How do I copy the contents of one stream to another?
Asked Answered
K

13

571

What is the best way to copy the contents of one stream to another? Is there a standard utility method for this?

Kropp answered 23/10, 2008 at 15:16 Comment(1)
Maybe more importantly at this point, how do you copy the contents "streamably", meaning that it only copies the source stream as something consumes the destination stream...?Poisoning
E
743

From .NET 4.5 on, there is the Stream.CopyToAsync method

input.CopyToAsync(output);

This will return a Task that can be continued on when completed, like so:

await input.CopyToAsync(output)

// Code from here on will be run in a continuation.

Note that depending on where the call to CopyToAsync is made, the code that follows may or may not continue on the same thread that called it.

The SynchronizationContext that was captured when calling await will determine what thread the continuation will be executed on.

Additionally, this call (and this is an implementation detail subject to change) still sequences reads and writes (it just doesn't waste a threads blocking on I/O completion).

From .NET 4.0 on, there's is the Stream.CopyTo method

input.CopyTo(output);

For .NET 3.5 and before

There isn't anything baked into the framework to assist with this; you have to copy the content manually, like so:

public static void CopyStream(Stream input, Stream output)
{
    byte[] buffer = new byte[32768];
    int read;
    while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
    {
        output.Write (buffer, 0, read);
    }
}

Note 1: This method will allow you to report on progress (x bytes read so far ...)
Note 2: Why use a fixed buffer size and not input.Length? Because that Length may not be available! From the docs:

If a class derived from Stream does not support seeking, calls to Length, SetLength, Position, and Seek throw a NotSupportedException.

Efflorescence answered 23/10, 2008 at 15:16 Comment(12)
Note that this is not the fastest way to do it. In the provided code snippet, you have to wait for the Write to complete before a new block is read. When doing the Read and Write asynchronously this waiting will disappear. In some situation this will make the copy twice as fast. However it will make the code a lot more complicated so if speed is not an issue, keep it simple and use this simple loop. This question on StackOverflow has some code that illustrates the async Read/Write: #1541158 Regards, SebastiaanCivilly
Is 32,768 the recommended byte size? I've seen smaller amounts before, such as 4,096.Zabaglione
@Zabaglione 32,768 is a copy with 32 KB chunks. 4096 uses 4 KB chunks. It'll be slightly slower but use less memory.Efflorescence
FWIW: input.CopyTo(output) does the same thing as this CopyStream method, except it uses 4K chunks and does some defensive input validation around stream state. Oh, and like @Jon suggested, they used the while((bytes = this.Read(...)) != 0) idiom.Stelliform
Would it be advisable to check for input.CanRead and output.CanWrite?Enneastyle
FWIW, in my testing I've found that 4096 is actually faster than 32K. Something to do with how the CLR allocates chunks over a certain size. Because of this, the .NET implementation of Stream.CopyTo apparently uses 4096.Removable
If you are having troubles like I was. add this before the copyTo() line. sourceStream.Seek(0, SeekOrigin.Begin);Burnell
If you want to know how CopyToAsync is implemented or make modifications like I did (I needed to be able specify the maximum number of bytes to copy) then it's available as CopyStreamToStreamAsync in "Samples for Parallel Programming with the .NET Framework" code.msdn.microsoft.com/ParExtSamplesCostive
@Burnell I was also having trouble when the input was a MemoryStream -- did not notice your comment, but was able to fix using .WriteTo rather than .CopyTo. I think with your suggestion you have to also check .CanSeek before trying to seek.Poisoning
FIY, optimal buffer size iz 81920 bytes, not 32768Flood
@Removable latest referecnceSource shows that it actually uses 81920 bytes buffer.Flood
Shouldn't await input.CopyToAsync(output) be await input.CopyToAsync(output).ContinueWith(t => /*do something with output*/);? I think this make more sense because it is likely the developer wants to work with the output stream.Nutrition
G
74

MemoryStream has .WriteTo(outstream);

and .NET 4.0 has .CopyTo on normal stream object.

