Matthew Watson Matthew Watson - 1 month ago 9
C# Question

How to use string.Format() to format a hex number surrounded by curly brackets?

Input:

uint hex = 0xdeadbeef;


Required output:
string result = "{deadbeef}"


First approach: Explicitly add the
{
and
}
; this works:

result = "{" + string.Format("{0:x}", hex) + "}"; // -> "{deadbeef}"


Output as decimal rather than hex using escaped curly brackets:

result = string.Format("{{{0}}}", hex); // -> "{3735928559}"


Seems promising, now all we need to do is add the
:x
hex specifer as per the first approach above:

result = string.Format("{{{0:x}}}", hex); // -> "{x}"


Oh dear, adding the
':x
has made it output
"{x}"
rather than the
"{deadbeef}"
that I wanted.

So my question is: Must I solve this by explicitly adding the
{
and
}
as per the first example, or is there a way to do it using composite formatting and escaping the curly brackets?

Also note that this also affects string interpolation which (after all) is just converted by the compiler into a call to
string.Format()
.

(This may well be duplicate question, but I have been unable to find a duplicate so far...)

edited

Answer

See "Escaping Braces" in http://msdn.microsoft.com/en-us/library/txafckwd(v=vs.110).aspx - basically your workaround is a solution.

From there:

The way escaped braces are interpreted can lead to unexpected results. For example, consider the format item "{{{0:D}}}", which is intended to display an opening brace, a numeric value formatted as a decimal number, and a closing brace. However, the format item is actually interpreted in the following manner:

1.The first two opening braces ("{{") are escaped and yield one opening brace.
2. The next three characters ("{0:") are interpreted as the start of a format item.
3. The next character ("D") would be interpreted as the Decimal standard numeric format specifier, but the next two escaped braces ("}}") yield a single brace. Because the resulting string ("D}") is not a standard numeric format specifier, the resulting string is interpreted as a custom format string that means display the literal string "D}".
4. The last brace ("}") is interpreted as the end of the format item.
5. The final result that is displayed is the literal string, "{D}". The numeric value that was to be formatted is not displayed.

and as a solution, adjusted to your example:

uint hex = 0xdeadbeef;
string output = string.Format("{0}{1:x}{2}", 
                             "{", hex, "}");
Console.WriteLine(output);
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