new learner's quick question,
what's the meaning of "#" in C# codes?
Examples:
ClientDataSource.Where = #"it.ClientID==1";
cont.Notes = #"";
Response.Redirect(#"~/Default.aspx");
Thanks!
That is a verbatim string literal.
MSDN describes it as such:
Use verbatim strings for convenience and better readability when the string text contains backslash characters, for example in file paths. Because verbatim strings preserve new line characters as part of the string text, they can be used to initialize multiline strings. Use double quotation marks to embed a quotation mark inside a verbatim string.
# can also be used to create identifiers that match reserved words: 2.4.2 Identifiers
For example:
var class = "Reading"; // compiler error
var #class = "Math"; // works
#"...." denotes a verbatim string literal. C# does not process any escape characters in the string, except for "" (to allow including of the " character in the string).
This makes it easier and cleaner to handle strings that would otherwise need to have a bunch of escapes to deal with properly. File/folders paths, for example.
string filePathRegular = "C:\\Windows\\foo\\bar.txt";
string filePathVerbatim = #"C:\Windows\foo\bar.txt";
It's also very useful in writing Regular Expressions, and probably many other things.
It's worth noting that C# also uses the # character as a prefix to allow reserved words to be used as identifiers. For example, Html Helpers in ASP.Net MVC can take an anonymous object containing HTML attributes for the tags they create. So you might see code like this:
<%= Html.LabelFor(m => m.Foo, new { #class = "some-css-class" } ) %>
The # is needed here because class is otherwise a reserved word.
The verbatim string literal allows you to put text inside of a string that would otherwise be treated differently by the compiler. For example, if I were going to write a file path and assign it to a variable, I might do something like so:
myString = "C:\\Temp\\Test.txt";
The reason I have to have the double slashes is because I am escaping out the slash so it isn't treated as an command. If I use the verbatim string literal symbol, my code could look as follows:
myString = #"C:\Temp\Test.txt";
It makes it easier to write strings when you are dealing with special characters.
Related
I want to write something like this C:\Users\UserName\Documents\Tasks in a textbox:
txtPath.Text = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)+"\Tasks";
I get the error:
Unrecognized escape sequence.
How do I write a backslash in a string?
The backslash ("\") character is a special escape character used to indicate other special characters such as new lines (\n), tabs (\t), or quotation marks (\").
If you want to include a backslash character itself, you need two backslashes or use the # verbatim string:
var s = "\\Tasks";
// or
var s = #"\Tasks";
Read the MSDN documentation/C# Specification which discusses the characters that are escaped using the backslash character and the use of the verbatim string literal.
Generally speaking, most C# .NET developers tend to favour using the # verbatim strings when building file/folder paths since it saves them from having to write double backslashes all the time and they can directly copy/paste the path, so I would suggest that you get in the habit of doing the same.
That all said, in this case, I would actually recommend you use the Path.Combine utility method as in #lordkain's answer as then you don't need to worry about whether backslashes are already included in the paths and accidentally doubling-up the slashes or omitting them altogether when combining parts of paths.
To escape the backslash, simply use 2 of them, like this:
\\
If you need to escape other things, this may be helpful..
There is a special function made for this Path.Combine()
var folder = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
var fullpath = path.Combine(folder,"Tasks");
Just escape the "\" by using + "\\Tasks" or use a verbatim string like #"\Tasks"
txtPath.Text = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)+"\\\Tasks";
Put a double backslash instead of a single backslash...
even though this post is quite old I tried something that worked for my case .
I wanted to create a string variable with the value below:
21541_12_1_13\":null
so my approach was like that:
build the string using verbatim
string substring = #"21541_12_1_13\"":null";
and then remove the unwanted backslashes using Remove function
string newsubstring = substring.Remove(13, 1);
Hope that helps.
Cheers
I have a Regex that I now need to moved into C#. I'm getting errors like this
Unrecognized escape sequence
I am using Regex.Escape -- but obviously incorrectly.
string pattern = Regex.Escape("^.*(?=.{7,})(?=.*[a-zA-Z])(?=.*(\d|[!##$%\?\(\)\*\&\^\-\+\=_])).*$");
hiddenRegex.Attributes.Add("value", pattern);
How is this correctly done?
The error you're getting is coming at compile time correct? That means C# compiler is not able to make sense of your string. Prepend # sign before the string and you should be fine. You don't need Regex.Escape.
