Fixes System.Drawing.Image.FromStream(...) throws "Parameter is not valid." when Stream.Read does not return full file length

src/System.Private.Windows.Core/src/Windows/Win32/System/Com/ComManagedStream.cs

@@ -122,12 +122,26 @@ HRESULT ISequentialStream.Interface.Read(void* pv, uint cb, uint* pcbRead)
         ActualizeVirtualPosition();
 
         Span<byte> buffer = new(pv, checked((int)cb));
-        int read = _dataStream.Read(buffer);
+
+        // Stream.Read can legally return fewer bytes than requested (e.g. chunked/network streams).
+        // Some callers (GDI+) treat a short read as failure, so fill the buffer until EOF.
+        int read = 0;
+        while (read < buffer.Length)
+        {
+            int bytesRead = _dataStream.Read(buffer[read..]);
+            if (bytesRead == 0)
+            {
+                break;
+            }
+
+            read += bytesRead;
+        }
 
         if (pcbRead is not null)
             *pcbRead = (uint)read;
 
-        return HRESULT.S_OK;
+        // Per IStream::Read, a short read caused by reaching EOF is signaled with S_FALSE.
+        return read < buffer.Length ? HRESULT.S_FALSE : HRESULT.S_OK;
     }
 
     HRESULT IStream.Interface.Read(void* pv, uint cb, uint* pcbRead)

src/System.Private.Windows.Core/tests/System.Private.Windows.Core.Tests/Windows/Win32/System/Com/ComManagedStreamTests.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Windows.Win32.Foundation;
+
 namespace Windows.Win32.System.Com.Tests;
 
 public class ComManagedStreamTests
@@ -22,6 +24,56 @@ public void Ctor_SeekableStream_UsesOriginalStream()
         comManagedStream.GetDataStream().Should().BeSameAs(seekableStream);
     }
 
+    [Fact]
+    public unsafe void Read_StreamReturnsShortReads_FillsBuffer()
+    {
+        // Streams are allowed to return fewer bytes than requested. Simulate a chunked/network stream
+        // and verify the wrapper keeps reading until the requested buffer is filled. See issue #14064.
+        byte[] sourceBytes = new byte[1024];
+        for (int index = 0; index < sourceBytes.Length; index++)
+        {
+            sourceBytes[index] = (byte)index;
+        }
+
+        using ChunkingStream shortReadStream = new(sourceBytes, chunkSize: 100);
+        ComManagedStream comManagedStream = new(shortReadStream);
+
+        byte[] destinationBuffer = new byte[sourceBytes.Length];
+        uint bytesReadCount;
+        fixed (byte* destinationPointer = destinationBuffer)
+        {
+            ((IStream.Interface)comManagedStream).Read(destinationPointer, (uint)destinationBuffer.Length, &bytesReadCount).Should().Be(HRESULT.S_OK);
+        }
+
+        bytesReadCount.Should().Be((uint)sourceBytes.Length);
+        destinationBuffer.Should().Equal(sourceBytes);
+    }
+
+    [Fact]
+    public unsafe void Read_RequestPastEndOfStream_ReturnsOnlyAvailableBytes()
+    {
+        // When more bytes are requested than remain, only the available bytes are returned (no throw on EOF),
+        // and a short read caused by EOF is reported with S_FALSE. See issue #14064.
+        byte[] sourceBytes = new byte[50];
+        for (int index = 0; index < sourceBytes.Length; index++)
+        {
+            sourceBytes[index] = (byte)index;
+        }
+
+        using ChunkingStream shortReadStream = new(sourceBytes, chunkSize: 10);
+        ComManagedStream comManagedStream = new(shortReadStream);
+
+        byte[] destinationBuffer = new byte[sourceBytes.Length * 2];
+        uint bytesReadCount;
+        fixed (byte* destinationPointer = destinationBuffer)
+        {
+            ((IStream.Interface)comManagedStream).Read(destinationPointer, (uint)destinationBuffer.Length, &bytesReadCount).Should().Be(HRESULT.S_FALSE);
+        }
+
+        bytesReadCount.Should().Be((uint)sourceBytes.Length);
+        destinationBuffer.Take(sourceBytes.Length).Should().Equal(sourceBytes);
+    }
+
     private class TestStream : MemoryStream
     {
         private readonly bool _canSeek;
@@ -33,4 +85,60 @@ public TestStream(bool canSeek, int numBytes) : base(new byte[numBytes])
             _canSeek = canSeek;
         }
     }
+
+    // Seekable stream that never returns more than a fixed chunk per Read, simulating chunked/network streams.
+    // Derives from Stream (not MemoryStream) so the span-based Read routes through this Read override on all targets.
+    private sealed class ChunkingStream : Stream
+    {
+        private readonly byte[] _sourceData;
+        private readonly int _maxBytesPerRead;
+        private int _position;
+
+        public ChunkingStream(byte[] sourceData, int chunkSize)
+        {
+            _sourceData = sourceData;
+            _maxBytesPerRead = chunkSize;
+        }
+
+        public override bool CanRead => true;
+        public override bool CanSeek => true;
+        public override bool CanWrite => false;
+        public override long Length => _sourceData.Length;
+
+        public override long Position
+        {
+            get => _position;
+            set => _position = (int)value;
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            int bytesToRead = Math.Min(Math.Min(count, _maxBytesPerRead), _sourceData.Length - _position);
+            if (bytesToRead <= 0)
+            {
+                return 0;
+            }
+
+            Array.Copy(_sourceData, _position, buffer, offset, bytesToRead);
+            _position += bytesToRead;
+            return bytesToRead;
+        }
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            _position = origin switch
+            {
+                SeekOrigin.Begin => (int)offset,
+                SeekOrigin.Current => _position + (int)offset,
+                SeekOrigin.End => _sourceData.Length + (int)offset,
+                _ => _position,
+            };
+
+            return _position;
+        }
+
+        public override void Flush() { }
+        public override void SetLength(long value) => throw new NotSupportedException();
+        public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
+    }
 }