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How to use “\” in a string without making it an escape sequence - C#?
Why is it giving me an error in C# when I use a string like: "\themes\default\layout.png"? At the "d" and "l" location? It says unrecognized escape sequence. And how do I stop it from giving me an error when I use: "\"?
Thans
You need to escape it with an additional \:
string value = "\\themes\\default\\layout.png";
or use the # symbol:
string value = #"\themes\default\layout.png";
which will avoid you from doubling all \.
Or if you are dealing with paths (which is what it seems you are) you could use the Path.Combine method:
string value = Path.Combine(#"\", "themes", "default", "layout.jpg");
You're using a backslash to escape 't' and 'd'. If you want to escape the actual backslash you need to do so:
"\\themes\\default\\layout.png"
"Regular" string literals treat the \ character as a special character, used for escape sequences to insert quickly special characters in strings - \n, for example, is used to insert the newline character, \" is used to insert the " character without terminating the string, and so on.
Because of this, to insert a backslash into a "normal" string you have to insert the corresponding escape sequence, which, unsurprisingly, is \\; you would then write in your case:
"\\themes\\default\\layout.png"
Failing to escape the backslashes will result in weird results or errors like the ones you got, since the compiler will try to interpret the couple backslash-letter that follows it as an escape sequence; if such sequence is defined you'll get unwanted characters (e.g. the first \t is escaped to a tab character), if it's not (like \l) you'll get an error about an undefined escape sequence.
Another option, if you don't need to escape any character, is to use the so-called "verbatim" strings literals: if you prefix the string with an # character the escape sequences will be disabled, and the string you write will be taken verbatim by the compiler. The only exception to this rule is for quotes, that can be inserted inside the verbatim string via the "quote escape sequence", i.e. "". In your case you would write:
#"\themes\default\layout.png"
For more info about regular vs verbatim string literals have a look at their documentation.
The backslash is treated as an escape character. Either escape the backslash itslef in the string like so:
"\\themes\\default\\layout.png"
or disable escaping altogether using a verbatim string literal:
#"\themes\default\layout.png"
Related
I have a trial version of ReSharper and it always suggests that I switch regular strings to verbatim strings. What is the difference?
A verbatim string is one that does not need to be escaped, like a filename:
string myFileName = "C:\\myfolder\\myfile.txt";
would be
string myFileName = #"C:\myfolder\myfile.txt";
The # symbol means to read that string literally, and don't interpret control characters otherwise.
This is covered in section 2.4.4.5 of the C# specification:
2.4.4.5 String literals
C# supports two forms of string literals: regular string literals and verbatim string literals.
A regular string literal consists of zero or more characters enclosed in double quotes, as in "hello", and may include both simple escape sequences (such as \t for the tab character) and hexadecimal and Unicode escape sequences.
A verbatim string literal consists of an # character followed by a double-quote character, zero or more characters, and a closing double-quote character. A simple example is #"hello". In a verbatim string literal, the characters between the delimiters are interpreted verbatim, the only exception being a quote-escape-sequence. In particular, simple escape sequences and hexadecimal and Unicode escape sequences are not processed in verbatim string literals. A verbatim string literal may span multiple lines.
In other words the only special character in a #"verbatim string literal" is the double-quote character. If you wish to write a verbatim string containing a double-quote you must write two double-quotes. All other characters are interpreted literally.
You can even have literal new lines in a verbatim string literal. In a regular string literal you cannot have literal new lines. Instead you must use for example "\n".
Verbatim strings literals are often useful for embedding filenames and regular expressions in the source code, because backslashes in these types of strings are common and would need to be escaped if a regular string literal were used.
There is no difference at runtime between strings created from regular string literals and strings created from a verbatim string literals - they are both of type System.String.
There is no runtime difference between a string and verbatim string. They're only different at compile time. The compiler accepts fewer escape sequences in a verbatim string so what-you-see-is-what-you-get other than a quote escape.
You can also use the verbatim character, #, to tell the compiler to treat a keyword as a name:
var #if = "if";
//okay, treated as a name
Console.WriteLine(#if);
//compiler err, if without # is a keyword
Console.WriteLine(if);
var #a = "a";
//okay
Console.WriteLine(#a);
//also okay, # isn't part of the name
Console.WriteLine(a);
You can have multiline string too using verbatim strings:
Console.WriteLine(#"This
is
a
Test
for stackoverflow");
without # you got an error.
In VB14 there is a new feature called Multiline Strings, it's like verbatim strings in C#.
Pro tip: VB string literals are now exactly like C# verbatim strings.
Regular strings use special escape sequences to translate to special characters.
/*
This string contains a newline
and a tab and an escaped backslash\
*/
Console.WriteLine("This string contains a newline\nand a tab\tand an escaped backslash\\");
Verbatim strings are interpreted as is, without translating any escape sequences:
/*
This string displays as is. No newlines\n, tabs\t or backslash-escapes\\.
*/
Console.WriteLine(#"This string displays as is. No newlines\n, tabs\t or backslash-escapes\\.");
If you want to suppress the ReSharper warnings, you can use:
Localizable(false)
For things like parameters, file locations, etc., this could be a good solution.
So in microsoft visual studio I have a string that is compiled into a regex. My string is "#(\d+(.\d+)?)=(\d+(.\d+)?)". I cannot compile my program because I get an error saying that \d is a unrecognized escape character. How do I tell it to shut up and let me regex like a pro?
