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Strings and use of \

I have stringBuilder & string class, storing a path:
StringBuilder name = new StringBuilder();
name.Append(#"NETWORK\MyComputer");
String serverName = name.ToString(); //this converts the \ to a \\
I have tried a number of things, but it always results in the string having \
Using serverName.Replace("\\", #"\"); doesn't work, it leaves it as a \
servername.Replace("\\", "\""); adds a " to the string, which is still not correct.
Please assist.

If you are concerned at a single back slash being shown as a double back slash then don't be - that is simply the way it is shown to you in the debugger.
The back slash is a special character, that 'specialness' is turned off by doubling it up. Alternatively the # symbol can be prefixed to the string in source code which avoids having to use it.

Use
name.Append(Path.Combine("NETWORK", "MyComputer");
In strings \ is an escape sequence. So \ in debugger will be \\
Acc.to MSDN
Character combinations consisting of a backslash (\) followed by a letter or by a combination of digits are called "escape sequences." To represent a newline character, single quotation mark, or certain other characters in a character constant, you must use escape sequences. An escape sequence is regarded as a single character and is therefore valid as a character constant.
Escape sequences are typically used to specify actions such as carriage returns and tab movements on terminals and printers. They are also used to provide literal representations of nonprinting characters and characters that usually have special meanings, such as the double quotation mark ("). The following table lists the ANSI escape sequences and what they represent.
Read Escape Sequences

I don't think your code can be compiled. Because \ is an escape character, thus the string "\" will be wrong. The #"\" is right because the # (literal) has ignored that escape and tread it as a normal character.
See more here

C#, escape double quote not working as expected

I'm trying to escape quotes in an xpath string like so:
var mktCapNode = htmlDoc.DocumentNode.SelectSingleNode("//*[#id=""yfs_j10_a""]");
The actual string I want passed is:
//*[#id="yfs_j10_a"]
This gives a compiler errors: ) expected and ; expected
I'm sure it's simple but I'm stumped. Any ideas?

You need to make this a verbatim string to use the "" as an escape
#"//*[#id=""yfs_j10_a""]"
For a normal string literal you need to use backslashes to escape the double quotes
"//*[#id=\"yfs_j10_a\"]"

Or use the escape char '\':
"//*[#id=\"yfs_j10_a\"]"

In C# the \ character is used to escape (see documentation).
This is different from VB where there are no escape characters except "" which escapes to ".
This means in C# you do not need vbCrLf to start a new line or vbTab to add a tab character to a string. Instead use "\r\n" and "\t".
You can also make the string a literal using the # character, but I do not think this works with the quotation mark.

Add the # prefix to your string.
#"//*[#id=""yfs_j10_a""]"
or escape the quotes with a \
"//*[#id=\"yfs_j10_a\"]"

Why does .NET add an additional slash to the already existent slashes in a path?

I've noticed that C# adds additional slashes (\) to paths. Consider the path C:\Test. When I inspect the string with this path in the text visualiser, the actual string is C:\\Test.
Why is this? It confuses me, as sometimes I may want to split the path up (using string.Split()), but have to wonder which string to use (one or two slashes).

The \\ is used because the \ is an escape character and is need to represent the a single \.
So it is saying treat the first \ as an escape character and then the second \ is taken as the actual value. If not the next character after the first \ would be parsed as an escaped character.
Here is a list of available escape characters:
\' - single quote, needed for character literals
\" - double quote, needed for string literals
\\ - backslash
\0 – Null
\a - Alert
\b - Backspace
\f - Form feed
\n - New line
\r - Carriage return
\t - Horizontal tab
\v - Vertical quote
\u - Unicode escape sequence for character
\U - Unicode escape sequence for surrogate pairs.
\x - Unicode escape sequence similar to "\u" except with variable length.
EDIT: To answer your question regarding Split, it should be no issue. Use Split as you would normally. The \\ will be treated as only the one character of \.

