Fix nullable property deserialization in OpenAPI v3 provider

src/SwaggerProvider.DesignTime/v3/DefinitionCompiler.fs

@@ -268,6 +268,10 @@ type DefinitionCompiler(schema: OpenApiDocument, provideNullable) as this =
                     | true -> getReq schemaObj
                     |> Set.ofSeq
 
+                // Helper to check if a schema has the Null type flag (OpenAPI 3.0 nullable)
+                let isSchemaNullable (schema: IOpenApiSchema) =
+                    not (isNull schema) && schema.Type.HasValue && schema.Type.Value.HasFlag(JsonSchemaType.Null)
+
                 // Generate fields and properties
                 let members =
                     let generateProperty = generateProperty(UniqueNameGenerator())
@@ -279,7 +283,13 @@ type DefinitionCompiler(schema: OpenApiDocument, provideNullable) as this =
                         if String.IsNullOrEmpty propName then
                             failwithf $"Property cannot be created with empty name. TypeName:%A{tyName}; SchemaObj:%A{schemaObj}"
 
-                        let isRequired = schemaObjRequired.Contains propName
+                        // Check if the property is nullable (OpenAPI 3.0 nullable becomes Null type flag in 3.1)
+                        let isNullable = isSchemaNullable propSchema
+
+                        // A property is "required" for type generation if it's in the required list AND not nullable.
+                        // Nullable properties must be wrapped as Option<T>/Nullable<T> to represent null values,
+                        // even if they're in the required list (required + nullable means must be present but can be null).
+                        let isRequired = schemaObjRequired.Contains propName && not isNullable
 
                         let pTy =
                             compileBySchema ns (ns.ReserveUniqueName tyName (nicePascalName propName)) propSchema isRequired ns.RegisterType false
@@ -303,14 +313,17 @@ type DefinitionCompiler(schema: OpenApiDocument, provideNullable) as this =
                 let ctorParams, fields =
                     let required, optional =
                         List.zip (List.ofSeq schemaObjProperties) members
-                        |> List.partition(fun (x, _) -> schemaObjRequired.Contains x.Key)
+                        |> List.partition(fun (x, _) -> 
+                            let isNullable = isSchemaNullable x.Value
+                            schemaObjRequired.Contains x.Key && not isNullable)
 
                     required @ optional
                     |> List.map(fun (x, (f, p)) ->
                         let paramName = niceCamelName p.Name
+                        let isNullable = isSchemaNullable x.Value
 
                         let prParam =
-                            if schemaObjRequired.Contains x.Key then
+                            if schemaObjRequired.Contains x.Key && not isNullable then
                                 ProvidedParameter(paramName, f.FieldType)
                             else
                                 let paramDefaultValue = this.GetDefaultValue f.FieldType

tests/SwaggerProvider.ProviderTests/Schemas/v3/nullable-date.yaml

@@ -0,0 +1,31 @@
+openapi: 3.0.0
+info:
+  title: Nullable Date Test API
+  version: 1.0.0
+paths:
+  /test:
+    get:
+      operationId: getTest
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/PersonDto'
+components:
+  schemas:
+    PersonDto:
+      type: object
+      required:
+        - id
+        - name
+      properties:
+        id:
+          type: string
+        name:
+          type: string
+        birthDate:
+          type: string
+          format: date
+          nullable: true

tests/SwaggerProvider.ProviderTests/SwaggerProvider.ProviderTests.fsproj

@@ -26,6 +26,7 @@
     <Compile Include="v3\Swagger.I0181.Tests.fs" />
     <Compile Include="v3\Swagger.I0219.Tests.fs" />
     <Compile Include="v3\Swagger.I0279.Tests.fs" />
+    <Compile Include="v3\Swagger.NullableDate.Tests.fs" />
     <Compile Include="v3\Swashbuckle.ReturnControllers.Tests.fs" />
     <Compile Include="v3\Swashbuckle.ReturnTextControllers.Tests.fs" />
     <Compile Include="v3\Swashbuckle.UpdateControllers.Tests.fs" />

tests/SwaggerProvider.ProviderTests/TestNullableDate.fsx

@@ -0,0 +1,20 @@
+#r "nuget: SwaggerProvider"
+open SwaggerProvider
+
+[<Literal>]
+let Schema = __SOURCE_DIRECTORY__ + "/Schemas/v3/nullable-date.yaml"
+
+type TestApi = OpenApiClientProvider<Schema>
+
+// Check the type of BirthDate property
+let personType = typeof<TestApi.PersonDto>
+let birthDateProp = personType.GetProperty("BirthDate")
+
+printfn "BirthDate property type: %A" birthDateProp.PropertyType
+printfn "Is generic: %b" birthDateProp.PropertyType.IsGenericType
+
+if birthDateProp.PropertyType.IsGenericType then
+    let genericTypeDef = birthDateProp.PropertyType.GetGenericTypeDefinition()
+    printfn "Generic type definition: %A" genericTypeDef
+    printfn "Is Option: %b" (genericTypeDef = typedefof<Option<_>>)
+    printfn "Is Nullable: %b" (genericTypeDef = typedefof<System.Nullable<_>>)

tests/SwaggerProvider.ProviderTests/v3/Swagger.NullableDate.Tests.fs

@@ -0,0 +1,26 @@
+module Swagger.NullableDate.Tests
+
+open SwaggerProvider
+open Xunit
+open FsUnitTyped
+
+[<Literal>]
+let Schema = __SOURCE_DIRECTORY__ + "/../Schemas/v3/nullable-date.yaml"
+
+type TestApi = OpenApiClientProvider<Schema>
+
+[<Fact>]
+let ``PersonDto should have nullable birthDate property``() =
+    let personType = typeof<TestApi.PersonDto>
+    let birthDateProp = personType.GetProperty("BirthDate")
+    birthDateProp |> shouldNotEqual null
+
+    // The property should be Option<DateTimeOffset> (default) or Nullable<DateTimeOffset> (with PreferNullable=true)
+    let propType = birthDateProp.PropertyType
+    propType.IsGenericType |> shouldEqual true
+
+    let genericTypeDef = propType.GetGenericTypeDefinition()
+    let hasNullableWrapper = 
+        genericTypeDef = typedefof<Option<_>> || genericTypeDef = typedefof<System.Nullable<_>>
+
+    hasNullableWrapper |> shouldEqual true