Begin your string with #, that causes the compiler to leave (almost) all characters alone, unescaped (the exception is ", which can be escaped as ""):
#"#(\d+(.\d+)?)=(\d+(.\d+)?"
The problem is that c# does not like the \d inside the string. Use a verbatim string instead
string pattern = #"#(\d+(.\d+)?)=(\d+(.\d+)?)";
The "#" denotes it. C# will not look for escape sequences in the string. If you have to escape a " use two "".
Of cause you can use normal strings. but then you will have to escape the backslashes
string pattern = "#(\\d+(.\\d+)?)=(\\d+(.\\d+)?)";
If you're using a normal string, you need to escape your backslashes, like so:
"#(\\d+(.\\d+)?)=(\\d+(.\\d+)?)"
Basically, you're putting a literal string into C#; the C# compiler sees the string first, and tries to interpret \d as an escape sequence (which doesn't exist, hence error). Therefore, you use \\d to get the C# compiler to see the string as \d, which then gets passed to the regex engine (which does recognize \d as something meaningful). (yes, if you want to match a literal backslash in your regex pattern, you need to use \\\\)
But in C#, you have the alternative of just prepending the string with # to get the compiler to leave the string alone (though " still needs escaping), so that would be like this:
#"#(\d+(.\d+)?)=(\d+(.\d+)?)"
You could also use a verbatim string literal (I prefer to use these because of readability).
Use #"(#\d+(.\d+)?)=(\d+(.\d+)?)"
The #" sign indicates that the string shouldn't interpret escaped characters (A character prefixed by a \) until the closing " is reached.
Note: You can match a single " in your search pattern by double quoting instead "". For instance you can match "Hello" by using the pattern #"""\w+"""
I'm doing a match comparison on some escaped strings:
Regex.IsMatch("\\Application.evtx", "DebugLogs\\ConfigurationServices.log");
I don't see why I'm getting:
"parsing "DebugLogs\ConfigurationServices.log" - Unrecognized escape sequence \C."
The \C is escaped?
The edit really fooled a lot of people, including me!
'\' is a special character in regular expressions - it effectively is an escape character or denotes an escape sequence.
So the RegEx engine sees DebugLogs*\C*onfigurationServices.log which is, indeed, an unrecognized escape sequence. \A actually is an existing escape sequence.
So you need to escape the escape character. The simplest way to do this is to double the number of slashes used:
Regex.IsMatch("\\\\Application.evtx", "DebugLogs\\\\ConfigurationServices.log");
Which the RegEx engine will see as comparisons betweeen "\\Appplication.evtx" and "DebugLogs\\ConfigurationServices.log" - now the backslash has been escaped and has no special meaning.
Regex.IsMatch(#"\\Application.evtx", #"DebugLogs\\ConfigurationServices.log");
works fine too and is more readable.
The \ character is the escape character in strings. For example if you'd like to do a carriage return, you'd use \r. To get around this either use literal strings
#"\Application.evtx"
Or escape the escape character
"\\Application.evtx"
You probably want
Regex.IsMatch(#"\Application.evtx", #"DebugLogs\ConfigurationServices.log");
without the "#" C# will treat \C as an escape sequence similar to the way it would convert \n in to a newline character however \C is not a recognised/valid escape sequence.
I read some C# article to combine a path using Path.Combine(part1,part2).
It uses the following:
string part1 = #"c:\temp";
string part2 = #"assembly.txt";
May I know what is the use of # in part1 and part2?
# is not related to any method.
It means that you don't need to escape special characters in the string following to the symbol:
#"c:\temp"
is equal to
"c:\\temp"
Such string is called 'verbatim' or #-quoted. See MSDN.
As other have said its one way so that you don't need to escape special characters and very useful in specifying file paths.
string s1 =#"C:\MyFolder\Blue.jpg";
One more usage is when you have large strings and want it to be displayed across multiple lines rather than a long one.
string s2 =#"This could be very large string something like a Select query
which you would want to be shown spanning across multiple lines
rather than scrolling to the right and see what it all reads up";
As stated in C# Language Specification 4.0:
2.4.4.5 String literals
C# supports two forms of string
literals: regular string literals and
verbatim string literals. A regular
string literal consists of zero or
more characters enclosed in double
quotes, as in "hello", and may include
both simple escape sequences (such as
\t for the tab character), and
hexadecimal and Unicode escape
sequences. A verbatim string literal
consists of an # character followed by
a double-quote character, zero or more
characters, and a closing double-quote
character. A simple example is
#"hello". In a verbatim string
literal, the characters between the
delimiters are interpreted verbatim,
the only exception being a
quote-escape-sequence. In particular,
simple escape sequences, and
hexadecimal and Unicode escape
sequences are not processed in
verbatim string literals.
It denotes a verbatim string literal, and allows you to use certain characters that normally have special meaning, for example \, which is normally an escape character, and new lines. For this reason it's very useful when dealing with Windows paths.
Without using #, the first line of your example would have to be:
string part1 = "c:\\temp";
More information here.
With # you dont have to escape special characters.
So you would have to write "c:\\temp" without #
If more presise it is called 'verbatim' strings. You could read here about it:
http://msdn.microsoft.com/en-us/library/aa691090(v=vs.71).aspx
The # just indicates a different way of specifying a string such that you do not have to escape characters with . the only caveat is that double quotes need to be "" to represent a single ".
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!