A call to BeginUpdate must be accompanied by a subsequent call to EndUpdate, or desynchronization can occur. One mean of making sure that both calls stay synchronized is to use a finally block:

try 
{
  i.BeginUpdate();
  // Do some modifications to the folder
}
finally 
{
  i.EndUpdate();
}

In C#, the preferred way is to use the AutoBatchUpdate class in a using statement.

Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

IBatchUpdateable Interface
IBatchUpdateable Members
Overload List