For example \LM\W3SVC\2) in the IIS Metabase should be identical (case sensitive) in all the web servers in the web farm. See KB 325056 for details
SQLServer - In v1, there is a bug so that iif you specify integrated security in the connection string (e.g. "trusted_connection=true", or "integrated security=sspi"), it won't work if you also turn on impersonation in asp.net. This problem is (wrongly) described in KB 324479. Unfortunately, the "Description" and the "Cause" sections in the KB are quite wrong and misleading. But anyway, there is a QFE fix for it, and the fix will also be available when SP3 ships. The problem is fixed in v1.1. - Also, make sure your objects are serializable. Otherwise, your request will hang! See KB 312112 for details. The SQLServer mode hanging problem was fixed in v1.1. The QFE fix for KB 324479 also contains the fix for this problem. The problem will be fixed in v1 SP3 too. - For session state to be maintained across different web servers in the web farm, the Application Path of the website (For example \LM\W3SVC\2) in the IIS Metabase should be identical (case sensitive) in all the web servers in the web farm.

Handle paging in Gridview Control

Step 1:
Connect database and access any table to display in Gridview.

'Declare below code in Partial Class _Default
Dim con As SqlConnection
Dim cmd As SqlCommand
Dim adp As SqlDataAdapter
Dim ds As DataSet

'write code at page_load event
con = New SqlConnection(ConfigurationManager.AppSettings("cn"))
Dim sql As String
sql = "select * from tbl_logo_info"
cmd = New SqlCommand(sql, con)
adp = New SqlDataAdapter(cmd)
ds = New DataSet()
adp.Fill(ds, "tbl_adminuser")

GridView1.DataSource = ds
GridView1.DataBind()

Step 2:
'write below code at pageindexchaging event
Protected Sub GridView1_PageIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewPageEventArgs) Handles GridView1.PageIndexChanging

GridView1.PageIndex = e.NewPageIndex
GridView1.DataBind()

End Sub