Programming C#, Second Edition

Custom Attributes

You are free to create your own custom attributes and use them at runtime as you see fit. Suppose, for example, that your development organization wants to keep track of bug fixes. You already keep a database of all your bugs, but you’d like to tie your bug reports to specific fixes in the code.

You might add comments to your code along the lines of:

// Bug 323 fixed by Jesse Liberty 1/1/2005.

This would make it easy to see in your source code, but there is no enforced connection to Bug 323 in the database. A custom attribute might be just what you need. You would replace your comment with something like this:

[BugFixAttribute(323,"Jesse Liberty","1/1/2005", 
Comment="Off by one error")]

You could then write a program to read through the metadata to find these bug-fix notations and update the database. The attribute would serve the purposes of a comment, but would also allow you to retrieve the information programmatically through tools you’d create.

Declaring an Attribute

Attributes, like most things in C#, are embodied in classes. To create a custom attribute, derive your new custom attribute class from System.Attribute:

public class BugFixAttribute : System.Attribute

You need to tell the compiler which kinds of elements this attribute can be used with (the attribute target). Specify this with (what else?) an attribute:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Property, ...