Fix issue 14458: Clipboard.GetText(TextDataFormat.Rtf) does not retrieve RTF that lacks a null terminating character

src/System.Private.Windows.Core/src/System/Private/Windows/Ole/Composition.NativeToManagedAdapter.cs

@@ -114,8 +114,8 @@ private static bool TryGetDataFromHGLOBAL<T>(
 
             object? value = request.Format switch
             {
-                DataFormatNames.Text or DataFormatNames.Rtf or DataFormatNames.OemText =>
-                    ReadStringFromHGLOBAL(hglobal, unicode: false),
+                DataFormatNames.Text or DataFormatNames.OemText => ReadStringFromHGLOBAL(hglobal, unicode: false),
+                DataFormatNames.Rtf => ReadRegisteredFormatStringFromHGLOBAL(hglobal, Encoding.Default),
                 DataFormatNames.Html or DataFormatNames.Xaml => ReadUtf8StringFromHGLOBAL(hglobal),
                 DataFormatNames.UnicodeText => ReadStringFromHGLOBAL(hglobal, unicode: true),
                 DataFormatNames.FileDrop => ReadFileListFromHDROP((HDROP)(nint)hglobal),
@@ -197,10 +197,6 @@ private static unsafe string ReadStringFromHGLOBAL(HGLOBAL hglobal, bool unicode
             //
             // CF_TEXT, CF_OEMTEXT, CF_UNICODETEXT, and CFSTR_FILENAME are supposed to have a null terminator.
             // If we cannot find one in the buffer, assume it is corrupted and return an empty string.
-            //
-            // Can't find the explicit docs for CF_RTF, but we've always treated it as null terminated.
-            // The RichText control itself null terminates but looks like it doesn't require it.
-            // Given our prior and "normal" behavior, we'll continue to expect a null terminator.
 
             try
             {
@@ -242,6 +238,40 @@ private static unsafe string ReadStringFromHGLOBAL(HGLOBAL hglobal, bool unicode
             }
         }
 
+        private static unsafe string ReadRegisteredFormatStringFromHGLOBAL(HGLOBAL hglobal, Encoding encoding)
+        {
+            void* buffer = PInvokeCore.GlobalLock(hglobal);
+            if (buffer is null)
+            {
+                throw new Win32Exception();
+            }
+
+            try
+            {
+                int size = checked((int)PInvokeCore.GlobalSize(hglobal));
+                if (size == 0)
+                {
+                    throw new Win32Exception();
+                }
+
+                ReadOnlySpan<byte> bytes = new((byte*)buffer, size);
+
+                // Registered format strings may be null-terminated, but the terminator is optional.
+                // If present, stop at the first null byte rather than decoding the entire allocation.
+                int nullIndex = bytes.IndexOf((byte)0);
+                if (nullIndex >= 0)
+                {
+                    bytes = bytes[..nullIndex];
+                }
+
+                return bytes.IsEmpty ? string.Empty : encoding.GetString(bytes);
+            }
+            finally
+            {
+                PInvokeCore.GlobalUnlock(hglobal);
+            }
+        }
+
         private static string ReadUtf8StringFromHGLOBAL(HGLOBAL hglobal)
         {
             void* buffer = PInvokeCore.GlobalLock(hglobal);

src/System.Private.Windows.Core/tests/System.Private.Windows.Core.Tests/System/Private/Windows/Ole/NativeToManagedAdapterTests.cs

@@ -4,6 +4,7 @@
 using System.ComponentModel;
 using System.Formats.Nrbf;
 using System.Private.Windows.BinaryFormat;
+
 using Windows.Win32;
 using Windows.Win32.Foundation;
 using Windows.Win32.System.Com;
@@ -19,6 +20,8 @@
     System.Private.Windows.Ole.TestFormat>;
 using DataFormats = System.Private.Windows.Ole.DataFormatsCore<System.Private.Windows.Ole.TestFormat>;
 
+using System.Text;
+
 namespace System.Private.Windows.Ole;
 
 public unsafe class NativeToManagedAdapterTests
@@ -99,6 +102,19 @@ public void GetData_CustomType_BinaryFormattedData_AsSerializationRecord()
         data!.TypeName.AssemblyQualifiedName.Should().Be("System.Int32[], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
     }
 
+    [Fact]
+    public void GetData_RtfWithoutNullTerminator_ReturnsRtfText()
+    {
+        const string rtf = "{\\rtf1\\ansi Test}";
+        MemoryStream stream = new(Encoding.Default.GetBytes(rtf));
+        using HGlobalNativeDataObject dataObject = new(stream, (ushort)DataFormats.GetOrAddFormat(DataFormatNames.Rtf).Id);
+
+        var composition = Composition.Create(ComHelpers.GetComPointer<IDataObject>(dataObject));
+        object? data = composition.GetData(DataFormatNames.Rtf);
+
+        data.Should().Be(rtf);
+    }
+
 #if NET
     [Fact]
     public void GetData_CustomType_BinaryFormattedJson_AsSerializationRecord()