I Learnt Today…

2010-04-01

We frequently get to fetch data from the database, update them and then write them back to the database.

Most of the time we only have to write them back as individual records.

How about updating a whole database table in the memory and having to synchronize all changes to the actual data table?

My initial thought was this would be full of complex coding. However, thanks to Microsoft, there’s nothing much to be done at all.

So how are we gonna do this is…

We will use a data adaptor to fill our data table as usual.

The only new thing is the use of a Command Binder.

Command Binder: A command binder is capable to detect changes that have occurred to a table (in the memory) and then automatically generate appropriate SQL statements to save those changes into the actual data table (in the database).

Following is a simple example: Scenario: In my application I had to take a database table name and present data in a data grid and then save all changes made by the user.

What we need: a data table an adaptor a connection a command builder I have defined them at the form level so I can use them across the form from different events.

private DataTable _tbl;

private SqlDataAdapter _adptr;

private SqlConnection _conn;

private SqlCommandBuilder _cbldr;

Step1: Initialize connection and retrieve data from the database

_tbl = new DataTable();
_conn = FetchData.GetOpenConnection();
_adptr = new SqlDataAdapter("Select * from " + DatabaseTableName, _conn);
_cbldr = new SqlCommandBuilder(_adptr);
_adptr.Fill(_tbl);

Step2: Let the user to change data (in here simply bind the table to a grid)

dgMainGrid.DataSource = _tbl;

Step3: Save (synchronize) changes to the actual database table Even though this is a complex process and undoubtedly would take a lot of effort to do manually, Thanks to .net framework all we need is a line of code.

_adptr.Update(_tbl);

Once you call the adaptor to update the table, it will use the command builder attached to it to generate all necessary SQL command building. The database table is now up-to-date!

Was this post helpful to you? How can I improve? – Your comment is highly appreciated!

Cassian Menol Razeek

2008-12-04

Today I had to take care of a problem of a certain company (CIS) that I consult. They had a problem of keeping the values modified in a template field of a gridview when the user moves from one page of the grid to another.

Template fields allow us to add custom columns into a datagrid or gridview. For an instance, we can add a grid column which has a textbox in each cell.

When the gridview has several pages, the gridview holds only the rows that are displayed in its Rows (collection) property. So we cannot access the values in other pages using the gridview.Rows property.

Then I thought that I could access the whole collection of rows by referring to the DataSource property of the gridview. This is a good idea because even though the gridview only shows the rows on the current page for display purposes, the data source property of the gridview holds the whole collection of rows in it.

ASP.Net clears the data source of any control at post backs. This is done to optimize performance of communications. In addition, the state of each control is stored in ViewState so it is not necessary to keep the datasource between postbacks.

The problem is worse now because I could not use both datasource and the direct row collection from the gridview.

Of course we can use either cache or session to keep the datasouce.

Session["DataSource"] = dt;  // store our table in the session

Then to synchronize (update) the datasource which was stored in the session we have to:

• Go to each row in the gridview
• Get the matching data row from the data table
• Update the fields of the data table row

Since we have enabled paging for the gridview, we have to do this when ever user changes the page he/she is viewing to preserve his/her modifications.

So we need to write our code in the PageIndexChanging event handler of the GridView.

In my example I have two columns of the grid view called “ID” and “Name” and the name is the only template column I have used so I am only updating that column in the stored data table.

Code:

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
// get the datatable from the
DataTable dTable = (DataTable)Session["dataSource"];

// now we will iterate through all rows of the grid
// then get the matching row from the data table (datasource of our grid)
// and append the updated data (by the user) to the selected data row
foreach (GridViewRow grv in this.GridView1.Rows)
{
// get the matching row from the data table
DataRow dRow = dTable.Rows.Find(this.GridView1.DataKeys[grv.RowIndex].Value);

// set values of updated columns : here I have only let the user to edit "Name" column
dRow["Name"] = ((TextBox)grv.FindControl("txtName")).Text;
}

// go to the next page of the grid
this.GridView1.PageIndex = e.NewPageIndex;
this.GridView1.DataSource = (DataTable)Session["DataSource"];
this.GridView1.DataBind();

// show the whole collection of data in the second grid (used only to display)
this.GridView2.DataSource = (DataTable)Session["DataSource"];
this.GridView2.DataBind();
}

I have used a different gridview called GridView2 to show the whole data source without paging.

So don’t misunderstand the use of GridView2 in the last section of the code. It’s used only do display the whole set of data (without paging).

I cannot upload my code into this blog because it seems like zip files are not supported. If you like to take a glance of my code just add a comment so I’ll send the code to you via email.

Was this post helpful to you? How can I improve? – Your comment is highly appreciated!

Cassian Menol Razeek