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src/mscorlib/shared/System/IO/PinnedBufferMemoryStream.cs

@@ -29,12 +29,11 @@ internal PinnedBufferMemoryStream(byte[] array)
         {
             Debug.Assert(array != null, "Array can't be null");
 
-            int len = array.Length;
-
             _array = array;
             _pinningHandle = GCHandle.Alloc(array, GCHandleType.Pinned);
             // Now the byte[] is pinned for the lifetime of this instance.
             // But I also need to get a pointer to that block of memory...
+            int len = array.Length;
             fixed (byte* ptr = &MemoryMarshal.GetReference((Span<byte>)array))
                 Initialize(ptr, len, len, FileAccess.Read);
         }

src/mscorlib/src/System/IO/Stream.cs

@@ -767,16 +767,17 @@ public virtual int Read(Span<byte> destination)
 
         // Reads one byte from the stream by calling Read(byte[], int, int). 
         // Will return an unsigned byte cast to an int or -1 on end of stream.
-        // This implementation should be overridden by any 
+        // This implementation does not perform well because it allocates a new
+        // byte[] each time you call it, and should be overridden by any 
         // subclass that maintains an internal buffer.  Then, it can help perf
         // significantly for people who are reading one byte at a time.
         public virtual int ReadByte()
         {
-            Span<byte> bytes = stackalloc byte[1];
-            int r = Read(bytes);
+            byte[] oneByteArray = new byte[1];
+            int r = Read(oneByteArray, 0, 1);
             if (r == 0)
                 return -1;
-            return bytes[0];
+            return oneByteArray[0];
         }
 
         public abstract void Write(byte[] buffer, int offset, int count);
@@ -793,14 +794,15 @@ public virtual void Write(ReadOnlySpan<byte> source)
         }
 
         // Writes one byte from the stream by calling Write(byte[], int, int).
-        // This implementation should be overridden by any 
+        // This implementation does not perform well because it allocates a new
+        // byte[] each time you call it, and should be overridden by any 
         // subclass that maintains an internal buffer.  Then, it can help perf
         // significantly for people who are writing one byte at a time.
         public virtual void WriteByte(byte value)
         {
-            Span<byte> bytes = stackalloc byte[1];
-            bytes[0] = value;
-            Write(bytes);
+            byte[] oneByteArray = new byte[1];
+            oneByteArray[0] = value;
+            Write(oneByteArray, 0, 1);
         }
 
         public static Stream Synchronized(Stream stream)

src/mscorlib/src/System/StubHelpers.cs

@@ -46,7 +46,7 @@ unsafe static internal byte ConvertToNative(char managedChar, bool fBestFit, boo
 
         static internal char ConvertToManaged(byte nativeChar)
         {
-            Span<byte> bytes = stackalloc byte[1];
+            Span<byte> bytes = new Span<byte>(ref nativeChar, 1);
             bytes[0] = nativeChar;
             string str = Encoding.Default.GetString(bytes);
             return str[0];