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Transforming a string into a escaped char

The goal is to have a string input (coming from the frontend), and this string should be transformed to act as a escaped char in the backend.
In the following example I want the user to write "\" + "t", and the backend should interpret it as "\t" (= tab char):
var inputStr = #"\t"; // The input is a string written by a user: "\t" (backslash char + t char == #"\t" != "\t")
var outputStr = SomeOperation(inputStr); // ???
Console.WriteLine("A" + outputStr + "B <= should be tab separated");
I have tried:
var outputStr = inputStr.Replace("\", "");

This isn't something that is built in. Ultimately, "\t" == (a string of length 1 containing a tab character) is implemented by the C# compiler, not the runtime. There isn't a pre-existing implementation of this in the runtime, in part because each language (VB.NET, C#, F#, etc) can have their own rules.
You would need to write your own implementation with your own definitions of escape characters. Fortunately, it is mostly an exercise in .Replace(...). There are some edge cases to think about - in particular for ordering - though; for example, if \\ becomes \ and \n becomes newline; does \\n become \n? or does it become \(newline)? done naively, it can end up as just (newline) - i.e. foo.Replace(#"\\",#"\").Replace(#"\n","\n")

You can do something like this:
void Main()
{
Debug.Assert(ReplaceChar("hello\tworld", #"\t") == "helloworld"); // passed
}
string ReplaceChar(string str, string userInput)
{
switch (userInput)
{
case #"\t":
return str.Replace("\t","");
}
return str;
}

I have finally found an easy way to do it:
Regex.Unescape(inputStr);
See the documentation of the Regex.Unescape function for more details.
Example:
var ouptutStr = Regex.Unescape("\\t");
// ✓ Result: outputStr == "\t"

var outputStr = Char.Parse("\t").ToString();
gives
A B <= should be tab separated
It isn't seen here but in console it looks properly.

The fastest way to trim string in C#

I need to trim paths in million strings like this:
C:\workspace\my_projects\my_app\src\my_component\my_file.cpp
to
src\my_component\my_file.cpp
I.e. remove absolute part of the path, what is the fastest way to do that?
My try using regex:
Regex.Replace(path, #"(.*?)\src", ""),

I wouldn't go with regex for this, use the plain old method.
If the path prefix is always the same:
const string partToRemove = #"C:\workspace\my_projects\my_app\";
if (path.StartsWith(partToRemove, StringComparison.OrdinalIgnoreCase))
path = path.Substring(partToRemove.Length);
If the prefix is variable, you can get the last index of \src\:
var startIndex = path.LastIndexOf(#"\src\", StringComparison.OrdinalIgnoreCase);
if (startIndex >= 0)
path = path.Substring(startIndex + 1);

define the regex with a new and reuse it
there is a (significant) cost to creating the regex
string input = "This is text with far too much " +
"whitespace.";
string pattern = "\\s+";
string replacement = " ";
Regex rgx = new Regex(pattern);
string result = rgx.Replace(input, replacement);

I'm not sure if you need speed here, but if you always get the full path, you could do a simple .Substring()
var path = #"C:\workspace\my_projects\my_app\src\my_component\my_file.cpp";
Console.WriteLine(path.Substring(32));
However, I think you should sanitize your input first; in this case, the Uri class could do the parsing step:
var root = #"C:\workspace\my_projects\my_app\";
var path = #"C:\workspace\my_projects\my_app\src\my_component\my_file.cpp";
var relative = new Uri(root).MakeRelativeUri(new Uri(path));
Console.WriteLine(relative.OriginalString.Replace("/", "\\"));
Notice here the Uri will change the \ with a /: that's the .Replace reason.

Cant think any faster than this
path.Substring(33);
What is before src is constant. and it starts from index 33.
C:\workspace\my_projects\my_app\src\my_component\my_file.cpp
^
How ever if its not always constant. you can find it once. and do the rest inside loop.
int startInd = path.IndexOf(#"\src\") + 1;
// Do this inside loop. 1 million times
path.Substring(startInd);

If your files will all end in "src/filename.ext" you could use the Path class in the .NET framework for it and get around all caveats you could have with pathes and filenames:
result = "src\" + Path.GetFileName(path);
So you should first double-check that the conversion is the thing that takes to long.

Proper way in C# to combine an arbitrary number of strings into a single string

I breezed through the documentation for the string class and didn't see any good tools for combining an arbitrary number of strings into a single string. The best procedure I could come up with in my program is
string [] assetUrlPieces = { Server.MapPath("~/assets/"),
"organizationName/",
"categoryName/",
(Guid.NewGuid().ToString() + "/"),
(Path.GetFileNameWithoutExtension(file.FileName) + "/")
};
string assetUrl = combinedString(assetUrlPieces);
private string combinedString ( string [] pieces )
{
string alltogether = "";
foreach (string thispiece in pieces) alltogether += alltogether + thispiece;
return alltogether;
}
but that seems like too much code and too much inefficiency (from the string addition) and awkwardness.

