This is from the Prashn Maarg, a Sanskrit authority in Astrology. This chapter also references the book named “Yaatraa” on dreams and the Vaalmeeki Raamaayan. The following are passages may be interesting to know in relation to dream , and are from the Chapter 31 of volume 2 of the book.

“Dreams are of 7 types:

1) Dreshtah : See in a dream what you have seen while awake.

2) Sruthah: See in a dream what you have heard while awake.

3) Anubhootah: See in a dream what you have smelt, tasted, or touched while awake.

4) Praarthitah: See in a dream what you have desired while awake.

5) Kalpitah: See in a dream what you have day-dreamed about: something which you would have seen and heard and desired earlier and forgotten .

6) Bhavijah: A dream which involves none of the above.

7) Doshajah: Dream depending upon diseases involving the body (eg weaknesses)

Of these 7 types of dreams, only Bhavijah kind of dream has any fruit in the future. And even of these Bhavijah dreams, if seen within 5 hours of sunset, it will bear fruit only after a very long time. Dreams seen at sunrise will bear fruit the same day .

--If after the dream , the person does not sleep then the result of the dream will be negligible .
--If after the dream the person sleeps again, it will surely bear fruit.
--If you speak about the dream to someone and you hear negative things about it, then the fruits of the dream will be negligible.
--Dreams seen during day time, dreams forgotten soon after waking up, dream seen over a very long time, and those seen for a very short while, do not bear fruit.

Many things seen in a dream have been described to be good, out of which (1) girls, (2) Devtaas, (3) people etc; (it s a very big list). All these dreams bring longevity of life, health and wealth

The night is divided into 4 “Yaam” - A dream seen ---  
In the first Yaam will bear fruit within 1 year,
In the 2nd Yaam will bear the fruit in 6 months time,
In the 3rd Yaam will bear the fruit within 3 months, and
In the 4th Yaam  will bear the fruit within a month.
And a dream seen at sunrise brings the fruit immediately

(There are slightly different opinions also, but there is general agreement )

After seeing a dream if one has the very strong feeling that something bad is going to happen then the dream indicate inauspicious incidents.”

It has not been said anywhere that if one sees a particular thing it would be that particular thing that would bear the fruit of the dream. For example (From same chapter of same book) : “If one dreams that one has received sandal, Shankh, pearl, betel leaf, then he will beget children and wealth.

If someone sees the dream of drinking ghee or Kheer from a dirty lotus leaf,  in the middle of a lake, then he will beget knowledge which will be respected by the great. . . Therefore what is seen in the dream is not how it will bear fruit. But it will indicate something else. For example - the result of seeing Devtaas etc (mentioned above)”

Therefore seeing Shiv Ling in a dream does not necessarily indicate that one should worship the same Shiv Ling as the fruit of the dream.