Added async flushing of binary write streams

Author: levitation

Diff for: BinaryFileExtensions.cs

@@ -127,6 +127,11 @@ ex is IOException
 
                             await stream.WriteAsync(contents, i, writeBufferLength, cancellationToken);
                         }
+
+                        //make the flush operation explicit so it can be async
+                        //This is a use case for IAsyncDisposable as proposed in async streams, a candidate feature for C# 8.0, combined with a using statement enhancement that that proposal didn't cover. C# 7.x and below don't have a "nice" way to do asynchronous dispose, so the implicit Dispose() call from your using statement does the flush synchronously. The workaround is directly calling and awaiting on FlushAsync() so that the synchronous Dispose() doesn't have to synchronously flush.
+                        //https://stackoverflow.com/questions/21987533/streamwriter-uses-synchronous-invocation-when-calling-asynchronous-methods
+                        await stream.FlushAsync(cancellationToken); 
                     }
 
                     if (createTempFileFirst)

Diff for: FileExtensions.cs

@@ -117,7 +117,7 @@ private static async Task<string> InternalReadAllTextAsync(string path, Encoding
                 StringBuilder sb = new StringBuilder();
                 while (true)
                 {
-                    int read = await sr.ReadAsync(buffer, 0, buffer.Length).ConfigureAwait(false);
+                    int read = await sr.ReadAsync(buffer, 0, buffer.Length);  //TODO: add cancellationToken here?
                     if (read == 0)
                     {
                         return sb.ToString();
@@ -226,12 +226,12 @@ private static async Task InternalWriteAllTextAsync(StreamWriter sw, string cont
                 {
                     int batchSize = Math.Min(DefaultBufferSize, count - sourceOffset);
                     contents.CopyTo(sourceOffset, buffer, 0, batchSize);
-                    await sw.WriteAsync(buffer, 0, batchSize);
+                    await sw.WriteAsync(buffer, 0, batchSize);  //TODO: add cancellationToken here?
                     sourceOffset += batchSize;
                 }
 
                 cancellationToken.ThrowIfCancellationRequested();
-                await sw.FlushAsync();
+                await sw.FlushAsync();  //TODO: add cancellationToken here?
             }
             finally
             {