See What's the # in front of a string in C#?
var pattern = new Regex(#"^.*(?=.{7,})(?=.*[a-zA-Z])(?=.*(\d|[!##$%\?\(\)\*\&\^\-\+\=_])).*$");
pattern.IsMatch("Your input string to test the pattern against");
The error you are getting is due to the fact that your string contains invalid escape sequences (e.g. \d). To fix this, either escape the backslashes manually or write a verbatim string literal instead:
string pattern = #"^.*(?=.{7,})(?=.*[a-zA-Z])(?=.*(\d|[!##$%\?\(\)\*\&\^\-\+\=_])).*$";
Regex.Escape would be used when you want to embed dynamic content to a regular expression, not when you want to construct a fixed regex. For example, you would use it here:
string name = "this comes from user input";
string pattern = string.Format("^{0}$", Regex.Escape(name));
You do this because name could very well include characters that have special meaning in a regex, such as dots or parentheses. When name is hardcoded (as in your example) you can escape those characters manually.
See this java code :-
s = s.replaceAll( "\\\\", "\\\\\\\\" ).replaceAll( "\\$", "\\\\\\$" );
I sorta don't understand it. It's a regex replace all.
I've tried the following C# code...
text = text.RegexReplace("\\\\", "\\\\\\\\");
text = text.RegexReplace("\\$", "\\\\\\$");
But if i have the following unit test :-
} ul[id$=foo] label:hover {
The java code returns: } ul[id\$=foo] label:hover {
My c# code returns: } ul[id\\\$=foo] label:hover {
So i'm not sure I understand why my c# code is putting more \'s in, mainly with regards to how these control characters are being represented.. ??
Update:
So, when i use XXX's idea of just using text.Replace(..), this works.
eg.
text = text.Replace("\\\\", "\\\\\\\\");
text = text.Replace("\\$", "\\\\\\$");
But I was hoping to stick with RegEx... to try and keep it as close to the java code as possible.
The extension method being used is...
public static string RegexReplace(this string input,
string pattern,
string replacement)
{
return Regex.Replace(input, pattern, replacement);
}
hmm...
Java needs all $ signs escaped in its replace string - "\\\\\\$" means \\ and \$. Without it it throws an error: http://www.regular-expressions.info/refreplace.html (look for "$ (unescaped dollar as literal text)").
Remember $1, $0 etc are replaced the text with captured groups, so there are a part of the syntax on the second argument to replaceAll. C# has a slightly different syntax, and doesn't require the extra slash, which it takes literally.
You could write:
text = text.RegexReplace(#"\\", #"\\");
text = text.RegexReplace(#"\$", #"\$");
Or,
text = text.RegexReplace(#"[$\\]", #"\$&");
I think it's the equivalent of this C# code:
text = text.Replace(#"\", #"\\");
text = text.Replace("$", #"\$");
The # indicates a verbatim string in C#, meaning that the backslashes in strings don't have to be escaped with more backslashes. In other words, the code replaces a single backslash with a double backslash and then replaces a dollarsign with a backslash followed by a dollarsign.
If you were to use the regex function, it would be something like this:
text = text.RegexReplace(#"\\", #"\\");
text = text.RegexReplace(#"\$", #"\$$");
Note that in the regex pattern (the first parameter), backslashes are special, while in the replacement (the second parameter) it is the dollarsigns that are special.
The code quotes the backslashes and '$' characters in the original string.
Java regex parsing: http://download.oracle.com/javase/1.4.2/docs/api/java/util/regex/Pattern.html
C#: http://msdn.microsoft.com/en-us/library/xwewhkd1.aspx
I think that in Java, you have to escape the \ character by using \, but in C#, you don't. Try taking out half of the \ in your C# version.
I want to assign a xml code into a string variable.
I can do this without escaping single or double-quotes by using triple-quote in python.
Is there a similar way to do this in F# or C#?
F# 3.0 supports triple quoted strings. See Visual Studio F# Team Blog Post on 3.0 features.
The F# 3.0 Spec Strings and Characters section specifically mentions the XML scenario:
A triple-quoted string is specified by using three quotation marks
(""") to ensure that a string that includes one or more escaped
strings is interpreted verbatim. For example, a triple-quoted string
can be used to embed XML blobs:
As far as I know, there is no syntax corresponding to this in C# / F#. If you use #"str" then you have to replace quote with two quotes and if you just use "str" then you need to add backslash.
