Searching takes O(1) time if there is no collisons in the hashtable , so it is incorrect to sya that searching in a hashtable takes O(1) or constant time.
See how Hashtable works on MSDN?
EDIT
mgiuca explains it beautifully and i am just adding one more Collosion Avoidance technique which is called Chaining.
IN this technique , We maintain a linklist of values at each location so when you have a collosion , your value will be added to the Linklist at that position so when you are searching for a value there may be scenario that you need to search the value in whole link list so in that case searching will not be O(1) operation.
