Fix DateOnly: check target runtime version instead of design-time host

src/SwaggerProvider.DesignTime/Provider.OpenApiClient.fs

@@ -103,7 +103,8 @@ type public OpenApiClientTypeProvider(cfg: TypeProviderConfig) as this =
                                      |> Seq.map(fun e -> $"%s{e.Message} @ %s{e.Pointer}")
                                      |> String.concat "\n")
 
-                        let defCompiler = DefinitionCompiler(schema, preferNullable)
+                        let useDateOnly = cfg.SystemRuntimeAssemblyVersion.Major >= 6
+                        let defCompiler = DefinitionCompiler(schema, preferNullable, useDateOnly)
 
                         let opCompiler =
                             OperationCompiler(schema, defCompiler, ignoreControllerPrefix, ignoreOperationId, preferAsync)

src/SwaggerProvider.DesignTime/Provider.SwaggerClient.fs

@@ -86,7 +86,8 @@ type public SwaggerTypeProvider(cfg: TypeProviderConfig) as this =
 
                         let schema = SwaggerParser.parseSchema schemaData
 
-                        let defCompiler = DefinitionCompiler(schema, preferNullable)
+                        let useDateOnly = cfg.SystemRuntimeAssemblyVersion.Major >= 6
+                        let defCompiler = DefinitionCompiler(schema, preferNullable, useDateOnly)
 
                         let opCompiler =
                             OperationCompiler(schema, defCompiler, ignoreControllerPrefix, ignoreOperationId, preferAsync)

src/SwaggerProvider.DesignTime/v2/DefinitionCompiler.fs

@@ -150,7 +150,7 @@ and NamespaceAbstraction(name: string) =
                 Some ty)
 
 /// Object for compiling definitions.
-type DefinitionCompiler(schema: SwaggerObject, provideNullable) as this =
+type DefinitionCompiler(schema: SwaggerObject, provideNullable, useDateOnly: bool) as this =
     let definitionToSchemaObject = Map.ofSeq schema.Definitions
     let definitionToType = Collections.Generic.Dictionary<_, _>()
     let nsRoot = NamespaceAbstraction("Root")
@@ -353,11 +353,15 @@ type DefinitionCompiler(schema: SwaggerObject, provideNullable) as this =
                 | Double -> typeof<double>
                 | String -> typeof<string>
                 | Date ->
-                    // Runtime detection: design-time assembly targets netstandard2.0 so
-                    // compile-time NET6_0_OR_GREATER is not available; DateOnly exists on .NET 6+
-                    System.Type.GetType("System.DateOnly")
-                    |> Option.ofObj
-                    |> Option.defaultValue typeof<DateTime>
+                    // Use DateOnly only when the target runtime supports it (.NET 6+).
+                    // We check useDateOnly (derived from cfg.SystemRuntimeAssemblyVersion) rather than
+                    // probing the design-time host process, which may differ from the consumer's runtime.
+                    if useDateOnly then
+                        System.Type.GetType("System.DateOnly")
+                        |> Option.ofObj
+                        |> Option.defaultValue typeof<DateTime>
+                    else
+                        typeof<DateTime>
                 | DateTime -> typeof<DateTime>
                 | File -> typeof<byte>.MakeArrayType 1
                 | Enum(_, "string") -> typeof<string>

src/SwaggerProvider.DesignTime/v3/DefinitionCompiler.fs

@@ -165,7 +165,7 @@ and NamespaceAbstraction(name: string) =
                 Some ty)
 
 /// Object for compiling definitions.
-type DefinitionCompiler(schema: OpenApiDocument, provideNullable) as this =
+type DefinitionCompiler(schema: OpenApiDocument, provideNullable, useDateOnly: bool) as this =
     let pathToSchema =
         if isNull schema.Components then
             Map.empty
@@ -497,11 +497,15 @@ type DefinitionCompiler(schema: OpenApiDocument, provideNullable) as this =
                         // for `multipart/form-data` : https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#considerations-for-file-uploads
                         typeof<IO.Stream>
                     | HasFlag JsonSchemaType.String, "date" ->
-                        // Runtime detection: design-time assembly targets netstandard2.0 so
-                        // compile-time NET6_0_OR_GREATER is not available; DateOnly exists on .NET 6+
-                        System.Type.GetType("System.DateOnly")
-                        |> Option.ofObj
-                        |> Option.defaultValue typeof<DateTimeOffset>
+                        // Use DateOnly only when the target runtime supports it (.NET 6+).
+                        // We check useDateOnly (derived from cfg.SystemRuntimeAssemblyVersion) rather than
+                        // probing the design-time host process, which may differ from the consumer's runtime.
+                        if useDateOnly then
+                            System.Type.GetType("System.DateOnly")
+                            |> Option.ofObj
+                            |> Option.defaultValue typeof<DateTimeOffset>
+                        else
+                            typeof<DateTimeOffset>
                     | HasFlag JsonSchemaType.String, "date-time" -> typeof<DateTimeOffset>
                     | HasFlag JsonSchemaType.String, "uuid" -> typeof<Guid>
                     | HasFlag JsonSchemaType.String, _ -> typeof<string>

tests/SwaggerProvider.Tests/Schema.Parser.Tests.fs

@@ -13,7 +13,7 @@ module V2 =
     let testSchema schemaStr =
         let schema = SwaggerParser.parseSchema schemaStr
 
-        let defCompiler = DefinitionCompiler(schema, false)
+        let defCompiler = DefinitionCompiler(schema, false, Environment.Version.Major >= 6)
         let opCompiler = OperationCompiler(schema, defCompiler, true, false, true)
         opCompiler.CompileProvidedClients(defCompiler.Namespace)
         ignore <| defCompiler.Namespace.GetProvidedTypes()
@@ -35,7 +35,7 @@ module V3 =
                     |> Seq.map (fun e -> e.Message)
                     |> String.concat ";\n- ")*)
         try
-            let defCompiler = DefinitionCompiler(schema, false)
+            let defCompiler = DefinitionCompiler(schema, false, Environment.Version.Major >= 6)
             let opCompiler = OperationCompiler(schema, defCompiler, true, false, true)
             opCompiler.CompileProvidedClients(defCompiler.Namespace)
             defCompiler.Namespace.GetProvidedTypes()