Collection was modified; enumeration operation might not execute.

Problem:

Iterating through a loop of DataTable.Rows and you try and delete a row and get:

Collection was modified; enumeration operation might not execute.

foreach (DataRow dr in dt.Rows)
{
   if ((int)dr["ID"] > 5)
      dr.Delete();
}

Simplest Solution:

Don't loop on the original collection of Rows, but instead do a Select("") and pull those DataRows into an Array and loop on that.  Now you can Delete() to your heart's content.

foreach (DataRow dr in dt.Select(""))
{
   if ((int)dr["ID"] > 5)
      dr.Delete();
}