TNValidate is a fluent validation library for .Net. It enables you to write validation logic in a way that somewhat resembles natural language. This is not only intended to make it a little easier for developers to scan, but also means non-programmers have a better chance of being able to understand and modify the constraints being placed on data.
TNValidate collects together validation failures and has a default set of error messages, which you can present to end users. Currently we support English, Swedish, Brazilian and Portuguese languages; if you want something else, see the section on adding languages. However, often you will want to customize the error that is given to the end user for a specific failure, which is made quick and easy too.
Finally, TNValidate is open source, made available under the MIT license. This enables you to use it in your free or non-free software, and does not obligate you to contribute back any changes. However, if you are in an environment where you are allowed to do so, contributions are welcome (especially in the area of adding support for more languages).