Map [MinLength]/[MaxLength] on dictionary properties to minProperties / maxProperties by KitKeen · Pull Request #3922 · domaindrivendev/Swashbuckle.AspNetCore

KitKeen · 2026-04-24T06:25:28Z

Problem

Applying [MinLength(1)] to a property typed as a dictionary (IDictionary<,>, IReadOnlyDictionary<,>, etc.) produces "minLength": 1 on the generated schema:

public class MyModel
{
    [Required]
    [MinLength(1)]
    public IReadOnlyDictionary<string, string?> Values { get; init; }
}

"minLength" on an object schema is invalid per the OpenAPI 3.x spec — it's defined only for string schemas. Linters such as vacuum flag this (the original reporter hit it exactly that way). The equivalent constraint for object schemas is "minProperties".

Fix

OpenApiSchemaExtensions.Apply{Min,Max,}LengthAttribute already branches on schema.Type to pick MinItems / MaxItems for arrays. This PR adds a parallel branch for JsonSchemaTypes.Object so dictionary schemas get MinProperties / MaxProperties instead of the meaningless MinLength / MaxLength.

No changes to the MinLengthRouteConstraint / MaxLengthRouteConstraint handlers — route parameters are primitives and cannot be dictionaries, so the existing Array/otherwise split is correct there.

Before / after

public class MyModel
{
    [Required]
    [MinLength(1)]
    public IReadOnlyDictionary<string, string?> Values { get; init; }
}

Before:

"MyModel": {
  "type": "object",
  "required": [ "values" ],
  "properties": {
    "values": {
      "type": "object",
      "additionalProperties": { "type": "string", "nullable": true },
      "minLength": 1
    }
  }
}

After:

"MyModel": {
  "type": "object",
  "required": [ "values" ],
  "properties": {
    "values": {
      "type": "object",
      "additionalProperties": { "type": "string", "nullable": true },
      "minProperties": 1
    }
  }
}

Tests

Added to OpenApiSchemaExtensionsTests:

MinLength on dictionary schema → minProperties
MaxLength on dictionary schema → maxProperties
Length(min,max) on dictionary schema → min+maxProperties
Regression guards: MinLength on string schema still maps to minLength; on array schema still maps to minItems.

Scope

No public API changes.
No dependency changes.
Behavior change is limited to schemas whose Type includes Object — previously those would receive MinLength/MaxLength, which was invalid per the OpenAPI spec, so any consumer relying on the old output was already producing non-conforming documents.

Checklist

Bug fix includes a test that fails without the fix.
No public API surface change.
Builds & tests pass locally. (Local SDK is 8.0; repo requires 10.0 — relying on CI.)

…erties / maxProperties Fixes domaindrivendev#3915. OpenAPI defines minLength/maxLength only for string schemas; for object schemas (including dictionaries) the equivalent constraints are minProperties/maxProperties. Before this change, applying [MinLength(1)] to a property typed as IReadOnlyDictionary<TKey, TValue> produced "minLength": 1 on the schema, which is invalid per the spec and is flagged by linters such as vacuum. The three attribute handlers (MinLength, MaxLength, Length) already switch to MinItems/MaxItems when the schema is an Array. Extend the same branching to recognise Object schemas and emit MinProperties/MaxProperties instead of MinLength/MaxLength. Route constraint handlers (MinLengthRouteConstraint, MaxLengthRouteConstraint) are intentionally left alone: route parameters are primitives and cannot be dictionaries, so the existing two-branch array/string logic is correct there. Tests added in OpenApiSchemaExtensionsTests: - MinLength on dictionary → minProperties - MaxLength on dictionary → maxProperties - Length on dictionary → min+maxProperties - Regression guards: MinLength on string still maps to minLength, on array still maps to minItems.

