Report and update deprecated APIs in C#

When evolving an API, you have to introduce breaking changes sooner or later. The traditional way of dealing with that is to mark the deprecated types and members with the [Obsolete] attribute and explain how to migrate to the new API using the attribute's message.

JetBrains Fleet provides a more elegant solution that allows API users to find and automatically convert the old API to a new one. As the API author, you need to mark the obsolete type or member with the [CodeTemplate] attribute from JetBrains.Annotations where you can specify a search pattern to match the old API and a replacement pattern for it. The attribute will act as a custom code inspection with the corresponding quick-fix.

[CodeTemplateAttribute] is recognized by all other JetBrains products that analyze C# code, for example, JetBrains Rider JetBrains ReSharper.

Let's see how it works using an example:

  public class MyAssert
  {
    // Deprecated API. Usages look like:
    // MyAssert.IsTrue(args.Length > 0);
    public static void IsTrue(bool condition)
    {
      if (!condition)
        throw new Exception("Assertion failed");
    }

    // New API. Usages should look like:
    // MyAssert.That(args.Length > 0, Is.True);
    public static void That<T>(T value, Constraint<T> constraint)
    {
      // ...
    }
  }

  public class Constraint<T> { }

  class Is
  {
    public static Constraint<bool> True => null;
    public static Constraint<bool> False => null;
  }

    

Let's annotate the deprecated IsTrue() method with the [CodeTemplate] attribute:

    [CodeTemplate(
      searchTemplate: "$member$($expr$)",
      Message = "The API is deprecated, use 'MyAssert.That' instead",
      ReplaceTemplate = "MyAssert.That($expr$, Is.True)",
      ReplaceMessage = "Convert to 'MyAssert.That'")]
    public static void IsTrue(bool condition)
    {
      if (!condition)
        throw new Exception("Assertion failed");
    }

    

Now JetBrains Fleet will report all usages of MyAssert.IsTrue() and suggest a migration fix:

JetBrains Fleet: Using [CodeTemplate] attribute to suggest migration fixes for deprecated APIs

MyAssert.IsTrue(args.Length > 0);

MyAssert.That(args.Length > 0, Is.True);

Last modified: 15 April 2024