string.Format throws System.Format exception on HTML + javascript

Question

I'm running string.Format on a readonly string that contains a bit of HTML + javascript but I get a System.FormatException instead.

This is my format string:

<script type="text/javascript">
    function {0}_showHideFieldWindow() {
        if ({0}.IsCustomizationWindowVisible()) {
            {0}.HideCustomizationWindow();
        } else {
            {0}.ShowCustomizationWindow();
        }
    }
</script>

All i'm doing is passing in the object name. Like this:

string.Format(javascript, "grid");

Andrew Hare · Accepted Answer · 2009-04-30 20:41:01Z

12

Since you have curly braces in the string you need to escape them by doubling them up ({{ and }}) to prevent the formatter from thinking they are tokens.

Your string initialization should look something like this:

String javascript = @"<script type=""text/javascript"">
            function {0}_showHideFieldWindow() {{
            if ({0}.IsCustomizationWindowVisible()) {{
                {0}.HideCustomizationWindow();
            }} else {{
                {0}.ShowCustomizationWindow();
            }}
        }}
    </script>";

answered Apr 30, 2009 at 20:41

Andrew Hare

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6 Comments

Tomas Aschan Over a year ago

Just out of curiosity, isn't the standard escape character in C# a backslash (\)? That is, shouldn't the curly braces be escaped with "\{\}" instead of "{{}}"? I do understand that the double braces work, but is it best practice?

John Rasch Over a year ago

Since he's using the @ symbol before the string, a backslash will be interpreted literally as a backslash so nothing would be escaped

Andrew Hare Over a year ago

John is correct - since it is a verbatim string there are a few escapes that you can do and all of them involve doubling up the character (i.e. { become {{, " becomes "", etc.).

James Couvares Over a year ago

A backslash is used to escape illegal C# characters. A brace is legal in a C# string, It's the string.Format method that expects the string in its own legal format.

Jonathan Rupp Over a year ago

@Tomas: the \ is the escape character for string literals -- ie. it's used to enter 'special' characters into a string literal. For a format string however, you double the character you want to escape. Why? Well, because that's what the API says. Why did the API designers do that? Probably because they also had to make it work for people using VB.Net. Which is easier -- explaining to a VB-er to put \{, or explaining to C# users to use {{? I'd guess the latter is far easier.

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Keltex · Accepted Answer · 2009-04-30 20:42:29Z

6

String.Format needs the extra brackets to be escaped. You might be better off doing something like this, which might be more readable than escaping each bracket if you don't need all of String.Format's functionality:

mystring.Replace("{0}","grid");

answered Apr 30, 2009 at 20:42

Keltex

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