FormatException when passing DateTime.toString() to AppendFormat C# .NET

Question

I'm trying to do the following:

 var policyBuilder = new StringBuilder();    

 var expiration = DateTime.UtcNow.AddDays(1).ToString("s") + "Z";

 policyBuilder.AppendFormat("{ \"expiration\": \"{0}\",\n", expiration);

However, the last line throws the following exception:

 An exception of type 'System.FormatException' occurred in mscorlib.dll 
 but was not handled in user code

 Additional information: Input string was not in a correct format.

'expiration' is a string, so why am I getting this error?

Thanks

I would guess that you probably _do_ want the `{` character in the output, as it appears you're trying to build up a JSON string using a `StringBuilder`. There are better ways to do this. — James Thorpe, Feb 25 '16 at 12:16
Yes. I figured out that it works fine if I add another { at the start of the string like this: policyBuilder.AppendFormat("{{ \"expiration Why is this? — djst, Feb 25 '16 at 12:19
@djst: that's explained in the link i've provided in my answer ;) — Tim Schmelter, Feb 25 '16 at 12:22
Yep, I'm trying to build a string using the json format. I wont be serializing it. Its for doing AWS a3 policy signing. [link]http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html#sigv4-HTTPPOSTExpiration — djst, Feb 25 '16 at 12:23
_"I wont be serializing it"_ Well, you don't serialise the JSON string itself - the idea is you'd build an object that has the properties you want, and serialise _that_. Even without doing that though, there are things around that let you build up a JSON string without serialising objects that will handle this sort of stuff for you (braces and quoting key names/values etc) — James Thorpe, Feb 25 '16 at 12:25
@JamesThorpe thanks for the help and for setting that straight. — djst, Feb 25 '16 at 12:28

score 1 · Answer 1 · answered Feb 25 '16 at 12:18

If you want a { at the beginning you have to use two:

policyBuilder.AppendFormat("{{ \"expiration\": \"{0}\",\n", 10);

See: Escaping Braces in Composite Formatting

Opening and closing braces are interpreted as starting and ending a format item. Consequently, you must use an escape sequence to display a literal opening brace or closing brace. Specify two opening braces ("{{") in the fixed text to display one opening brace ("{"), or two closing braces ("}}") to display one closing brace ("}"). Braces in a format item are interpreted sequentially in the order they are encountered. Interpreting nested braces is not supported. ....

FormatException when passing DateTime.toString() to AppendFormat C# .NET

1 Answers1