A couple agents keep asking my current salary

Thanks everyone for replying.One more question.

A couple agents keep asking my current salary, and I'm very uncomfortable to tell them. They say the company will check my W-2 anyways. I told that the company should make offer based on my skills and market value, rather than on how much i'm making right now (especially the unversity usually pays about 20% lower than the market value). So I refuse to tell them my current salary. Tell them withdraw my application or submit it without that info. Am I right or it's a business rule to tell how much I'm making up-front?