using (DgnDocument targetDgnDocument = DgnDocument.CreateForLocalFile("fileName"))
{
DgnFile pDgn = DgnFile.Create(targetDgnDocument, DgnFileOpenMode.ReadWrite).DgnFile; StatusInt openForWriteStatus;
pDgn.LoadDgnFile(out openForWriteStatus);
if (pDgn.IsOpen)
{
DimensionStyleCollection dimstyles = pDgn.GetDimensionStyles();
foreach (Bentley.DgnPlatformNET.DimensionStyle item in dimstyles)
{
item.Name = "SS_" + item.Name; //by setting this property changes are not reflected in the file
}
} pDgn.ProcessChanges(DgnSaveReason.UserInitiated);
pDgn.Release();
}