Whether or Not To Finish the Manuscript Before Approaching Agents or Publishers
I'll say it simply: don't bother approaching an agent or an editor until you've completed your manuscript.
Why? Because anybody can say they're going to write this amazing novel that's going to be an instant classic and overnight bestseller. But how many can actually produce that?
You may have the best idea in the world, but no one—not even you—really knows whether or not you can produce a completed manuscript that will be universally hailed as a staggering work of heartbreaking genius.
You're an unknown. I mean, would the Dallas Cowboys take some joker's word for it that he's a great quarterback, and just drop him in the starting lineup for Sunday's game? (All Dallas Cowboys haters just keep your comments to yourself [grin].)
No, they'd probably laugh for a few minutes. Then they'd give him a football and tell him to toss some passes. If he looked really good, they'd probably give him the number of some junior league team somewhere to maybe have a tryout.
Yet that's what you're doing if you have nothing but an idea or a few sample chapters to show an agent or editor.
Your completed manuscript is your résumé, your references, your portfolio, your interview, your training, your qualifications, and your experience all rolled into one.
With it, doors open. Without it, they stay shut.
Finish your manuscript.
(Then get some feedback on it. Consider getting an editorial review, especially if you've been striking out with agents and editors and you don't know why.)