Fix up a few places in DeflateStream

Two changes:
- Get rid of some dead code in DeflateStream
- In debug builds, DeflateStream is asserting when the underlying stream returns bad data; in release it's throwing an exception.  I've changed it to always throw; this will also help to avoid future AVs if we decide to switch any more of the implementation to unsafe code.

Author: stephentoub

src/System.IO.Compression/src/System/IO/Compression/DeflateStream.cs

@@ -42,39 +42,32 @@ public DeflateStream(Stream stream, CompressionMode mode, bool leaveOpen)
         {
             if (stream == null)
                 throw new ArgumentNullException("stream");
-
-            if (CompressionMode.Compress != mode && CompressionMode.Decompress != mode)
-                throw new ArgumentException(SR.ArgumentOutOfRange_Enum, "mode");
-
-            _stream = stream;
-            _mode = mode;
-            _leaveOpen = leaveOpen;
-
-            switch (_mode)
+            
+            switch (mode)
             {
                 case CompressionMode.Decompress:
-
-                    if (!_stream.CanRead)
+                    if (!stream.CanRead)
                     {
                         throw new ArgumentException(SR.NotReadableStream, "stream");
                     }
-
                     _inflater = new Inflater();
-
                     break;
 
                 case CompressionMode.Compress:
-
-                    if (!_stream.CanWrite)
+                    if (!stream.CanWrite)
                     {
                         throw new ArgumentException(SR.NotWriteableStream, "stream");
                     }
-
                     _deflater = CreateDeflater(null);
-
                     break;
-            }  // switch (_mode)
 
+                default:
+                    throw new ArgumentException(SR.ArgumentOutOfRange_Enum, "mode");
+            }
+
+            _stream = stream;
+            _mode = mode;
+            _leaveOpen = leaveOpen;
             _buffer = new byte[DefaultBufferSize];
         }
 
@@ -120,22 +113,16 @@ private static IDeflater CreateDeflater(CompressionLevel? compressionLevel)
                 deflatorType = s_forcedTestingDeflaterType;
 #endif
 
-            switch (deflatorType)
+            if (deflatorType == WorkerType.ZLib)
             {
-                case WorkerType.Managed:
-                    return new DeflaterManaged();
-
-                case WorkerType.ZLib:
-                    if (compressionLevel.HasValue)
-                        return new DeflaterZLib(compressionLevel.Value);
-                    else
-                        return new DeflaterZLib();
-
-                default:
-                    // We do not expect this to ever be thrown.
-                    // But this is better practice than returning null.
-                    Environment.FailFast("Program entered an unexpected state.");
-                    return null; // we'll not reach here
+                return compressionLevel.HasValue ?
+                    new DeflaterZLib(compressionLevel.Value) :
+                    new DeflaterZLib();
+            }
+            else
+            {
+                Debug.Assert(deflatorType == WorkerType.Managed);
+                return new DeflaterManaged();
             }
         }
 
@@ -221,7 +208,6 @@ public override long Position
         public override void Flush()
         {
             EnsureNotDisposed();
-            return;
         }
 
         public override Task FlushAsync(CancellationToken cancellationToken)
@@ -273,9 +259,13 @@ public override int Read(byte[] array, int offset, int count)
                 Debug.Assert(_inflater.NeedsInput(), "We can only run into this case if we are short of input");
 
                 int bytes = _stream.Read(_buffer, 0, _buffer.Length);
-                if (bytes == 0)
+                if (bytes <= 0)
                 {
-                    break;      //Do we want to throw an exception here?
+                    break;
+                }
+                else if (bytes > _buffer.Length)
+                {
+                    throw new InvalidDataException(SR.GenericInvalidData);
                 }
 
                 _inflater.SetInput(_buffer, 0, bytes);
@@ -386,6 +376,10 @@ private async Task<int> ReadAsyncCore(Task<int> readTask, byte[] array, int offs
                         // This indicates the base stream has received EOF
                         return 0;
                     }
+                    else if (bytesRead > _buffer.Length)
+                    {
+                        throw new InvalidDataException(SR.GenericInvalidData);
+                    }
 
                     cancellationToken.ThrowIfCancellationRequested();