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Add type: "null" downcasting when in oneOf and anyOf for OpenAPI v3 #2645

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Add type: "null" downcasting when in oneOf and anyOf for OpenAPI v3 #2645

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c532c4e
Add type: null downcasting with oneOf and anyOf for OpenApi v3
desjoerd Dec 12, 2025
f72d77c
Apply suggestions from code review
desjoerd Dec 15, 2025
f9984c9
Initial plan
Copilot Dec 15, 2025
cf00a1e
Fix PR review comments: replace Enum.IsDefined with power-of-2 check …
Copilot Dec 15, 2025
fadc853
Rewrite type checking logic to avoid cryptic binary operators
Copilot Dec 15, 2025
3f2402f
Use bitwise operator to remove Null flag from type comparison
Copilot Dec 15, 2025
b8ad06f
Merge pull request #1 from desjoerd/copilot/fix-pr-comments-2645
desjoerd Dec 15, 2025
59240b3
Prefer the original type before the inferred type
desjoerd Dec 16, 2025
8195ff6
Merge branch 'fix/support-v2/gh-2629' of https://github.com/desjoerd/…
desjoerd Dec 16, 2025
9a2ee7e
Remove useless comment
desjoerd Dec 16, 2025
263e3ca
Use HasMultipleTypes helper method
desjoerd Dec 16, 2025
3673612
Improve readability of ProcessCompositionForNull
desjoerd Dec 16, 2025
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86 changes: 74 additions & 12 deletions src/Microsoft.OpenApi/Models/OpenApiSchema.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -438,23 +438,39 @@

// enum
var enumValue = Enum is not { Count: > 0 }
&& !string.IsNullOrEmpty(Const)
&& !string.IsNullOrEmpty(Const)
&& version < OpenApiSpecVersion.OpenApi3_1
? new List<JsonNode> { JsonValue.Create(Const)! }
: Enum;
writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));

// Handle oneOf/anyOf with null type for v3.0 downcast
IList<IOpenApiSchema>? effectiveOneOf = OneOf;
IList<IOpenApiSchema>? effectiveAnyOf = AnyOf;
bool hasNullInComposition = false;
JsonSchemaType? inferredType = null;

if (version == OpenApiSpecVersion.OpenApi3_0)
{
(effectiveOneOf, var inferredOneOf, var nullInOneOf) = ProcessCompositionForNull(OneOf);
hasNullInComposition |= nullInOneOf;
inferredType = inferredOneOf ?? inferredType;
(effectiveAnyOf, var inferredAnyOf, var nullInAnyOf) = ProcessCompositionForNull(AnyOf);
hasNullInComposition |= nullInAnyOf;
inferredType = inferredAnyOf ?? inferredType;
}

// type
SerializeTypeProperty(writer, version);
SerializeTypeProperty(writer, version, inferredType);

// allOf
writer.WriteOptionalCollection(OpenApiConstants.AllOf, AllOf, callback);

// anyOf
writer.WriteOptionalCollection(OpenApiConstants.AnyOf, AnyOf, callback);
writer.WriteOptionalCollection(OpenApiConstants.AnyOf, effectiveAnyOf, callback);

// oneOf
writer.WriteOptionalCollection(OpenApiConstants.OneOf, OneOf, callback);
writer.WriteOptionalCollection(OpenApiConstants.OneOf, effectiveOneOf, callback);

// not
writer.WriteOptionalObject(OpenApiConstants.Not, Not, callback);
Expand Down Expand Up @@ -493,7 +509,7 @@
// nullable
if (version == OpenApiSpecVersion.OpenApi3_0)
{
SerializeNullable(writer, version);
SerializeNullable(writer, version, hasNullInComposition);
}

// discriminator
Expand Down Expand Up @@ -766,14 +782,17 @@
writer.WriteEndObject();
}

private void SerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion version)
private void SerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion version, JsonSchemaType? inferredType = null)
{
if (Type is null)
// Use original type or inferred type when the explicit type is not set
var typeToUse = Type ?? inferredType;

if (typeToUse is null)
{
return;
}

var unifiedType = IsNullable ? Type.Value | JsonSchemaType.Null : Type.Value;
var unifiedType = IsNullable ? typeToUse.Value | JsonSchemaType.Null : typeToUse.Value;
var typeWithoutNull = unifiedType & ~JsonSchemaType.Null;

switch (version)
Expand Down Expand Up @@ -804,8 +823,8 @@
private static void WriteUnifiedSchemaType(JsonSchemaType type, IOpenApiWriter writer)
{
var array = (from JsonSchemaType flag in jsonSchemaTypeValues
where type.HasFlag(flag)
select flag.ToFirstIdentifier()).ToArray();
where type.HasFlag(flag)
select flag.ToFirstIdentifier()).ToArray();
if (array.Length > 1)
{
writer.WriteOptionalCollection(OpenApiConstants.Type, array, (w, s) =>
Expand All @@ -822,9 +841,9 @@
}
}

private void SerializeNullable(IOpenApiWriter writer, OpenApiSpecVersion version)
private void SerializeNullable(IOpenApiWriter writer, OpenApiSpecVersion version, bool hasNullInComposition = false)
{
if (IsNullable)
if (IsNullable || hasNullInComposition)
{
switch (version)
{
Expand All @@ -838,6 +857,49 @@
}
}

/// <summary>
/// Processes a composition (oneOf or anyOf) for null types, filtering out null schemas and inferring common type.
/// </summary>
/// <param name="composition">The list of schemas in the composition.</param>
/// <returns>A tuple with the effective list, inferred type, and whether null is present in composition.</returns>
private static (IList<IOpenApiSchema>? effective, JsonSchemaType? inferredType, bool hasNullInComposition)
ProcessCompositionForNull(IList<IOpenApiSchema>? composition)
{
if (composition is null || !composition.Any(static s => s.Type is JsonSchemaType.Null))
{
// Nothing to patch
return (composition, null, false);
}

var nonNullSchemas = composition
.Where(static s => s.Type is null or not JsonSchemaType.Null)
.ToList();

if (nonNullSchemas.Count > 0)
{
JsonSchemaType commonType = 0;

foreach (var schema in nonNullSchemas)
{
commonType |= schema.Type.GetValueOrDefault() & ~JsonSchemaType.Null;
}

if (HasMultipleTypes(commonType) || commonType == 0)

Check warning

Code scanning / CodeQL

Constant condition Warning

Condition is always false because of
call to method HasMultipleTypes
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@desjoerd desjoerd Dec 16, 2025

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Hmm... So HasMultipleTypes returns true for 0?? I don't think this is correct

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@desjoerd desjoerd Dec 16, 2025

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Or at least, it would not match with the intent. If there are multiple or 0 we should not give a inferred type

{
return (nonNullSchemas, null, true);
}
else
{
// Single common type
return (nonNullSchemas, commonType, true);
}
Comment on lines +887 to +895

Check notice

Code scanning / CodeQL

Missed ternary opportunity Note

Both branches of this 'if' statement return - consider using '?' to express intent better.
}
else
{
return (null, null, true);
}
}

#if NET5_0_OR_GREATER
private static readonly Array jsonSchemaTypeValues = System.Enum.GetValues<JsonSchemaType>();
#else
Expand Down
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