From 6df22b1c8773f7bfdff8b4357b8d86f122b82e05 Mon Sep 17 00:00:00 2001
From: Wei Hu <live1206@gmail.com>
Date: Tue, 3 Mar 2026 07:24:52 +0000
Subject: [PATCH] Fix: Handle empty namespace in CodeWriter to prevent
 'global::.' output

When a CSharpType has an empty namespace (which can occur when Roslyn
cannot resolve a type reference, such as in Custom code that inherits
from generated types), the CodeWriter was outputting 'global::.'
followed by the type name, resulting in invalid C# syntax.

This fix adds a check to only append the namespace and dot separator
when the namespace is not empty.

Fixes https://github.com/microsoft/typespec/issues/9880

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../src/Writers/CodeWriter.cs                 |  7 ++-
 .../test/Writers/CodeWriterTests.cs           | 49 +++++++++++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs
index f57b5435a4d..24156f94b49 100644
--- a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs
+++ b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs
@@ -658,8 +658,11 @@ private void AppendType(CSharpType type, bool isDeclaration, bool writeTypeNameO
                 UseNamespace(type.Namespace);
 
                 AppendRaw("global::");
-                AppendRaw(type.Namespace);
-                AppendRaw(".");
+                if (!string.IsNullOrEmpty(type.Namespace))
+                {
+                    AppendRaw(type.Namespace);
+                    AppendRaw(".");
+                }
                 if (type.DeclaringType is not null)
                     AppendRaw($"{type.DeclaringType.Name}.");
                 AppendRaw(type.Name);
diff --git a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/CodeWriterTests.cs b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/CodeWriterTests.cs
index 6fec1f24139..cfa5e02c23e 100644
--- a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/CodeWriterTests.cs
+++ b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/CodeWriterTests.cs
@@ -727,5 +727,54 @@ public void CodeWriter_WriteMethod_AbstractMethodWithoutBody()
             var result = codeWriter.ToString(false);
             Assert.AreEqual(expected, result);
         }
+
+        // Regression test for https://github.com/microsoft/typespec/issues/9880
+        // When a CSharpType has an empty namespace (can happen when Roslyn cannot resolve the type),
+        // the CodeWriter should not output "global::." (global prefix followed by empty namespace and dot).
+        [Test]
+        public void TypeWithEmptyNamespaceDoesNotOutputGlobalDot()
+        {
+            // Create a CSharpType with an empty namespace (simulating an unresolved error type)
+            var typeWithEmptyNamespace = new CSharpType(
+                "UnresolvedType",
+                "", // empty namespace
+                isValueType: false,
+                isNullable: false,
+                declaringType: null,
+                args: [],
+                isPublic: true,
+                isStruct: false);
+
+            using var writer = new CodeWriter();
+            writer.Append($"{typeWithEmptyNamespace}");
+            var result = writer.ToString(false);
+
+            // Should contain "global::UnresolvedType" (without extra dot after global::)
+            Assert.That(result, Does.Contain("global::UnresolvedType"),
+                "Type with empty namespace should be prefixed with 'global::' directly followed by the type name");
+        }
+
+        // Verifies that types with proper namespaces still work correctly
+        [Test]
+        public void TypeWithNamespaceOutputsCorrectly()
+        {
+            var typeWithNamespace = new CSharpType(
+                "ResolvedType",
+                "Sample.Models",
+                isValueType: false,
+                isNullable: false,
+                declaringType: null,
+                args: [],
+                isPublic: true,
+                isStruct: false);
+
+            using var writer = new CodeWriter();
+            writer.Append($"{typeWithNamespace}");
+            var result = writer.ToString(false);
+
+            // Should contain the full qualified name
+            Assert.That(result, Does.Contain("global::Sample.Models.ResolvedType"),
+                "Type with namespace should include the full namespace in output");
+        }
     }
 }