When we perform queries and retrieve objects like the product instances above, LINQ to SQL will by default keep track of any changes or updates we later make to these objects.  We can make any number of queries and changes we want using a LINQ to SQL DataContext, and these changes will all be tracked together. 

Note: LINQ to SQL change tracking happens on the consuming caller side - and not in the database.  This means that you are not consuming any database resources when using it, nor do you need to change/install anything in the database to enable it.

After making the changes we want to the objects we've retrieved from LINQ to SQL, we can then optionally call the "SubmitChanges()" method on our DataContext to apply the changes back to the database.  This will cause LINQ to SQL to dynamically calculate and execute the appropriate SQL code to update the database.

For example, I could write the below code to update the price and # of units in stock of the "Chai" product in the database:

Figure 4

When I call northwind.SubmitChanges() above, LINQ to SQL will dynamically construct and execute a SQL "UPDATE" statement that will update the two product property values we modified above.

I could then write the below code to loop over unpopular, expensive products and set the "ReorderLevel" property of them to zero:

Figure 5

When I call northwind.SubmitChanges() above, LINQ to SQL will calculate and execute an appropriate set of UPDATE statements to modify the products who had their ReorderLevel property changed.

Note that if a Product's property values weren't changed by the property assignments above, then the object would not be considered changed and LINQ to SQL would therefore not execute an update for that product back to the database.  For example - if the "Chai" product's unitprice was already $2 and the number of units in stock was 4, then calling SubmitChanges() would not cause any database update statements to execute.  Likewise, only those products in the second example whose ReorderLevel was not already 0 would be updated when the SubmitChanges() method was called.