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C# safe way to convert between bytes and other types

What is the safest way to guarantee that the following operation will be performed correctly:
When I read in 4 bytes as a uint32, I will write it out to a text file.
Later I will open this text file, read the number I wrote out previously, and then convert it back into the 4 bytes for use in other processing.

There is the BitConverter class to help you convert between primitive types and bytes.

Since you are storing this as a string, there isn't a whole lot to this. Obviously there is no issue converting the number into a string using .ToString(). So the only question I assume is how to go back in a reliable fashion. The solution is to use uint.Parse. i.e.:
var s = "12343632423432";
uint i = uint.Parse(s);
(PS: BitConverter is not helpful for conversion from strings)

How to properly write a UDP packet

I am trying to rewrite some of my code from a C++ program I wrote a while ago, but I am not sure if/how I can write to a byte array properly, or if I should be using something else. The code I am trying to change to C# .NET is below.
unsigned char pData[1400];
bf_write g_ReplyInfo("SVC_ReplyInfo", &pData, 1400);
void PlayerManager::BuildReplyInfo()
{
// Delete the old packet
g_ReplyInfo.Reset();
g_ReplyInfo.WriteLong(-1);
g_ReplyInfo.WriteByte(73);
g_ReplyInfo.WriteByte(g_ProtocolVersion.GetInt());
g_ReplyInfo.WriteString(iserver->GetName());
g_ReplyInfo.WriteString(iserver->GetMapName());
}

BinaryWriter might work, although strings are written with a preceding 7-bit encoded length, which I suspect the client won't be able to handle. You'll probably have to convert strings to bytes and then either add a length word or 0-terminate it.
No need to manually convert numbers to bytes. If you have a long that you want to write as a byte, just cast it. That is, if your BinaryWriter is bw, then you can write bw.Write((byte)longval);. To write -1 as a long: bw.Write((long)(-1)).

Socket Programming - Sending/Receiving hex and strings

I have the following code in C#:
Console.WriteLine("Connecting to server...");
TcpClient client = new TcpClient("127.0.0.1", 25565);
client.Client.Send(BitConverter.GetBytes(0x02));
client.Client.Send(BitConverter.GetBytes(0x0005));
client.Client.Send(Encoding.UTF8.GetBytes("wedtm"));
Console.Write("{0:x2}", client.GetStream().ReadByte());
For the life of me, I can't figure out how to transpose this to ruby. Any help here?
This is what I have so far, but it's not working as expected:
require 'socket'
s = TCPSocket.open("127.0.0.1", 25565)
s.write(0x02)
s.write(0x0005)
s.write("wedtm".bytes)
response = s.recvfrom(2)
puts "Response Size #{response.size}: #{response.to_s}"
The response should be 0x02
EDIT:
I'm assuming I have to use String#unpack on this, however, I can't figure out how to get "wedtm" to output to the appropriate \x000\x000\x000\x000 format.

There are at least two things to consider here:
Network byte order is big-endian. This means that you should always think in single bytes or arrays of bytes, as bytes are not subject to being shuffled around while larger types are.
C#'s BitConverter.GetBytes(int16) returns 2 bytes in little-endian format and GetBytes(int32) returns 4 bytes in little-endian format
Without knowing any Ruby or its string format, I'd guess you need to do something like this for the first part:
s.write("\x02\x00".bytes)
s.write("\x05\x00\x00\x00".bytes)
The second part should be okay.
WireShark is an invaluable tool when debugging network code and/or reverse engineering networking protocols, record the traffic of the C# app and compare the difference with yours.

What's the best way to read mixed (i.e. text and binary) data?

I need to be able to read a file format that mixes binary and non-binary data. Assuming I know the input is good, what's the best way to do this? As an example, let's take a file that has a double as the first line, a newline (0x0D 0x0A) and then ten bytes of binary data afterward. I could, of course, calculate the position of the newline, then make a BinaryReader and seek to that position, but I keep thinking that there has to be a better way.

You can use System.IO.BinaryReader. The problem with this though is you must know what type of data you are going to be reading before you call any of the Read methods.
Read(byte[], int, int)
Read(char[], int, int)
Read()
Read7BitEncodedInt()
ReadBoolean()
ReadByte()
ReadBytes(int)
ReadChar()
ReadChars()
ReadDecimal()
ReadDouble()
ReadInt16()
ReadInt32()
ReadInt64()
ReadSByte()
ReadSingle()
ReadString()
ReadUInt16()
ReadUInt32()
ReadUInt64()
And of course the same methods exist for writing in System.IO.BinaryWriter.

Is this file format already fixed? If it's not, it's a really good idea to change to use a length-prefixed format for the strings. Then you can read just the right amount and convert it to a string.
Otherwise, you'll need to read chunks from the file, scan for the newline, and decode the right amount of data or (if you don't find the newline) either buffer it somewhere else (e.g. a MemoryStream) or just remember the starting point and rewind the stream appropriately. It will be ugly, but that's just because of the deficiency of the file format.
I would suggest you don't "over-decode" (i.e. decode the arbitrary binary data after the string) - while it may well not do any harm, in some encodings you could be reading an impossible sequence of binary data, which then starts getting into the realms of DecoderFallbacks and the like.

I've had to deal with that when reading HTTP requests coming in over the wire on Compact Framework. My solution was to roll my own non-buffering ASCII-only StreamReader, so that it was safe to interleave calls to both the StreamReader and the underlying Stream.

How to send data to port in binary format

I am newbie in Low level programming. In my project(C#.NET) we are using EDBS protocol(7 bit format) for communication and i have the data in bit format to send like 00101010 so we would please guide me how to send these to port.I know that the serial port class accepts data in binary format but dont know how convert the bit format into byte .
Thanks in advance
prem

Use the BitConverter class to create basic types (like bytes or ints) from bytes.
http://msdn.microsoft.com/en-us/library/system.bitconverter.aspx
There is also a class called BitArray that can be used to store bit representations:
Convert from BitArray to Byte

.Net only receives data in byte (the smallest unit), you can use BitConverter to convert various value type to byte of array and vice verse.

There is a class called BitVector32 which lets you encapsulate your bit values into 32 bit number. Sorry about yesterday's answer, I just found this today.