Add test for StringBuilder null terminator implementation detail (#21800)

* Remove some commented out code.

* Add test verifying that we put a null terminator 2-past the end of the native buffer allocated for a StringBuilder.

3 files changed, 14 insertions, 1 deletions

diff --git a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTest.cs b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTest.cs
index cc7369f155..f7684a3463 100644
--- a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTest.cs
+++ b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTest.cs
diff --git a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestNative.cpp b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestNative.cpp
index df9a6d3649..833c3166d8 100644
--- a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestNative.cpp
+++ b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestNative.cpp
@@ -153,3 +153,11 @@ extern "C" DLL_EXPORT BOOL STDMETHODCALLTYPE ReverseP_MarshalStrB_Out(Test_Del_M
     return TRUE;
 
 }
+
+// Verify that we append extra null terminators to our StringBuilder native buffers.
+// Although this is a hidden implementation detail, it would be breaking behavior to stop doing this
+// so we have a test for it. In particular, this detail prevents us from optimizing marshalling StringBuilders by pinning.
+extern "C" DLL_EXPORT BOOL STDMETHODCALLTYPE Verify_NullTerminators_PastEnd(LPCWSTR buffer, int length)
+{
+    return buffer[length+1] == W('\0');
+}
diff --git a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestPInvokeDef.cs b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestPInvokeDef.cs
index f54397ba6b..d8648df2e8 100644
--- a/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestPInvokeDef.cs
+++ b/tests/src/Interop/StringMarshalling/LPTSTR/LPTSTRTestPInvokeDef.cs
@@ -63,5 +63,10 @@ namespace NativeDefs
         [DllImport(NativeBinaryName)]
         public static extern bool ReverseP_MarshalStrB_InOut(Del_MarshalStrB_InOut d, [MarshalAs(UnmanagedType.LPTStr)]string s);
 
+        [DllImport(NativeBinaryName)]
+        public static extern bool Verify_NullTerminators_PastEnd(StringBuilder builder, int length);
+
+        [DllImport(NativeBinaryName, EntryPoint = "Verify_NullTerminators_PastEnd")]
+        public static extern bool Verify_NullTerminators_PastEnd_Out([Out] StringBuilder builder, int length);
     }
-}
\ No newline at end of file
+}