[Civil3D API] After loading .NET dll, If Civil3D doesn't work finish.

After you call StartTransaction function, if you don't commit then Civil3D doesn't work finish.
So you can use StartTransaction like following.

using (Transaction ts = Application.DocumentManager.MdiActiveDocument.Database.TransactionManager.StartTransaction())
{
     ... 
     ...
}

When you use using statement. Transaction.commit() is called automatically at the end of block.