Add ATS nullable scalar support

src/Aspire.Hosting.CodeGeneration.Python/AtsPythonCodeGenerator.cs

@@ -209,7 +209,7 @@ private string MapTypeRefToPython(AtsTypeRef? typeRef)
             return wrapperClassName;
         }
 
-        return typeRef.Category switch
+        var mappedType = typeRef.Category switch
         {
             AtsTypeCategory.Primitive => MapPrimitiveType(typeRef.TypeId),
             AtsTypeCategory.Enum => MapEnumType(typeRef.TypeId),
@@ -225,6 +225,19 @@ private string MapTypeRefToPython(AtsTypeRef? typeRef)
             AtsTypeCategory.Unknown => "typing.Any",  // Unknown types use 'Any' since they're not in the ATS universe
             _ => "typing.Any"  // Fallback for any unhandled categories
         };
+        return ApplyNullableType(typeRef, mappedType);
+    }
+
+    private static string ApplyNullableType(AtsTypeRef typeRef, string mappedType)
+    {
+        if (typeRef.IsNullable != true || typeRef.Category is not (AtsTypeCategory.Primitive or AtsTypeCategory.Enum))
+        {
+            return mappedType;
+        }
+
+        return typeRef.TypeId is AtsConstants.Void or AtsConstants.Any or AtsConstants.CancellationToken
+            ? mappedType
+            : $"{mappedType} | None";
     }
 
     /// <summary>

src/Aspire.Hosting.CodeGeneration.TypeScript/AtsTypeScriptCodeGenerator.cs

@@ -204,7 +204,7 @@ private string MapTypeRefToTypeScript(AtsTypeRef? typeRef)
             return GetInterfaceName(wrapperClassName);
         }
 
-        return typeRef.Category switch
+        var mappedType = typeRef.Category switch
         {
             AtsTypeCategory.Primitive => MapPrimitiveType(typeRef.TypeId),
             AtsTypeCategory.Enum => MapEnumType(typeRef.TypeId),
@@ -220,6 +220,19 @@ private string MapTypeRefToTypeScript(AtsTypeRef? typeRef)
             AtsTypeCategory.Unknown => "any",  // Unknown types use 'any' since they're not in the ATS universe
             _ => "any"  // Fallback for any unhandled categories
         };
+        return ApplyNullableType(typeRef, mappedType);
+    }
+
+    private static string ApplyNullableType(AtsTypeRef typeRef, string mappedType)
+    {
+        if (typeRef.IsNullable != true || typeRef.Category is not (AtsTypeCategory.Primitive or AtsTypeCategory.Enum))
+        {
+            return mappedType;
+        }
+
+        return typeRef.TypeId is AtsConstants.Void or AtsConstants.Any or AtsConstants.CancellationToken
+            ? mappedType
+            : $"{mappedType} | null";
     }
 
     /// <summary>

src/Aspire.Hosting.RemoteHost/AtsCapabilityScanner.cs

@@ -1052,6 +1052,7 @@ public static List<AtsCapabilityInfo> ScanCapabilities(
 
         // Collect public properties for the DTO interface
         var properties = new List<AtsDtoPropertyInfo>();
+        var nullabilityContext = new NullabilityInfoContext();
 
         foreach (var prop in type.GetProperties(BindingFlags.Public | BindingFlags.Instance))
         {
@@ -1074,6 +1075,7 @@ public static List<AtsCapabilityInfo> ScanCapabilities(
             {
                 continue;
             }
+            propTypeRef = WithNullability(propTypeRef, prop.PropertyType, nullabilityContext.Create(prop).ReadState);
 
             IReadOnlyList<AtsCallbackParameterInfo>? callbackParameters = null;
             AtsTypeRef? callbackReturnType = null;
@@ -1083,7 +1085,7 @@ public static List<AtsCapabilityInfo> ScanCapabilities(
             }
 
             var propDescription = GetXmlDocSummary(xmlDoc, $"P:{type.FullName}.{prop.Name}");
-            var isOptional = !prop.CanWrite || Nullable.GetUnderlyingType(prop.PropertyType) is not null;
+            var isOptional = !prop.CanWrite || propTypeRef.IsNullable == true;
 
             properties.Add(new AtsDtoPropertyInfo
             {
@@ -1492,6 +1494,7 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
         var exposeAllProperties = HasExposePropertiesAttribute(contextType);
         var exposeAllMethods = HasExposeMethodsAttribute(contextType);
         var typeExportAttr = GetAspireExportAttribute(contextType);
+        var nullabilityContext = new NullabilityInfoContext();
 
