S3993 — Custom attributes should be marked with "System.AttributeUsageAttribute"

Why is this an issue?

When defining custom attributes, AttributeUsageAttribute must be used to indicate where the attribute can be applied. This will:

• indicate how the attribute can be used
• prevent it from being used at invalid locations

How to fix it

Code examples

Noncompliant code example

public sealed class MyAttribute : Attribute // Noncompliant - AttributeUsage is missing
{
    private string text;

    public MyAttribute(string text)
    {
        this.text = text;
    }

    public string Text => text;
}

Compliant solution

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Interface | AttributeTargets.Delegate)]
public sealed class MyAttribute : Attribute
{
    private string text;

    public MyAttribute(string text)
    {
        this.text = text;
    }

    public string Text => text;
}

Resources

Documentation

• Microsoft Learn - Create custom attributes
• Microsoft Learn - AttributeUsageAttribute class
• Microsoft Learn - Attribute class