You don't always need a patent to get your invention idea reviewed by a top company, and get it sold for cash.
For inventions that are not yet patented, or that contain trade secrets or features that are additional to the patent, I recommend considering the following procedure for submission to companies. (I am not giving legal advice, and you should always contact your attorney when you are dealing with legally binding contracts.)
I typically meet many company executives, especially at trade shows, and many are very willing to offer to sign a confidentiality agreement. They typically ask that one be sent to them at their office after the trade show. This is all fine and good, however it is my experience that 1 in 10 of these executives ever follow through with their promise. This, by itself, is not necessarily an indicator that the company is not a worthwhile candidate, for reason that I will explain later.
After returning from the trade show, I would send the company executive a confidential disclosure agreement and give them an opportunity to return it. If, after 30 days and one verbal reminder, they have not returned the confidential disclosure agreement, and if it is still part of the strategy to submit the invention to this company, then I would do so with a cover letter of confidentiality. The reason I would do this is because since the company already indicated that they are willing to receive the invention on a confidential basis, receiving a cover letter of confidentiality is not out of the realm of what they should expect.
In my cover letter to the company I usually state the invention is being submitted under the understanding that all submitted information shall remain property of the inventor, and the company shall not commercialize or use the invention without a written agreement between the parties. As long as I get this term, it usually doesn’t matter so much to me if the company also agrees to confidentiality, as I have never had a company abuse this in my 33 years in the invention business.
Attorneys have advised me that if I submit my invention to a company under these terms, and if the company accepts the submission without taking exception to the terms, then the procedure becomes and offer and acceptance, thereby potentially making the terms legally biding. (Again, no legal advice here, always consult your attorney)
If critical patent time bars are an issue, then I would lean on the confidential aspect. The book, The Inventor’s Bible, has more detailed information about submitting inventions to companies.
This vastly differs from the submission policy of large multi-national companies whose policy is either to not except non-patented inventions, or to receive inventions only after a non-confidential disclosure agreement is signed. One reason for a large corporation to not agree with confidentiality is because they may be compromised if the company were working on a similar idea simultaneous to the inventor’s submission. A confidential agreement may bring undeserving legal liability to the company. This may welcome a lawsuit from the inventor against the company; something companies want to avoid. I have heard several horror stories from corporate executives who have experienced this from unreasonable inventors.