I know they put rails on guns now. My own M&P 9c has them. But I personally would never attach a light to a gun and show an opponent where my head and hands are in the dark. Just use a maglight or something equally effective, and hold it out as far to one side as you can, if you must use a light. Keep it away from you head and body.

Bear in mind, I am thinking "self defense" here. Perhaps there are other uses for lights on guns? If you engaged in a firefight in the dark, or even before a fight might start, why give away your position and provide your potential assailant a beacon to show where you are at all times? Even when one fires in the dark, one must move quickly in some lateral direction, to avoid drawing fire.

Perhaps the police have a use for lights mounted on pistols, but does the armed citizen? Is it even a reasonable idea?

i think so. the whole point is that it doesnt give you something to shoot at accurately. i have a dark hallway from the the front door of my apt. to my bedroom. i would bet that if i flicked my mounted light on, any assailant coming down that hallway would be blinded -advantage me. he cant see a thing, i see him perfectly.

You shoot at the light. If the light is on the gun and the gun is in front of your head.. well. In a firefight at night you have to shoot and move to keep the enemy from shooting at your flash. Some people like those rail lights but I would rather they have them than me.

ill give you that, if someone is looking to hurt you and you have a flashlight they have a good idea were to go, but what about IDPA they do low/no light Challenges, that is why i am looking for a flashlight

If you use a flashlight in a hallway, you're as good as dead. Your space is too limited to keep the light from giving your opponent your exact position, as Teuthis has already pointed out.
In a larger space, you might use the FBI flashlight technique for searching, since it requires you to light the light when it's away from your face and body. Or, you might use the Harries technique, if you want to do some precision shooting at a target in a known position.

I strongly suggest that you learn to "slide shoot" all the way out to 10 yards. This is a very useful low-light technique that will not give your position away until you have actually fired a shot.
You don't need your sights, to hit a man-size target at across-the-room distances. All you need is to see the target's dark silhouette, and the dark silhouette of the rear end of your own pistol. Superimpose the two, and fire.
Assuming that your superimposition has placed your pistol in line with your adversary's vital parts, and assuming that your trigger control remains usefully smooth, you will make the desired hit and stop the fight.