         // Scan properties
         foreach (var property in contextType.GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static))
@@ -1591,6 +1594,9 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
                     // Skip properties with unmapped types
                     continue;
                 }
+                var propertyNullability = nullabilityContext.Create(property);
+                var getterTypeRef = WithNullability(propertyTypeRef!, propType, propertyNullability.ReadState);
+                var setterTypeRef = WithNullability(propertyTypeRef!, propType, propertyNullability.WriteState);
 
                 // Create type ref for the context type with full inheritance info
                 var contextTypeRef = CreateHandleTypeRef(contextType);
@@ -1616,7 +1622,7 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
                         OwningTypeName = typeName,
                         Description = propertyDescription,
                         Parameters = [
-                           new AtsParameterInfo
+                            new AtsParameterInfo
                             {
                                 Name = "context",
                                 Type = contextTypeRef,
@@ -1625,8 +1631,8 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
                                 IsCallback = false,
                                 DefaultValue = null
                             }
-                       ],
-                        ReturnType = propertyTypeRef!,
+                        ],
+                        ReturnType = getterTypeRef,
                         TargetTypeId = typeId,
                         TargetType = contextTypeRef,
                         TargetParameterName = "context",
@@ -1657,7 +1663,7 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
                         OwningTypeName = typeName,
                         Description = $"Sets the {property.Name} property",
                         Parameters = [
-                           new AtsParameterInfo
+                            new AtsParameterInfo
                             {
                                 Name = "context",
                                 Type = contextTypeRef,
@@ -1669,13 +1675,13 @@ private static ContextTypeCapabilitiesResult CreateContextTypeCapabilities(
                             new AtsParameterInfo
                             {
                                 Name = "value",
-                                Type = propertyTypeRef!,
+                                Type = setterTypeRef,
                                 IsOptional = false,
-                                IsNullable = false,
+                                IsNullable = setterTypeRef.IsNullable == true,
                                 IsCallback = false,
                                 DefaultValue = null
                             }
-                       ],
+                        ],
                         ReturnType = contextTypeRef,
                         TargetTypeId = typeId,
                         TargetType = contextTypeRef,
@@ -2506,6 +2512,32 @@ private static bool IsResourceBuilderType(Type type)
         Category = AtsTypeCategory.Primitive
     };
 
+    private static AtsTypeRef WithNullability(AtsTypeRef typeRef, Type declaredType, NullabilityState nullabilityState)
+    {
+        var isNullable = nullabilityState == NullabilityState.Nullable ||
+            Nullable.GetUnderlyingType(declaredType) is not null;
+        if (!isNullable)
+        {
+            return typeRef;
+        }
+
+        return new AtsTypeRef
+        {
+            TypeId = typeRef.TypeId,
+            ClrType = typeRef.ClrType,
+            Category = typeRef.Category,
+            IsInterface = typeRef.IsInterface,
+            IsNullable = true,
+            ElementType = typeRef.ElementType,
+            KeyType = typeRef.KeyType,
+            ValueType = typeRef.ValueType,
+            IsReadOnly = typeRef.IsReadOnly,
+            UnionTypes = typeRef.UnionTypes,
+            ImplementedInterfaces = typeRef.ImplementedInterfaces,
+            BaseType = typeRef.BaseType
+        };
+    }
+
     /// <summary>
     /// Creates an AtsTypeRef from a CLR type, optionally collecting enum types.
     /// </summary>

src/Aspire.TypeSystem/AtsCapabilityInfo.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
 
 namespace Aspire.TypeSystem;
 
@@ -34,6 +35,19 @@ public sealed class AtsTypeRef
     /// </summary>
     public bool IsInterface { get; init; }
 
