fix: fix generation of usings of date types for when present in nested classes

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Fixes generation of usings/imports in where Date Types are placed in a nested class. [#6286](https://github.com/microsoft/kiota/issues/6286)
+
 ## [1.24.1] - 2025-03-13
 
 ### Changed

src/Kiota.Builder/Refiners/CommonLanguageRefiner.cs

@@ -802,7 +802,7 @@ currentClass.StartBlock is ClassDeclaration currentClassDeclaration &&
 
 
             if (usingsToAdd.Length != 0)
-                (currentClass.Parent is CodeClass parentClass ? parentClass : currentClass).AddUsing(usingsToAdd); //lots of languages do not support imports on nested classes
+                (currentClass.Parent as CodeClass ?? currentClass).AddUsing(usingsToAdd); //lots of languages do not support imports on nested classes
         }
         CrawlTree(current, x => AddPropertiesAndMethodTypesImports(x, includeParentNamespaces, includeCurrentNamespace, compareOnDeclaration, codeTypeFilter));
     }
@@ -892,7 +892,7 @@ protected static void CorrectCoreTypes(CodeClass? parentClass, Dictionary<string
             if (!string.IsNullOrEmpty(replacement.Item1))
                 type.Name = replacement.Item1;
             if (replacement.Item2 != null)
-                parentClass.AddUsing((CodeUsing)replacement.Item2.Clone());
+                (parentClass.Parent as CodeClass ?? parentClass).AddUsing((CodeUsing)replacement.Item2.Clone()); //lots of languages do not support imports on nested classes
         }
     }
     protected static void InlineParentClasses(CodeElement currentElement, CodeElement parent)

tests/Kiota.Builder.Tests/Refiners/CSharpLanguageRefinerTests.cs

@@ -670,6 +670,33 @@ public async Task ReplacesDateOnlyByNativeTypeAsync()
         Assert.Equal("Date", method.ReturnType.Name);
     }
     [Fact]
+    public async Task ReplacesDateOnlyByNativeTypeInNestedClassAsync()
+    {
+        var model = root.AddClass(new CodeClass
+        {
+            Name = "model",
+            Kind = CodeClassKind.Model
+        }).First();
+        var nestedModel = model.AddInnerClass(new CodeClass
+        {
+            Name = "nestedModel",
+            Kind = CodeClassKind.Model
+        }).First();
+        var propertyInNestedModel = nestedModel.AddProperty(new CodeProperty
+        {
+            Name = "nestedModelProperty",
+            Type = new CodeType
+            {
+                Name = "DateOnly",
+                IsExternal = true// this is external from the Kiota abstractions
+            },
+        }).First();
+        await ILanguageRefiner.RefineAsync(new GenerationConfiguration { Language = GenerationLanguage.CSharp }, root);
+        Assert.NotEmpty(model.StartBlock.Usings); // using is added to outer class.
+        Assert.Empty(nestedModel.StartBlock.Usings); // using is not added to nested model
+        Assert.Equal("Date", propertyInNestedModel.Type.Name);
+    }
+    [Fact]
     public async Task ReplacesTimeOnlyByNativeTypeAsync()
     {
         var model = root.AddClass(new CodeClass