In any case, there is some encoding of ":
var str = #"allows
multiline, but still need to encode "" as two chars";
var str = "need to use backslahs \" here";
However, the best thing to do when you need to embed large strings (such as XML data) into your application is probably to use .NET resources (or store the data somewhere else, depending on your application). Embedding large string literals in program is generally not very recommended. Also, there used to be a plugin for pasting XML as a tree that constructs XElement objects for C#, but I'm not sure whether it still exists.
Although, I would personally vote to add """ as known from Python to F# - it is very useful, especially for interactive scripting.
In case someone ran into this question when looking for triple quote strings in C# (rather than F#), C#11 now has raw string literals and they're (IMO) better than Python's (due to how indentation is handled)!
Raw string literals are a new format for string literals. Raw string literals can contain arbitrary text, including whitespace, new lines, embedded quotes, and other special characters without requiring escape sequences. A raw string literal starts with at least three double-quote (""") characters. It ends with the same number of double-quote characters. Typically, a raw string literal uses three double quotes on a single line to start the string, and three double quotes on a separate line to end the string. The newlines following the opening quote and preceding the closing quote are not included in the final content:
string longMessage = """
This is a long message.
It has several lines.
Some are indented
more than others.
Some should start at the first column.
Some have "quoted text" in them.
""";
Any whitespace to the left of the closing double quotes will be removed from the string literal. Raw string literals can be combined with string interpolation to include braces in the output text. Multiple $ characters denote how many consecutive braces start and end the interpolation:
var location = $$"""
You are at {{{Longitude}}, {{Latitude}}}
""";
The preceding example specifies that two braces starts and end an interpolation. The third repeated opening and closing brace are included in the output string.
https://devblogs.microsoft.com/dotnet/csharp-11-preview-updates/#raw-string-literals
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11
As shoosh said, you want to use the verbatim string literals in C#, where the string starts with # and is enclosed in double quotation marks. The only exception is if you need to put a double quotation mark in the string, in which case you need to double it
System.Console.WriteLine(#"Hello ""big"" world");
would output
Hello "big" world
http://msdn.microsoft.com/en-us/library/362314fe.aspx
In C# the syntax is #"some string"
see here
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
when to use # in c# ?
F.e. string sqlSelect = #"SELECT * FROM Sales".
It means interpret the following string as literal. Meaning, the \ in the string will actually be a "\" in the output, rather than having to put "\\" to mean the literal character
Before string it allows different string formating rules. You can't use backslash to specify special symbols and "" (double quotes become quotes). I find this format very useful for regular expressions
Example
Console.WriteLine(#"\n""\/a"); // outputs \n"\/a
Console.WriteLine("\\n\"\"\\/a"); // outputs \n"\/a
You might also seen # symbol before variable. In such case it allows using special C# keywords as variables.
Example:
var #switch = 1;
var #if = "test";
It means there is no need to escape characters in such a string.
So if you want to write the path for c:\Windows, you can write it as
string path = "c:\\Windows"; // Note escaped '\'
OR
string path = #"c:\Windows"; // '\' need not be escaped
There are two types of string literals, regular and verbatim. The # symbol makes it a verbatim string literal.
MSDN: String literals (C#)
In C and C++, string has some special characters called "escape characters". For example \, & and the " itself is an escape character!
In the very normal way, you to print a statement like:
Nancy Said Hello World! & smiled
you had to set your string like next
string str = "Nancy said Hello World! \& smiled.";
But people in Microsoft made a new cool feature in C# compiler so you can escape the headache of handling the escape characters by adding # before any string, and the compiler will handle all the escape characters for you by himself. For the last example you can have this in C# like next:
string str = #"Nancy said Hello World! & smiled.";
Verbatim string literals start with #
and are also enclosed in double
quotation marks. For example:
#"good morning" // a string literal
Nicked from, have a look at the last few lines above the example for more information.
http://msdn.microsoft.com/en-us/library/362314fe.aspx
Used for string literal. It marks the string within the quote (") marks as the value without applying any interpretation for symbols in that string.
It allows you to have a string with a \ delimiter in it. #"C:\A\b\c\d\e\f" is legal.