[quote user="Mohammad Aryafar"]If I use mdlWorkDgn_openFile to open an existing model, will this be my current active model? [/quote]
No! As the name of the function suggests, it creates a working DGN model reference: that is, a DgnModelRefP that you can work on programmatically. That model is not the active model. It's not even displayed — a user can't see it. You should make no assumptions about a working model.
A lot of MicroStation applies only to an active model and its attachment. Write your code first to make stuff happen in the active model. If you feel the need to use a working model, that's the time to experiment with your code.
[quote user="Mohammad Aryafar"]If it's not possible to attach the raster to the file which is not active model [/quote]
Get your terminology correct: you don't attach anything to a file. A DGN file is a container of one or more models. You attach a vector or raster reference to a model, not a file.