GitHub - bsheldrick/validatum: An open-source library for building fluent validation functions for .NET.

// build a validator
var validator = new ValidatorBuilder<Employee>()
    .Required(e => e.FirstName)
    .Email(e => e.Email)
    .For(e => e.LastName, name =>
    {
        name.MinLength(5)
            .Equal("Smithers");
    })
    .Build();

// validate
var result = validator.Validate(
    new Employee
    {
        LastName = "Simpson",
        Email = "homer[at]springfieldnuclear.com"
    }
);

foreach (var rule in result.BrokenRules)
{
    Console.WriteLine($"[{rule.Rule}] {rule.Key}: {rule.Message}");
}