Bug 29337 - Stream.CopyTo(destination, bufferSize) adds '\0' characters in destination FileStream
Summary: Stream.CopyTo(destination, bufferSize) adds '\0' characters in destination Fi...
Alias: None
Product: Class Libraries
Classification: Mono
Component: mscorlib ()
Version: 3.12.0
Hardware: PC Linux
: --- normal
Target Milestone: Untriaged
Assignee: Marek Safar
Depends on:
Reported: 2015-04-23 10:12 UTC by ceco
Modified: 2015-04-27 16:01 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

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Description ceco 2015-04-23 10:12:50 UTC
* Problem DOES occur when calling Stream.CopyTo(destination, bufferSize) and the destination stream is FileStream created with useAsync = true. Then there are some '\0' characters added in the destination FileStream.

* Problem DOES NOT occur when calling Stream.CopyTo(destination) without specified bufferSize. This is valid with the aforementioned details about the destination FileStream instance.

* Problem DOES NOT occur when using Microsoft.NET on Windows. I don't use Mono on Windows so I cannot provide information for that use case.

Following is an example which demonstrates the problem on:

* Ubuntu 14.04 LTS
* Mono 3.12.1
* MonoDevelop 5.7

using System;
using System.Globalization;
using System.IO;
using System.Text;
using NUnit.Framework;

namespace ConsoleApplication1
    public static class Program
        public static void Main(string[] args)
            StringBuilder dataBuilder = new StringBuilder();
            for (int i = 0; i < 10000; ++i)
            string expectedData = dataBuilder.ToString();

            const string fileName = "test";
            const bool useAsync = true;
            const int bufferSize = 8192;

            byte[] bytes = Encoding.Default.GetBytes(expectedData);
            using (Stream dataStream = new MemoryStream(bytes))
            using (Stream fileStream = new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite, FileShare.None, bufferSize, useAsync))
                // this works fine
                // this fails
                dataStream.CopyTo(fileStream, bufferSize);

            string actualData = File.ReadAllText(fileName);
            Assert.That(actualData, Is.EqualTo(expectedData));
Comment 1 ceco 2015-04-23 10:14:51 UTC
Forgot to mention:

Problem DOES NOT occur when the destination FileStream is created with useAsync = false.
Comment 2 Marek Safar 2015-04-27 16:01:17 UTC
Fixed in Mono 4.0