+    /// <summary>
+    /// Gets or sets whether this type reference accepts a JSON null value at its current use site.
+    /// </summary>
+    /// <remarks>
+    /// Nullability is attached to the type reference as it appears in a capability or DTO property.
+    /// Nested element, key, and value type nullability is only represented when those nested
+    /// references were scanned from member metadata that exposes nullability information.
+    /// For example, a DTO property declared as <code>string?</code> produces a nullable string
+    /// type reference, while the same CLR <see cref="string" /> type on a non-nullable property does not.
+    /// </remarks>
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
+    public bool? IsNullable { get; init; }
+
     /// <summary>
     /// Gets or sets the element type reference for Array/List types.
     /// </summary>
@@ -307,6 +321,7 @@ public sealed class AtsParameterInfo
     /// <summary>
     /// Gets or sets whether this parameter is nullable.
     /// </summary>
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
     public bool IsNullable { get; init; }
 
     /// <summary>

tests/Aspire.Hosting.CodeGeneration.Python.Tests/AtsPythonCodeGeneratorTests.cs

@@ -293,6 +293,22 @@ public void GeneratedCode_UsesTypeHints()
         Assert.Contains(": str", aspirePy);
     }
 
+    [Fact]
+    public void GeneratedCode_NullableScalarProperties_GenerateNullableTypes()
+    {
+        var atsContext = CreateContextFromBothAssemblies();
+
+        var files = _generator.GenerateDistributedApplication(atsContext);
+        var aspirePy = files["aspire_app.py"];
+
+        Assert.Contains("OptionalField: str | None", aspirePy);
+        Assert.Contains("def description(self) -> str | None:", aspirePy);
+        Assert.Contains("def description(self, value: str | None) -> None:", aspirePy);
+        Assert.Contains("Name: str", aspirePy);
+        Assert.Contains("def name(self) -> str:", aspirePy);
+        Assert.Contains("def name(self, value: str) -> None:", aspirePy);
+    }
+
     [Fact]
     public void GeneratedCode_SanitizesPythonKeywordIdentifiers()
     {

tests/Aspire.Hosting.CodeGeneration.Python.Tests/Snapshots/AtsGeneratedAspire.verified.py

@@ -1,4 +1,4 @@
-#   -------------------------------------------------------------
+#   -------------------------------------------------------------
 #   Copyright (c) Microsoft Corporation. All rights reserved.
 #   Licensed under the MIT License. See LICENSE in project root for information.
 #
@@ -1543,7 +1543,7 @@ class TestConfigDto(typing.TypedDict, total=False):
     Name: str
     Port: int
     Enabled: bool
-    OptionalField: str
+    OptionalField: str | None
 
 class TestDeeplyNestedDto(typing.TypedDict, total=False):
     NestedData: AspireDict[str, AspireList[TestConfigDto]]
@@ -1655,16 +1655,16 @@ def handle(self) -> Handle:
         return self._handle
 
     @_uncached_property
-    def name(self) -> str:
+    def name(self) -> str | None:
         """Gets the Name property"""
         result = self._client.invoke_capability(
             'Aspire.Hosting.CodeGeneration.TypeScript.Tests.TestTypes/TestCallbackContext.name',
             {'context': self._handle}
         )
-        return typing.cast(str, result)
+        return typing.cast(str | None, result)
 
     @name.setter
-    def name(self, value: str) -> None:
+    def name(self, value: str | None) -> None:
         """Sets the Name property"""
         self._client.invoke_capability(
             'Aspire.Hosting.CodeGeneration.TypeScript.Tests.TestTypes/TestCallbackContext.setName',
@@ -1764,16 +1764,16 @@ def name(self, value: str) -> None:
         )
 
     @_uncached_property
-    def description(self) -> str:
+    def description(self) -> str | None:
         """Gets the Description property"""
         result = self._client.invoke_capability(
             'Aspire.Hosting.CodeGeneration.TypeScript.Tests.TestTypes/TestEnvironmentContext.description',
             {'context': self._handle}
         )
-        return typing.cast(str, result)
+        return typing.cast(str | None, result)
 
     @description.setter
-    def description(self, value: str) -> None:
+    def description(self, value: str | None) -> None:
         """Sets the Description property"""
         self._client.invoke_capability(
             'Aspire.Hosting.CodeGeneration.TypeScript.Tests.TestTypes/TestEnvironmentContext.setDescription',