What is the Diff b/w a Lifeline and an Object

[GraemeR]

In the context of a Sequence Diagram.

What is the difference between the use of LifeLines and Objects.

Both seem to be very similar in operation.

Why would one be chosen over the other?

Cheers Graeme.

[TrtnJohn]

A lifeline can be used as a general placeholder when there is no classifier for the object.  The intention of sequence diagrams are to model interactions between objects.  So, unless you are modeling something that doesn't have any classes already defined, I'd recommend always using an object.

[thomaskilian]

True. Also you can't change the element type of a lifeline which you can for objects.