I created a source generator to solve a problem that has annoyed me for several years.

The problem

If you have this code:

public partial class Product
{
    [Required]
    [StringLength(100)]
    public string? Name { get; set; }

    [Required]
    [Range(0.01, 999999.99)]
    public decimal Price { get; set; }
}

How do you get the values from your data annotations?

The solution

I’ve recently created DataAnnotationValuesExtractor to solve this specific problem. With this you just add an attribute to your models:

[DataAnnotationValues]
public partial class Product
{
    [Required]
    [StringLength(100)]
    public string? Name { get; set; }

    [Required]
    [Range(0.01, 999999.99)]
    public decimal Price { get; set; }
}

And then you can get values like this:

// Name length constraints
int maxNameLength = Product.Annotations.Name.MaximumLength; // 100
int minNameLength = Product.Annotations.Name.MinimumLength; // 0
bool nameRequired = Product.Annotations.Name.IsRequired; // true

// Price constraints
double minPrice = Product.Annotations.Price.Minimum; // 0.01
double maxPrice = Product.Annotations.Price.Maximum; // 999999.99
bool priceRequired = Product.Annotations.Price.IsRequired;

It almost like magic!

There is also a solution if you scaffolding your models, read more in the project on GitHub.

Summary

This was a fun little project that solved a real problem for me and that has annoyed since I started to use data annotations. I’ve seen solutions where you use reflection, but this solution is much better.