Introduction

Custom assembly attributes let you attach your own metadata to a compiled .NET assembly. They are useful when code, tools, or deployment pipelines need structured information that does not fit the built-in version and product attributes provided by the framework.

What assembly attributes are for

An assembly attribute is metadata compiled into the assembly manifest. Built-in attributes cover common cases such as title, version, and company name. Custom attributes extend that idea so you can define metadata specific to your application or tooling.

Typical uses include:

• embedding build environment information
• declaring a plugin category
• marking assemblies for internal scanners
• attaching ownership or support metadata

The important distinction is that assembly-level attributes describe the whole assembly, not just one type or member.

Define a custom attribute class

Every custom attribute derives from System.Attribute. A good attribute is small, explicit, and immutable after construction whenever possible.

1using System;
2
3[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false)]
4public sealed class DeploymentRingAttribute : Attribute
5{
6    public string Ring { get; }
7    public bool RequiresApproval { get; }
8
9    public DeploymentRingAttribute(string ring, bool requiresApproval)
10    {
11        Ring = ring;
12        RequiresApproval = requiresApproval;
13    }
14}

The AttributeUsage declaration matters. Here it limits the attribute to assemblies only.

Apply the attribute at assembly scope

Assembly attributes are usually placed in a source file near other assembly metadata, or in a project-wide metadata file.

using System.Reflection;

[assembly: DeploymentRing("internal", requiresApproval: true)]

In SDK-style projects, this line can live in any compiled .cs file. It does not need to be in the old AssemblyInfo.cs specifically.

Read the attribute with reflection

At runtime, you can inspect the loaded assembly and read the attribute values:

1using System;
2using System.Reflection;
3
4class Program
5{
6    static void Main()
7    {
8        Assembly assembly = Assembly.GetExecutingAssembly();
9        var attr = assembly.GetCustomAttribute<DeploymentRingAttribute>();
10
11        if (attr is null)
12        {
13            Console.WriteLine("DeploymentRingAttribute not found");
14            return;
15        }
16
17        Console.WriteLine($"Ring: {attr.Ring}");
18        Console.WriteLine($"RequiresApproval: {attr.RequiresApproval}");
19    }
20}

This is the normal access pattern for startup checks, plugin scanners, or diagnostic tools.

Keep the attribute contract stable

If tools depend on your custom attribute, treat it like a public contract. That means:

• avoid renaming properties casually
• prefer additive changes over breaking constructor changes
• document expected values
• version your consuming tools carefully

For example, if a deployment scanner expects Ring values such as internal, beta, and prod, changing those strings silently can break automation even though compilation still succeeds.

Attributes versus configuration

A custom assembly attribute is good for metadata that should travel with the binary. It is not a replacement for ordinary runtime configuration.

Good candidates for attributes:

• build provenance
• plugin type classification
• compatibility markers
• compile-time capabilities

Bad candidates:

• secrets
• environment-specific endpoints
• values that change per deployment without rebuilding

If the value changes often after compilation, configuration files or environment variables are usually a better fit.

Assembly attributes in libraries and plugins

Custom assembly attributes are especially useful in plugin architectures. A host can scan assemblies and decide which ones to load.

1using System;
2using System.Linq;
3using System.Reflection;
4
5var assemblies = AppDomain.CurrentDomain.GetAssemblies();
6
7foreach (var assembly in assemblies)
8{
9    var ring = assembly.GetCustomAttribute<DeploymentRingAttribute>();
10    if (ring != null)
11    {
12        Console.WriteLine($"{assembly.GetName().Name}: {ring.Ring}");
13    }
14}

This avoids hardcoding metadata maps in separate files when the metadata belongs naturally to the assembly itself.

Common Pitfalls

The most common mistake is using a custom assembly attribute for data that should really be runtime configuration. Another is forgetting to restrict AttributeUsage, which can make the attribute appear on targets where it does not belong. Developers also sometimes change constructor signatures after tooling has already started reflecting over the attribute, which breaks consumers unexpectedly. Overloading attributes with large amounts of structured data is another bad pattern because reflection code becomes fragile and hard to maintain. Finally, people often forget that the attribute becomes part of the assembly contract, so changing property names or expected values has downstream consequences.

Summary

• Custom assembly attributes attach application-specific metadata to an entire assembly.
• Define them by inheriting from System.Attribute.
• Use AttributeUsage to restrict where the attribute can be applied.
• Read them with reflection through Assembly.GetCustomAttribute.
• Use them for stable binary metadata, not for fast-changing runtime settings.
• Treat any reflected attribute used by tools as a real compatibility contract.