If you want to insert a separator between values, string.Join is your friend. If you just want to concatenate the strings, then you can use string.Concat:
string assetUrl = string.Concat(assetUrlPieces);
That's marginally simpler (and possibly more efficient, but probably insignificantly) than calling string.Join with an empty separator.
As noted in comments, if you're actually building up the array at the same point in the code that you do the concatenation, and you don't need the array for anything else, just use concatenation directly:
string assetUrl = Server.MapPath("~/assets/") +
"organizationName/" +
"categoryName/" +
Guid.NewGuid() + "/" +
Path.GetFileNameWithoutExtension(file.FileName) + "/";
... or potentially use string.Format instead.

I prefer using string.Join:
var result = string.Join("", pieces);
You can read about string.Join on MSDN

You want a StringBuilder, I think.
var sb = new StringBuilder(pieces.Count());
foreach(var s in pieces) {
sb.Append(s);
}
return sb.ToString();
Update
#FiredFromAmazon.com: I think you'll want to go with the string.Concat solution offered by others for
Its sheer simplicity
Higher performance. Under the hood, it uses FillStringChecked, which does pointer copies, whereas string.Join uses StringBuilder. See http://referencesource.microsoft.com/#mscorlib/system/string.cs,1512. (Thank you to #Bas).

string.Concat is the most appropriate method for what you want.
var result = string.Concat(pieces);
Unless you want to put delimiters between the individual strings. Then you'd use string.Join
var result = string.Join(",", pieces); // comma delimited result.

A simple way to do this with a regular for loop:
(since you can use the indices, plus I like these loops better than foreach loops)
private string combinedString(string[] pieces)
{
string alltogether = "";
for (int index = 0; index <= pieces.Length - 1; index++) {
if (index != pieces.Length - 1) {
alltogether += string.Format("{0}/" pieces[index]);
}
}
return alltogether;

Parsing syntactic string in C#

I am trying to parse a header and create method stubs from the interface/method declarations.
I want to take c++ com method declarations like this:
STDMETHOD(GetCubeMapSurface)(THIS_ D3DCUBEMAP_FACES FaceType,UINT Level,IDirect3DSurface9** ppCubeMapSurface) PURE;
Then modify it to generate a c++ method stub from it like this:
HRESULT __stdcall WrapIDirect3DCubeTexture9::GetCubeMapSurface(D3DCUBEMAP_FACES FaceType, UINT Level, IDirect3DSurface9 * * ppCubeMapSurface)
{
}
I am a little unsure if I should be using regex for this or using .net string functions, and I am confused on how exactly to implement it either way.
I have quite a few methods to do, so creating a tool seems like the right thing to do.
Can anyone help guide me in the right direction?
EDIT: I should have added that I was looking for some help on how I should be implementing it. I wasn't sure if I should be tokenizing all words/special chars and empty spaces and just go from there, using a regex like this and then just parsing and processing with it broken up.
"(\d[x0-9a-fA-F.UL]*|\w+|\s+|"[^"]*"|.)"
Although now it seems like overkill and that I was over analyzing this whole thing. I ended up quickly creating an implementation with .net string functions, and then seen that Caesay helped me out in the regex direction. So I came up with two implementations.
I have decided I will go with the regex implementation. Since I will be doing some other advanced processing and parsing, and regex would make that easier. The implementations are below.
String based implementation:
if (line.StartsWith(" STDMETHOD"))
{
string newstr = line.Replace(" STDMETHOD(", "HRESULT __stdcall WrapIDirect3DCubeTexture9::");
newstr = StringExtensions.RemoveFirst(newstr, ")");
newstr = newstr.Replace("THIS_ ", "");
newstr = newstr.Replace(" PURE;", Environment.NewLine + "{ " + Environment.NewLine + Environment.NewLine + "}");
textBox2.AppendText(newstr + Environment.NewLine);
}
String extension class taken from(C# - Simplest way to remove first occurrence of a substring from another string):
public static class StringExtensions
{
public static string RemoveFirst(this string source, string remove)
{
int index = source.IndexOf(remove);
return (index < 0)
? source
: source.Remove(index, remove.Length);
}
}
Now for the Regex implementation:
if (line.StartsWith(" STDMETHOD"))
{
Regex regex = new Regex(#"\(.*?\)");
MatchCollection matches = regex.Matches(line);
string newstr = String.Format(#"HRESULT __stdcall WrapIDirect3DCubeTexture9::{0}({1})", matches[0].Value.Trim('(', ')'), matches[1].Value.Trim('(', ')'));
newstr = newstr.Replace("THIS_ ", "");
textBox2.AppendText(newstr + Environment.NewLine + "{" + Environment.NewLine + Environment.NewLine + "}" + Environment.NewLine);
}