.NET 4.0:

instream.CopyTo(outstream);
Glynnis answered 22/6, 2010 at 2:41 Comment(4)
I do not see many samples on the web using these methods. Is this because they are fairly new or is there some limitations?Doradorado
It's because they are new in .NET 4.0. Stream.CopyTo() basically does exactly the same for loop that the approved answer does, with some additional sanity checks. The default buffer size is 4096, but there is also an overload to specify a larger one.Furtek
Stream need to be rewinded after copy: instream.Position = 0;Flabellate
In addition to rewinding the input stream, I also found a need to rewind the output stream: outstream.Position = 0;Judgment
A
33

I use the following extension methods. They have optimized overloads for when one stream is a MemoryStream.

    public static void CopyTo(this Stream src, Stream dest)
    {
        int size = (src.CanSeek) ? Math.Min((int)(src.Length - src.Position), 0x2000) : 0x2000;
        byte[] buffer = new byte[size];
        int n;
        do
        {
            n = src.Read(buffer, 0, buffer.Length);
            dest.Write(buffer, 0, n);
        } while (n != 0);           
    }

    public static void CopyTo(this MemoryStream src, Stream dest)
    {
        dest.Write(src.GetBuffer(), (int)src.Position, (int)(src.Length - src.Position));
    }

    public static void CopyTo(this Stream src, MemoryStream dest)
    {
        if (src.CanSeek)
        {
            int pos = (int)dest.Position;
            int length = (int)(src.Length - src.Position) + pos;
            dest.SetLength(length); 

            while(pos < length)                
                pos += src.Read(dest.GetBuffer(), pos, length - pos);
        }
        else
            src.CopyTo((Stream)dest);
    }
Aboveground answered 10/8, 2009 at 3:54 Comment(0)
P
3

.NET Framework 4 introduce new "CopyTo" method of Stream Class of System.IO namespace. Using this method we can copy one stream to another stream of different stream class.

Here is example for this.

    FileStream objFileStream = File.Open(Server.MapPath("TextFile.txt"), FileMode.Open);
    Response.Write(string.Format("FileStream Content length: {0}", objFileStream.Length.ToString()));

    MemoryStream objMemoryStream = new MemoryStream();

    // Copy File Stream to Memory Stream using CopyTo method
    objFileStream.CopyTo(objMemoryStream);
    Response.Write("<br/><br/>");
    Response.Write(string.Format("MemoryStream Content length: {0}", objMemoryStream.Length.ToString()));
    Response.Write("<br/><br/>");
Puissant answered 27/7, 2012 at 8:42 Comment(0)
M
2

There is actually, a less heavy-handed way of doing a stream copy. Take note however, that this implies that you can store the entire file in memory. Don't try and use this if you are working with files that go into the hundreds of megabytes or more, without caution.

public static void CopySmallTextStream(Stream input, Stream output)
{
  using (StreamReader reader = new StreamReader(input))
  using (StreamWriter writer = new StreamWriter(output))
  {
    writer.Write(reader.ReadToEnd());
  }
}

NOTE: There may also be some issues concerning binary data and character encodings.

Miniaturize answered 23/10, 2008 at 15:16 Comment(5)
The default constructor for StreamWriter creates a UTF8 stream without a BOM (msdn.microsoft.com/en-us/library/fysy0a4b.aspx) so there's no danger of encoding problems. Binary data almost certainly shouldn't be copied this way.Marabelle
one could easily argue that loading "the entire file in memory" is hardly considered "less heavy-handed".Getraer
i get outmemory exception because of thisPancreatotomy
This is not stream to stream. reader.ReadToEnd() puts everything in RAMKapellmeister
I renamed the method from 'CopyStream()' to 'CopySmallTextStream()'. Perhaps this will help make the caveats of this solution more obvious in downstream codebases.Miniaturize
P
1

The basic questions that differentiate implementations of "CopyStream" are:

  • size of the reading buffer
  • size of the writes
  • Can we use more than one thread (writing while we are reading).

The answers to these questions result in vastly different implementations of CopyStream and are dependent on what kind of streams you have and what you are trying to optimize. The "best" implementation would even need to know what specific hardware the streams were reading and writing to.

Polytypic answered 23/10, 2008 at 15:29 Comment(1)
... or the best implementation could have overloads to allow you to specify the buffer size, write size, and whether threads are allowed?Jaleesa
D
0

Unfortunately, there is no really simple solution. You can try something like that:

Stream s1, s2;
byte[] buffer = new byte[4096];
int bytesRead = 0;
while (bytesRead = s1.Read(buffer, 0, buffer.Length) > 0) s2.Write(buffer, 0, bytesRead);
s1.Close(); s2.Close();

But the problem with that that different implementation of the Stream class might behave differently if there is nothing to read. A stream reading a file from a local harddrive will probably block until the read operaition has read enough data from the disk to fill the buffer and only return less data if it reaches the end of file. On the other hand, a stream reading from the network might return less data even though there are more data left to be received.