Added: - End-to-end coverage via shared TypeWithValidationAttributes fixture — DictionaryWithMinMaxLength ([MinLength(1),MaxLength(3)]) and DictionaryWithLength ([Length(1,3)]) properties, asserted by both JsonSerializerSchemaGenerator and NewtonsoftSchemaGenerator tests. Reproduces the exact scenario from the issue (dictionary property on a model class) rather than only exercising ApplyValidationAttributes in isolation. - Enum-keyed dictionary shape — Object schema with known Properties and AdditionalPropertiesAllowed=false (see CreateDictionarySchema). MinLength must still route to MinProperties here. - Nullable dictionary shape — Type = Object | Null. HasFlag(Object) must still route to MinProperties. - Symmetry regression guards: MaxLength/Length on string map to MinLength/MaxLength, on array map to MinItems/MaxItems (previously only MinLength had explicit guards). Fixture updates are picked up by both Newtonsoft and JsonSerializer generator test suites because the shared TestSupport fixture drives both.

martincostello

Please make some test changes to that the OpenAPI snapshots demonstrate the required changes to the generated OpenAPI schemas.

martincostello · 2026-04-24T17:22:35Z

+    [Fact]
+    public static void ApplyValidationAttributes_MinLength_On_Dictionary_Maps_To_MinProperties()
+    {
+        // Arrange — dictionary schema is represented as an Object with AdditionalProperties


Please remove all the AI —s and replace with -s.

done, removed all AI markers

martincostello · 2026-04-24T17:24:19Z


    public string[] ArrayWithMinMaxLength { get; set; }

+    public IReadOnlyDictionary<string, string> DictionaryWithMinMaxLength { get; set; }


This name doesn't seem to match the purpose?

martincostello · 2026-04-24T17:24:23Z


    public string[] ArrayWithLength { get; set; }

+    public Dictionary<string, string> DictionaryWithLength { get; set; }


This name doesn't seem to match the purpose?

- Replace em-dashes with hyphens in OpenApiSchemaExtensionsTests.cs - Rename fixture property DictionaryWithMinMaxLength to IReadOnlyDictionaryWithMinMaxLength so the name matches its actual IReadOnlyDictionary<,> declared type - Add JSON-output test that serializes the schema for TypeWithValidationAttributes/TypeWithValidationAttributesViaMetadataType to OpenAPI 3.0 JSON and asserts the dictionary properties surface as minProperties/maxProperties in the produced OpenAPI document

…ttribute Per review: 'Length' as a noun does not describe a dictionary (which has a property/item count, not a length). The Attribute suffix makes it explicit that the property exists to test the [Length] attribute mapping rather than a conceptual 'length' of the dictionary.

martincostello · 2026-04-25T08:40:25Z


    public string[] ArrayWithLength { get; set; }

+    public Dictionary<string, string> DictionaryWithLengthAttribute { get; set; }


But it doesn't have an attribute...

martincostello · 2026-04-25T08:40:34Z


    public string[] ArrayWithMinMaxLength { get; set; }

+    public IReadOnlyDictionary<string, string> IReadOnlyDictionaryWithMinMaxLength { get; set; }


And there's no min or max.

Per follow-up review: in TypeWithValidationAttributesViaMetadataType the outer class is a bare property list (attributes live on the inner MetadataType class), so names that referenced attributes were misleading. Renamed to neutral, type-focused names that work in both fixture variants: - IReadOnlyDictionaryWithMinMaxLength -> BoundedReadOnlyDictionary - DictionaryWithLengthAttribute -> BoundedDictionary The bounded prefix describes the test purpose (a dictionary whose size is constrained); the readonly/mutable distinction reflects the actual type under test. No attribute presence is claimed by the property name.

KitKeen added 2 commits April 24, 2026 09:25

martincostello reviewed Apr 24, 2026

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KitKeen added 2 commits April 25, 2026 11:12

martincostello reviewed Apr 25, 2026

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Map [MinLength]/[MaxLength] on dictionary properties to minProperties / maxProperties#3922

Map [MinLength]/[MaxLength] on dictionary properties to minProperties / maxProperties#3922
KitKeen wants to merge 5 commits intodomaindrivendev:masterfrom
KitKeen:fix/minlength-dictionary-minproperties

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		public string[] ArrayWithMinMaxLength { get; set; }

		public IReadOnlyDictionary<string, string> DictionaryWithMinMaxLength { get; set; }


		public string[] ArrayWithLength { get; set; }

		public Dictionary<string, string> DictionaryWithLength { get; set; }

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KitKeen commented Apr 24, 2026

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