I will write you some code to help get you started.
If you start with a minimal output string containing the variables, it will be easier to see what needs to be done, so:
String.Format(#"HRESULT __stdcall WrapIDirect3DCubeTexture9::{0}({1})
{{
}}", "methodName", "arguments");
Here we can see there are two items we need to extract from the original string, the method name - and the arguments. I would suggest using a regex to match what is in the parenthesis in the original string. This will give you two matches, the method name - and the arguments. You will need to do post-processing on the arguments string but this will give an idea.

Newline char in exchange appointments

I need to put an adress into a appointment. The address is constructed out of several variables. Of course I also need some newlines. But "\n" doesnt result in an new line when i open the appointment in outlook.
Ok here is code snippet:
string address = name + "\n" + strasse + "\n" + plz.ToString() + " " + ort;
if ( telefon != "") {
address = address + "\nTelefon:: " + telefon;
}
if ( natel != "") {
address = address + "\nNatel: " + natel;
}
if ( mail != "") {
address = address + "\nE-Mail: " +mail;
}
Nothing special. The Problem is when i write this to the body of an appointment, then there aren't any actual newlines.

Its pretty hard to diagnose this without seeing at least an example of the string you are passing, but one thing that I tend to do in my C# code is to use the constant:
Environment.NewLine
Or I use the StringBuilder class with the AppendLine() call to add a newline.
Edit: Based on your code snippet, I would write it this way (it will be more performant as well). With your snippet, lots of strings are being allocated (because strings are immutable). The recommended approach in this case is to use StringBuilder.
StringBuilder address = new StringBuilder();
address.AppendLine(name);
address.AppendLine(strasse);
address.Append(plz.ToString()); // This may not be neccessary depending on the type of plz, StringBuilder has overloads that will convert base types to string for you
address.Append(" ");
address.Append(ort);
if (!string.IsNullOrEmpty(telefon))
{
address.AppendLine();
address.Append("Telefon:: ");
address.Append(telefon);
}
if (!string.IsNullOfEmpty(natel))
{
address.AppendLine();
address.Append("Natel: ");
address.Append(natel);
}
if (!string.IsNullOrEmpty(mail))
{
address.AppendLine();
address.Append("E-Mail: ");
address.Append(mail);
}
return address.ToString();
Note: If you are using .Net 4.0 you can use string.IsNullOrWhitespace instead of IsNullOrEmpty to check for not just an empty string, but one that contains only whitespace.
Edit 2 - Based on your answer of needing <br /> tags instead of newlines.
const string newLine = " <br /> ";
StringBuilder address = new StringBuilder();
address.Append(name);
address.Append(newLine);
address.Append(strasse);
address.Append(newLine);
address.Append(plz.ToString()); // This may not be neccessary depending on the type of plz, StringBuilder has overloads that will convert base types to string for you
address.Append(" ");
address.Append(ort);
if (!string.IsNullOrEmpty(telefon))
{
address.Append(newLine);
address.Append("Telefon:: ");
address.Append(telefon);
}
if (!string.IsNullOfEmpty(natel))
{
address.Append(newLine);
address.Append("Natel: ");
address.Append(natel);
}
if (!string.IsNullOrEmpty(mail))
{
address.Append(newLine);
address.Append("E-Mail: ");
address.Append(mail);
}
return address.ToString();

Ok i got it now. I found out that appointments are stored in html format.
So i tried to use the html entity for \r\n, .That didn't work. I finally solved the problem by using the br tag

While you're absolutely correct about using <br/>, newline is not the only thing Exchange eats in notes/appointment body.
I ended up with the following code:
Regex NewlineRegex = new Regex("(\r\n)|(\r)|(\n)");
string valueToWrite = NewlineRegex.Replace(
SecurityElement.Escape(fieldValue), "<br/>")
.Replace(" ", " ")
.Replace("&apos;", "'"); // &apos; is not in HTML.
And even after that you will read back an extra "\r\n" in the end of the body/notes, so I have to .TrimEnd() them after reading.

you should try "\r\n"

See http://www.infinitec.de/post/2009/08/25/Exchange-WebServices-Bug-with-Lineendings.aspx