> For the complete documentation index, see [llms.txt](https://validly.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://validly.gitbook.io/docs/basics/defining-validations.md).
# Defining Validations
Suppose you have a class representing an API request you want to validate:
```csharp
public class CreateUserRequest
{
public required string Username { get; init; }
public required string Password { get; init; }
public required string Email { get; init; }
public required int Age { get; init; }
public string? FirstName { get; init; }
public string? LastName { get; init; }
}
```
To enable validation capabilities, mark your DTO with the `[Validatable]` attribute and make it `partial`. This allows **Validly** to generate a `Validate` instance method for the class.
```csharp
[Validatable]
public partial class CreateUserRequest
{
// ...
}
```
Then, decorate each property with the appropriate validation attributes. Here’s an example:
```csharp
[LengthBetween(5, 20)] // Username must be between 5 and 20 characters.
public required string Username { get; init; }
[MinLength(12)] // Password must be at least 12 characters long.
public required string Password { get; init; }
[EmailAddress] // Email must be in a valid email format.
public required string Email { get; init; }
[Between(18, 120)] // Age must be between 18 and 120.
public required int Age { get; init; }
[NotEmpty] // First name, if provided, cannot be empty.
public string? FirstName { get; init; }
[NotEmpty] // Last name, if provided, cannot be empty.
public string? LastName { get; init; }
```
Custom Property Validation Code
Learn how to write custom code to validate a property
Before and After Validate Hooks
Learn how to hook into validation, how to write custom validation code of whole model, handle and/or replace validation result