Add support for JSON Schema const keyword

[Copilot]

JSON Schema draft 6 defines a const keyword that constrains a value to a single specific value. This was not parsed, validated, or reflected in generated C# or TypeScript code.

Changes

Core Schema (NJsonSchema)

• JsonSchema: Added Const (object?) property serialized as "const", with HasConstValue and ConstValueType helpers — the latter infers the semantic JSON type (string/integer/number/boolean) from the runtime JValue type
• ValidationErrorKind: Added ConstantValueMismatch
• JsonSchemaValidator: Added ValidateConst using JToken.DeepEquals for type-aware comparison (avoids false mismatches like 42 vs 42.0)

C# Code Generation

• CSharpTypeResolver: Infers C# type from const value when no type is specified (bypasses the IsAnyType short-circuit)
• PropertyModel: HasSetter returns false for const properties — they generate read-only auto-properties
• ValueGeneratorBase: GetDefaultValue uses const value as initializer regardless of useSchemaDefault; new GetConstantValue virtual method formats values with invariant-culture numeric precision

TypeScript Code Generation

• TypeScriptTypeResolver: Returns a TypeScript literal type for const schemas (bypasses the IsAnyType short-circuit)

Example

Schema:

{ "properties": { "cmdType": { "const": "person" } } }

Generated C#:

public string CmdType { get; } = "person";

Generated TypeScript (interface):

cmdType: "person";

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• http://json-schema.org:80/draft-04/schema
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpw5orSR.tmp 5359f0.ts k/NJ�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_5b71db70-6e0b-4323-8a58-0462a95fe18a.ts p_e1d355b2-5b5f-4f7c-be4a-5845322a7084.ts sonSchema/src/NJsonSchema.CodeGeneration.TypeScript.Tests/node_modules/.bin/sh Script.Tests/temp_942af1c4-c91b-49a7-9495-6337341e00bb.ts f07547.ts odules/.bin/sh d0838ca85c58.ts /hom�� sonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratiosh -6931-4676-ac64-/home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGenerati-c sh n.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts cts/bin/NJsonSch-c sonSchema.CodeGe"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_58df9e39-82c6-432d-9414-0617d679ff74.ts 17993a.ts (http block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests @@ /tmp/tmp98DHIF.tmp 5359f0.ts ema.�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_5b71db70-6e0b-4323-8a58-0462a95fe18a.ts 4ad6-af65-04c325728f28.ts sonSchema/src/NJsonSchema.CodeGeneration.TypeScript.Tests/node_modules/.bin/sh Script.Tests/temp_e8f714c7-8ad3-4469-91de-9a091ecd703c.ts f07547.ts odules/.bin/sh d0838ca85c58.ts /hom�� sonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratiosh c7b773.ts /../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_3801dcef-dea8-4af2-be90-68aef6"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratio n.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts 42bd196a038c.ts modules/@npmcli/"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_58df9e39-82c6-432d-9414-0617d679ff74.ts 17993a.ts (http block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpaOsh8a.tmp 4551-a834-46c97d8f3d6d.ts sonS�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts 28ad04d6570f.ts ode Script.Tests/temp_e8f714c7-8ad3-4469-91de-9a091ecd703c.ts eGeneration.Type-c odules/.bin/node"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_dc48cadc-4c17-4913-8b1a-964ae10c5db4.ts 5631235fea31.ts sonS�� ema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.Typetail 66665d2b6ece.ts (http block)
• schemas.sportradar.com
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpw5orSR.tmp 5359f0.ts k/NJ�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_5b71db70-6e0b-4323-8a58-0462a95fe18a.ts p_e1d355b2-5b5f-4f7c-be4a-5845322a7084.ts sonSchema/src/NJsonSchema.CodeGeneration.TypeScript.Tests/node_modules/.bin/sh Script.Tests/temp_942af1c4-c91b-49a7-9495-6337341e00bb.ts f07547.ts odules/.bin/sh d0838ca85c58.ts /hom�� sonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratiosh -6931-4676-ac64-/home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGenerati-c sh n.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts cts/bin/NJsonSch-c sonSchema.CodeGe"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_58df9e39-82c6-432d-9414-0617d679ff74.ts 17993a.ts (dns block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests @@ /tmp/tmp98DHIF.tmp 5359f0.ts ema.�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_5b71db70-6e0b-4323-8a58-0462a95fe18a.ts 4ad6-af65-04c325728f28.ts sonSchema/src/NJsonSchema.CodeGeneration.TypeScript.Tests/node_modules/.bin/sh Script.Tests/temp_e8f714c7-8ad3-4469-91de-9a091ecd703c.ts f07547.ts odules/.bin/sh d0838ca85c58.ts /hom�� sonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratiosh c7b773.ts /../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_3801dcef-dea8-4af2-be90-68aef6"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneratio n.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts 42bd196a038c.ts modules/@npmcli/"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_58df9e39-82c6-432d-9414-0617d679ff74.ts 17993a.ts (dns block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpaOsh8a.tmp 4551-a834-46c97d8f3d6d.ts sonS�� neration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_3d90c537-de83-4880-a971-2e38dd07403d.ts 28ad04d6570f.ts ode Script.Tests/temp_e8f714c7-8ad3-4469-91de-9a091ecd703c.ts eGeneration.Type-c odules/.bin/node"tsc" /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.TypeScript.Tests/temp_dc48cadc-4c17-4913-8b1a-964ae10c5db4.ts 5631235fea31.ts sonS�� ema.CodeGeneration.TypeScript.Tests/debug_net8.0/../../../../src/NJsonSchema.CodeGeneration.Typetail 66665d2b6ece.ts (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support of const keyword</issue_title>
<issue_description>I'm not a Json schema deep diver, but somehow I miss support for the const keyword (at least for C# code generation):

{
"title": "Person",
"type": "object",
"properties": {
"cmdType": {
"$comment": "This results in a string template literal when generating TS",
"const": "person"
},
...
}</issue_description>

Comments on the Issue (you are @copilot in this section)

@RicoSuter Ref: http://json-schema.org/understanding-json-schema/reference/generic.html#constant-values

• Fixes Support of const keyword #1329

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