Following C# code runing in .NET 2 causes error in Update() line

Npgsql.NpgsqlException was unhandled by user code

Why exception is not catched ?
catch must catch ALL exceptions.

   int RowsUpdated;
   try {
       RowsUpdated = datapter.Update(changes);
     }
   catch {
       System.Windows.Forms.MessageBox.Show("error");
       transact.Rollback();
     throw;
     }

error message details are:

Npgsql.NpgsqlException was unhandled by user code
Message="ERROR: 23505: duplicate key violates unique constraint \"konto_pkey\""
Source="Npgsql"
BaseMessage="duplicate key violates unique constraint \"konto_pkey\""
Code="23505"
Detail=""
ErrorSql="update \"firma1\".\"konto\" set \"kontonr\" = '2', ....
File="nbtinsert.c"
Hint=""
Line="277"
Position=""
Routine="_bt_check_unique"
Severity="ERROR"
Where=""

StackTrace:
at Npgsql.NpgsqlConnector.CheckErrors() in c:\Desenvolvimento\NpgsqlBuildOficial2.0\src\Npgsql\NpgsqlConnector.cs:line 359
at Npgsql.NpgsqlConnector.CheckErrorsAndNotifications() in c:\Desenvolvimento\NpgsqlBuildOficial2.0\src\Npgsql\NpgsqlConnector.cs:line 410
at Npgsql.NpgsqlCommand.ExecuteCommand() in c:\Desenvolvimento\NpgsqlBuildOficial2.0\src\Npgsql\NpgsqlCommand.cs:line 1479
at Npgsql.NpgsqlCommand.ExecuteReader(CommandBehavior cb) in c:\Desenvolvimento\NpgsqlBuildOficial2.0\src\Npgsql\NpgsqlCommand.cs:line 633
at Npgsql.NpgsqlCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior cb) in c:\Desenvolvimento\NpgsqlBuildOficial2.0\src\Npgsql\NpgsqlCommand.cs:line 601
at System.Data.Common.DbDataAdapter.UpdateRowExecute(RowUpdatedEventArgs rowUpdatedEvent, IDbCommand dataCommand, StatementType cmdIndex)
at System.Data.Common.DbDataAdapter.Update(DataRow[] dataRows, DataTableMapping tableMapping)