Factory Method Generator

Convert a static partial class into a keyed factory using [FactoryMethod], [FactoryCase], and [FactoryDefault]. The generator produces Create, TryCreate, and async siblings when needed.

Quick start

using PatternKit.Generators.Factories;

namespace Demo;

[FactoryMethod(typeof(string), CreateMethodName = "Make")]
public static partial class MimeFactory
{
    [FactoryCase("json")]
    public static string Json() => "application/json";

    [FactoryDefault]
    public static string Default() => "application/octet-stream";
}

Generated API (sync):

public static string Make(string key);
public static bool TryCreate(string key, out string value);

Async is generated when any case/default returns ValueTask<T> or Task<T>:

public static ValueTask<string> MakeAsync(string key);
public static ValueTask<(bool Success, string Result)> TryCreateAsync(string key);

Behavior

• Key matching
  • KeyType == string and CaseInsensitiveStrings == true (default) → String.Equals with OrdinalIgnoreCase.
  • Otherwise exact match (Ordinal for strings).
• Default path
  • If [FactoryDefault] exists, it is used when no case matches.
  • Otherwise Create throws ArgumentOutOfRangeException; TryCreate returns false and default.
• Parameters
  • All [FactoryCase]/[FactoryDefault] methods must be static with identical signatures (return + parameters). Parameters are forwarded into generated methods.
• Async rules
  • If any mapping is async, generated async APIs use ValueTask<T>.
  • Task<T> cases are wrapped in ValueTask<T>.
  • Synchronous cases are wrapped with ValueTask.FromResult.

Diagnostics

Id	Message	Fix
PKKF001	[FactoryMethod] type must be static partial	Mark the class static partial.
PKKF002	Factory methods must share the same signature	Align return type and parameters across all cases/default.
PKKF003	Duplicate factory key	Remove or rename the duplicate key.
PKKF004	Multiple default factory methods	Keep only one [FactoryDefault].
PKKF005	Invalid factory key value	Ensure the key literal converts implicitly to KeyType.
PKKF006	Factory methods must be static	Mark all [FactoryCase]/[FactoryDefault] methods static.

Tips

• Prefer small, pure mapping methods; let the generated factory handle validation and branching.
• For string keys that must be case-sensitive, set CaseInsensitiveStrings = false and test both Create and TryCreate.
• Use CreateMethodName to match existing APIs (Make, FromKey, etc.).