bufferasync.cs

static async Task<byte[]> ReadBytesAsync(string filePath)
{
    byte[] buffer;
    using(FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
    {
        buffer = new byte[fs.Length];
        if (fs.Length < Int32.MaxValue)
        {
            await fs.ReadAsync(buffer, 0, (int)fs.Length);
        }
        else
        {
            throw new Exception("File too large for this method"); 
        }
    }

    return buffer;
}