Custom Attributes — Metadata Annotations for Your Types

Custom attributes let you add declarative metadata to types, methods, properties, and parameters. The attribute is stored in the assembly’s metadata and read at runtime with reflection. ASP.NET Core is saturated with attributes: [ApiController], [HttpGet], [Route], [Authorize], [Required], [MaxLength] — all implemented with exactly the same attribute mechanism available to your own code. Building custom attributes for validation, documentation, auditing, and routing is a powerful way to make your code declarative and reduce repetition.

Defining a Custom Attribute #

// ── Define a custom attribute ──────────────────────────────────────────────
// Attributes must inherit from System.Attribute
[AttributeUsage(
    AttributeTargets.Property | AttributeTargets.Field,  // where it can be applied
    AllowMultiple = false,   // can appear at most once per target
    Inherited     = true)]   // derived classes inherit this attribute
public sealed class SlugAttribute : Attribute
{
    // Positional parameter (required) — set via constructor
    public int MaxLength { get; }

    // Named parameter (optional) — set via property initialiser
    public bool AutoGenerate { get; set; } = true;

    public SlugAttribute(int maxLength)
    {
        if (maxLength <= 0)
            throw new ArgumentOutOfRangeException(nameof(maxLength), "MaxLength must be positive.");
        MaxLength = maxLength;
    }
}

// ── Apply the attribute ────────────────────────────────────────────────────
public class Post
{
    public int Id { get; set; }

    public string Title { get; set; } = string.Empty;

    [Slug(50, AutoGenerate = true)]   // positional then named
    public string Slug { get; set; } = string.Empty;

    public string Body { get; set; } = string.Empty;
}

Reading Attributes at Runtime #

// ── Read a specific attribute from a property ─────────────────────────────
PropertyInfo? slugProp = typeof(Post).GetProperty(nameof(Post.Slug));
SlugAttribute? slugAttr = slugProp?.GetCustomAttribute<SlugAttribute>();

if (slugAttr is not null)
{
    Console.WriteLine($"MaxLength: {slugAttr.MaxLength}");     // 50
    Console.WriteLine($"AutoGenerate: {slugAttr.AutoGenerate}"); // true
}

// ── Build a validation framework using attributes ─────────────────────────
public static class AttributeValidator
{
    public static Dictionary<string, string[]> Validate(object model)
    {
        var errors = new Dictionary<string, string[]>();
        var type   = model.GetType();

        foreach (var prop in type.GetProperties())
        {
            var value = prop.GetValue(model);

            // Check [Required] attribute
            var required = prop.GetCustomAttribute<RequiredAttribute>();
            if (required is not null && value is null or "")
                errors[prop.Name] = [$"{prop.Name} is required."];

            // Check [MaxLength] attribute
            var maxLen = prop.GetCustomAttribute<MaxLengthAttribute>();
            if (maxLen is not null && value is string str && str.Length > maxLen.Length)
                errors[prop.Name] = [$"{prop.Name} max length is {maxLen.Length}."];

            // Check custom [Slug] attribute
            var slug = prop.GetCustomAttribute<SlugAttribute>();
            if (slug is not null && value is string slugVal)
            {
                if (slugVal.Length > slug.MaxLength)
                    errors[prop.Name] = [$"Slug exceeds max length of {slug.MaxLength}."];
                if (slugVal.Contains(' '))
                    errors[prop.Name] = ["Slug cannot contain spaces."];
            }
        }
        return errors;
    }
}

Common Mistakes #

Mistake 1 — Non-constant attribute argument (compile error) #

❌ Wrong — new DateTime(…) is not a compile-time constant:

[ValidFrom(new DateTime(2025, 1, 1))]   // compile error!

✅ Correct — use string representation or separate integer arguments:

[ValidFrom("2025-01-01")]   // parse the string inside the attribute

Mistake 2 — AttributeTargets mismatch (compile error at usage) #

❌ Wrong — applying a property attribute to a class:

[Slug(50)]   // on a class — compile error if AttributeTargets.Property only!

✅ Correct — verify the AttributeUsage is set to the correct target(s).