Merge pull request #6 from gregsdennis/corvus

add post about corvus.jsonschema

Author: gregsdennis

.jekyll-metadata

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+---
+title: "Dropping Project Support for Code Generation"
+date: 2024-02-11 09:00:00 +1200
+tags: [json-schema, codegen]
+toc: true
+pin: false
+---
+
+Some time ago, I released [my first attempt at code generation](/posts/exploring-codegen) from JSON Schemas.  However, I've decided to deprecate the library in favor of _Corvus.JsonSchema_.
+
+When I created _JsonSchema.Net.CodeGeneration_, I knew about _Corvus.JsonSchema_, but I thought it was merely an alternative validator.  I didn't truly understand its approach to supporting JSON Schema in .Net.
+
+Today we're going to take a look at this seeming competitor to see why it actually isn't one.
+
+## What is _Corvus.JsonSchema_?
+
+_Corvus.JsonSchema_ is a JSON Schema code generator that bakes validation logic directly into the model.
+
+To show this, consider the following schema.
+
+```json
+{
+  "type": "object",
+  "properties": {
+    "foo": {
+      "type": "integer",
+      "minimum": 0
+    }
+  }
+}
+```
+
+As one would expect, the library would generate a class with a single property: `int Foo`.  But it also generates an `.IsValid()` method that contains all of the validation logic.  So if you set `model.Foo = -1`, the `.IsValid()` method will return false.
+
+However _Corvus.JsonSchema_ has another trick up its sleeve.  But before we get into that, it will help to have some understanding of how _System.Text.Json_'s `JsonElement` works.
+
+## A quick review of `JsonElement`
+
+Under the hood, `JsonElement` captures the portion of the parsed JSON text by using spans.  This has a number of follow-on benefits:
+
+- By avoiding substringing, there are no additional heap allocations.
+- `JsonElement` can be a struct, which further avoids allocations, because it only maintains references to existing memory.
+- By holding onto the original text, the value can be interpreted different ways.  For example, numbers could be read as `double` or `decimal` or `integer`.
+
+As an example, consider this string:
+
+```json
+{ "foo": 42, "bar": [ "a string", false ] }
+```
+
+Five different `JsonElement`s would be created:
+
+- top-level object
+- number value under `foo`
+- array value under `bar`
+- first element of `bar` array
+- second element of `bar` array
+
+But the kicker is that everything simply references the original string.
+
+|Value|Backing span|
+|:-|:-|
+| top-level object | start: 0, length: 44 |
+| number value under `foo` | start: 9, length: 2 |
+| array value under `bar` | start: 20, length: 21 |
+| first element of `bar` array | start: 22, length: 10 |
+| second element of `bar` array | start: 34, length: 5 |
+
+## Back to the validator
+
+_Corvus.JsonSchema_ builds on this "backing data" pattern that `JsonElement` establishes.  Instead of creating a backing field that is the same type that the property exposes, which is the traditional approach for backing fields, the generated code will use a `JsonElement` for the backing field while the property is still strongly typed.
+
+This means that a model generated by the library can usually be deserialized without any extra allocations, resulting in very high performance!
+
+> For a much better explanation of what's going on inside the package than what I can provide, I recommend you watch their [showcase video](https://www.youtube.com/watch?v=aTcD-axJBac).
+{: .prompt-tip }
+
+## Keep moving forward
+
+Ever since I saw that video, I've lamented the fact that it's only available as a `dotnet` tool.  I've always envisioned this functionality as a Roslyn source generator.
+
+To that end, I've paired with [Matthew Adams](https://github.com/mwadams), one of the primary contributors to _Corvus.JsonSchema_, as co-mentor on a [project proposal](https://github.com/json-schema-org/community/issues/614) for JSON Schema's submission to Google's Summer of Code.  This project aims to wrap the existing library in an incremental source generator that uses JSON Schema files within a .Net project to automatically generate models at compile time.
+
+This is a great opportunity to learn about incremental source generators in .Net and build your open source portfolio.  If this sounds like a fun project, please make your interest known by commenting on the proposal issue linked above.
+
+(Even if it's not accepted by GSoc, we're probably going to do it anyway.)