Always check the documentation of the specific stream class you are using before using a generic solution.

Dinh answered 23/10, 2008 at 15:26 Comment(1)
The generic solution will work here - Nick's answer is a fine one. The buffer size is an arbitrary choice of course, but 32K sounds reasonable. I think Nick's solution is right not to close the streams though - leave that to the owner.Boettcher
H
0

There may be a way to do this more efficiently, depending on what kind of stream you're working with. If you can convert one or both of your streams to a MemoryStream, you can use the GetBuffer method to work directly with a byte array representing your data. This lets you use methods like Array.CopyTo, which abstract away all the issues raised by fryguybob. You can just trust .NET to know the optimal way to copy the data.

Hanna answered 23/10, 2008 at 17:24 Comment(0)
D
0

if you want a procdure to copy a stream to other the one that nick posted is fine but it is missing the position reset, it should be

public static void CopyStream(Stream input, Stream output)
{
    byte[] buffer = new byte[32768];
    long TempPos = input.Position;
    while (true)    
    {
        int read = input.Read (buffer, 0, buffer.Length);
        if (read <= 0)
            return;
        output.Write (buffer, 0, read);
    }
    input.Position = TempPos;// or you make Position = 0 to set it at the start
}

but if it is in runtime not using a procedure you shpuld use memory stream

Stream output = new MemoryStream();
byte[] buffer = new byte[32768]; // or you specify the size you want of your buffer
long TempPos = input.Position;
while (true)    
{
    int read = input.Read (buffer, 0, buffer.Length);
    if (read <= 0)
        return;
    output.Write (buffer, 0, read);
 }
    input.Position = TempPos;// or you make Position = 0 to set it at the start
Detribalize answered 22/3, 2009 at 16:44 Comment(1)
You shouldn't change the position of the input stream, because not all streams allow random access. In a network stream, for example, you cannot change position, only read and/or write.Mojave
H
0

Since none of the answers have covered an asynchronous way of copying from one stream to another, here is a pattern that I've successfully used in a port forwarding application to copy data from one network stream to another. It lacks exception handling to emphasize the pattern.

const int BUFFER_SIZE = 4096;

static byte[] bufferForRead = new byte[BUFFER_SIZE];
static byte[] bufferForWrite = new byte[BUFFER_SIZE];

static Stream sourceStream = new MemoryStream();
static Stream destinationStream = new MemoryStream();

static void Main(string[] args)
{
    // Initial read from source stream
    sourceStream.BeginRead(bufferForRead, 0, BUFFER_SIZE, BeginReadCallback, null);
}

private static void BeginReadCallback(IAsyncResult asyncRes)
{
    // Finish reading from source stream
    int bytesRead = sourceStream.EndRead(asyncRes);
    // Make a copy of the buffer as we'll start another read immediately
    Array.Copy(bufferForRead, 0, bufferForWrite, 0, bytesRead);
    // Write copied buffer to destination stream
    destinationStream.BeginWrite(bufferForWrite, 0, bytesRead, BeginWriteCallback, null);
    // Start the next read (looks like async recursion I guess)
    sourceStream.BeginRead(bufferForRead, 0, BUFFER_SIZE, BeginReadCallback, null);
}

private static void BeginWriteCallback(IAsyncResult asyncRes)
{
    // Finish writing to destination stream
    destinationStream.EndWrite(asyncRes);
}
Hewart answered 7/11, 2011 at 21:5 Comment(1)
Surely if the second read completes before the first write then you will write over the contents of bufferForWrite from the first read, before it is is written out.Pullen
F
0

For .NET 3.5 and before try :

MemoryStream1.WriteTo(MemoryStream2);
Falcone answered 29/6, 2013 at 16:52 Comment(1)
That only works if you're dealing with MemoryStreams though.Lophobranch
A
0

Easy and safe - make new stream from original source:

    MemoryStream source = new MemoryStream(byteArray);
    MemoryStream copy = new MemoryStream(byteArray);
Alvie answered 30/4, 2018 at 12:47 Comment(0)
A
-3

The following code to solve the issue copy the Stream to MemoryStream using CopyTo

Stream stream = new MemoryStream();

//any function require input the stream. In mycase to save the PDF file as stream document.Save(stream);

MemoryStream newMs = (MemoryStream)stream;

byte[] getByte = newMs.ToArray();

//Note - please dispose the stream in the finally block instead of inside using block as it will throw an error 'Access denied as the stream is closed'

Accuse answered 6/8, 2020 at 2:52 Comment(1)
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