How do you implement validation?

Question

Accepted Answer

**FluentValidation** — validation library.

**Validator:**
```csharp
public class UserValidator : AbstractValidator<User>
{
    public UserValidator()
    {
        RuleFor(x => x.Name)
            .NotEmpty()
            .MaximumLength(50);

RuleFor(x => x.Email)
            .NotEmpty()
            .EmailAddress();

RuleFor(x => x.Age)
            .InclusiveBetween(18, 120);
    }
}
```

**Usage:**
```csharp
var validator = new UserValidator();
var result = validator.Validate(user);
if (!result.IsValid)
{
    return Results.ValidationProblem(result.ToDictionary());
}
```

**Key:** FluentValidation for rules.

C#: How do you implement validation?