.Net is not adding anything to your string here. What your seeing is an effect of how the debugger chooses to display strings. C# strings can be represented in 2 forms
Verbatim Strings: Prefixed with an # sign and removes the need o escape \\ characters
Normal Strings: Standard C style strings where \\ characters need to escape themselves
The debugger will display a string literal as a normal string vs. a verbatim string. It's just an issue of display though, it doesn't affect it's underlying value.

Debugger visualizers display strings in the form in which they would appear in C# code. Since \ is used to escape characters in non-verbatum C# strings, \\ is the correct escaped form.

Okay, so the answers above are not wholly correct. As such I am adding my findings for the next person who reads this post.
You cannot split a string using any of the chars in the table above if you are reading said string(s) from an external source.
i.e,
string[] splitStrings = File.ReadAllText([path]).Split((char)7);
will not split by those chars. However internally created strings work fine.
i.e.,
string[] splitStrings = "hello\agoodbye".Split((char)7);
This may not hold true for other methods of reading text from a file. I am unsure as I have not tested with other methods. With that in mind, it is probably best not to use those chars for delimiting strings!

How do you use this # in C#?

simpleSound = new SoundPlayer(#"c:\Windows\Media\chimes.wav");
Why and what is the # for?

It's a literal string. Instead of having to escape the "\" by putting two of them "\" the compiler interprets the string "as is".
Say you wanted to print out the following text to the screen: "Hello \t world".
If you were to just do Console.WriteLine("Hello \t world"), then your output would be:
Hello world
notice the tab. That's because \t is interperted as a tab. If you use the literal though, like this:
Console.WriteLine(#"Hello \t world")
then your output would be:
"Hello \t world"

The # sign before a string means to treat the backslash as a normal character rather than the start of a special character (such as newline).

It identifies a string literal. It allows you to have the special character \ in the string without escaping it.

It is a verbatim string. A verbatim string allows you to include special characters like \, " etc. without using the escape sequence.

Another usage of # is that you can put it in front of a keyword, if you want to use a keyword as a variable name (even if it's not a good idea). Example:
public int #object = 1;

Defining the # symbol prior to the assigning a string value will prevent the need for doubling the backslash (\) in C#. The at symbol (#) simply ignores escape characters.

"c:\\my\\file.txt" could be done as #"c:\my\file.txt"
This means your \n and \t or whatever it is will also not be processed.

String.Format escaping VB vs C#

While searching on how to escape a single quote in String.Format, I found the answer at SO: Escaping single quote in String.Format()
It seems to be different for VB though. I tested it, and indeed C# needs
string s = DateTime.Now.ToString("MMM d \\'yy 'at' H:mmm");
while VB needs
Dim s As String = Now.ToString("MMM d \'yy 'at' H:mmm")
Why does C# need a double backslash, and VB a single backslash? This might be a bit of a silly question to C# users, but while I can read C#, I'm not used to writing it.

In C#, string literals can contain escape sequences such as \n for a new line or \t for a tab or \" for a quote. If you do not need the escaping, you can prefix the literal with # (eg: #"MMM ...") and get the same string a VB.
In VB, escaping is never allowed, so there is no need to escape the backslash.

The reason why is that C# supports escape sequences within string literals via the \ character. VB has no such escaping mechanisms and hence the single \ is interpretted as a \.
In C# you can get the same behavior by using verbatim strings
#"MMM d \'yy 'at' H:mmm"

In C# the backslash has a meaning (\n is newline \t tab ....).
So backlslahs itselft is an escape character - which you have to escape :)
Or place a AT-sign in front of the string - this makes a "non escaped string" (typically used for paths)

In c# \ will escape . Your text will become "MMM d \'yy 'at' H:mmm". You don't need to escape the ' character in a string. If you were to use " in the string on the other hand, you would need to escape it to not end your string "MMM d \"yy \"at\" H:mmm". Or you could also use the #"" string method which will automatically escape characters for you (not " though). So you could write #"this will not \n be two lines"