你也可以尝试一次只读入部分，并展开返回的字节数组:

public byte[] StreamToByteArray(string fileName)
{
    byte[] total_stream = new byte[0];
    using (Stream input = File.Open(fileName, FileMode.Open, FileAccess.Read))
    {
        byte[] stream_array = new byte[0];
        // Setup whatever read size you want (small here for testing)
        byte[] buffer = new byte[32];// * 1024];
        int read = 0;

        while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
        {
            stream_array = new byte[total_stream.Length + read];
            total_stream.CopyTo(stream_array, 0);
            Array.Copy(buffer, 0, stream_array, total_stream.Length, read);
            total_stream = stream_array;
        }
    }
    return total_stream;
}

像这样调用下一个函数

byte[] m_Bytes = StreamHelper.ReadToEnd (mystream);

功能:

public static byte[] ReadToEnd(System.IO.Stream stream)
{
    long originalPosition = 0;

    if(stream.CanSeek)
    {
         originalPosition = stream.Position;
         stream.Position = 0;
    }

    try
    {
        byte[] readBuffer = new byte[4096];

        int totalBytesRead = 0;
        int bytesRead;

        while ((bytesRead = stream.Read(readBuffer, totalBytesRead, readBuffer.Length - totalBytesRead)) > 0)
        {
            totalBytesRead += bytesRead;

            if (totalBytesRead == readBuffer.Length)
            {
                int nextByte = stream.ReadByte();
                if (nextByte != -1)
                {
                    byte[] temp = new byte[readBuffer.Length * 2];
                    Buffer.BlockCopy(readBuffer, 0, temp, 0, readBuffer.Length);
                    Buffer.SetByte(temp, totalBytesRead, (byte)nextByte);
                    readBuffer = temp;
                    totalBytesRead++;
                }
            }
        }

        byte[] buffer = readBuffer;
        if (readBuffer.Length != totalBytesRead)
        {
            buffer = new byte[totalBytesRead];
            Buffer.BlockCopy(readBuffer, 0, buffer, 0, totalBytesRead);
        }
        return buffer;
    }
    finally
    {
        if(stream.CanSeek)
        {
             stream.Position = originalPosition; 
        }
    }
}

Byte[] Content = new BinaryReader(file.InputStream).ReadBytes(file.ContentLength);

Stream s;
int len = (int)s.Length;
byte[] b = new byte[len];
int pos = 0;
while((r = s.Read(b, pos, len - pos)) > 0) {
    pos += r;
}

一个稍微复杂一点的解决方案是s.Length超过Int32.MaxValue。但是如果需要将这么大的流读入内存，则可能需要考虑另一种解决问题的方法。

编辑:如果你的流不支持长度属性，使用Earwicker的变通方法进行修改。

public static class StreamExtensions {
    // Credit to Earwicker
    public static void CopyStream(this Stream input, Stream output) {
        byte[] b = new byte[32768];
        int r;
        while ((r = input.Read(b, 0, b.Length)) > 0) {
            output.Write(b, 0, r);
        }
    }
}

[...]

Stream s;
MemoryStream ms = new MemoryStream();
s.CopyStream(ms);
byte[] b = ms.GetBuffer();

在. net Framework 4及以后版本中，Stream类有一个内置的CopyTo方法可供使用。

对于框架的早期版本，方便的helper函数是:

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

然后使用上面的方法之一复制到MemoryStream并调用GetBuffer:

var file = new FileStream("c:\\foo.txt", FileMode.Open);

var mem = new MemoryStream();

// If using .NET 4 or later:
file.CopyTo(mem);

// Otherwise:
CopyStream(file, mem);

// getting the internal buffer (no additional copying)
byte[] buffer = mem.GetBuffer();
long length = mem.Length; // the actual length of the data 
                          // (the array may be longer)

// if you need the array to be exactly as long as the data
byte[] truncated = mem.ToArray(); // makes another copy

编辑:最初我建议使用Jason的答案为支持长度属性的流。但是它有一个缺陷，因为它假设Stream会在一个Read中返回它的所有内容，这并不一定是正确的(例如，对于Socket来说就不是这样)。我不知道在BCL中是否有一个流实现的例子，它支持长度，但可能会以比你请求的更短的块返回数据，但任何人都可以继承流，这很容易是这样的情况。

在大多数情况下，使用上面的通解可能会更简单，但假设你确实想直接读入一个bigEnough数组:

byte[] b = new byte[bigEnough];
int r, offset;
while ((r = input.Read(b, offset, b.Length - offset)) > 0)
    offset += r;

也就是说，反复调用Read并移动将要存储数据的位置。

快速和肮脏的技巧:

    static byte[] StreamToByteArray(Stream inputStream)
    {
        if (!inputStream.CanRead)
        {
            throw new ArgumentException(); 
        }

        // This is optional
        if (inputStream.CanSeek)
        {
            inputStream.Seek(0, SeekOrigin.Begin);
        }

        byte[] output = new byte[inputStream.Length];
        int bytesRead = inputStream.Read(output, 0, output.Length);
        Debug.Assert(bytesRead == output.Length, "Bytes read from stream matches stream length");
        return output;
    }

测试:

    static void Main(string[] args)
    {
        byte[] data;
        string path = @"C:\Windows\System32\notepad.exe";
        using (FileStream fs = File.Open(path, FileMode.Open, FileAccess.Read))
        {
            data = StreamToByteArray(fs);
        }

        Debug.Assert(data.Length > 0);
        Debug.Assert(new FileInfo(path).Length == data.Length); 
    }

我会问，为什么你要把一个流读到一个字节[]，如果你想复制一个流的内容，我可以建议使用MemoryStream和写你的输入流到内存流。