An elegant way to consume (all bytes of a) BinaryReader?

Original Answer (Read Update Below!)

Simply do:

byte[] allData = read1.ReadBytes(int.MaxValue);

The documentation says that it will read all bytes until the end of the stream is reached.

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Update

Although this seems elegant, and the documentation seems to indicate that this would work, the actual implementation (checked in .NET 2, 3.5, and 4) allocates a full-size byte array for the data, which will probably cause an OutOfMemoryException on a 32-bit system.

Therefore, I would say that actually there isn't an elegant way.

Instead, I would recommend the following variation of @iano's answer. This variant doesn't rely on .NET 4:
Create an extension method for BinaryReader (or Stream, the code is the same for either).

public static byte[] ReadAllBytes(this BinaryReader reader)
{
    const int bufferSize = 4096;
    using (var ms = new MemoryStream())
    {
        byte[] buffer = new byte[bufferSize];
        int count;
        while ((count = reader.Read(buffer, 0, buffer.Length)) != 0)
            ms.Write(buffer, 0, count);
        return ms.ToArray();
    }
    
}

There is not an easy way to do this with BinaryReader. If you don't know the count you need to read ahead of time, a better bet is to use MemoryStream:

public byte[] ReadAllBytes(Stream stream)
{
    using (var ms = new MemoryStream())
    {
        stream.CopyTo(ms);
        return ms.ToArray();
    }
}

To avoid the additional copy when calling ToArray(), you could instead return the Position and buffer, via GetBuffer().