String.Format exception following upgrade from .Net 2.0 to .Net 4.5.2

Question

I have recently upgraded a ASP.Net forms application to .Net 4.5.2, after fixing some relatively trivial namespace issues I was able to build the solution successfully. However at runtime I have been receiving the following error:

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

Which when debugging is thrown by the following line:

string.Format("Init took {0:mm:ss}", (object) DateTime.Now.Subtract(renderStartTime))  

Where renderStartTime = DateTime.Now

I am somewhat puzzled why I am seeing this error since upgrading. Any thoughts?

Answer 1

It is a breaking change.

See: Formatting and Parsing Time Intervals in the .NET Framework 4

In the .NET Framework 3.5 and earlier versions, TimeSpan does not implement IFormattable, nor does it support format strings. Therefore, the “r” format string is ignored, and the parameterless TimeSpan.ToString method is called. In the .NET Framework 4, on the other hand, TimeSpan.ToString(String, IFormatProvider) is called and passed the unsupported format string, which causes the exception.

Just to expand on the answer, your original code in .Net framework 2.0 would not throw an exception but it will not give you the desired output, (minutes:seconds). Since the parameter less constructor would be call for TimeSpan, ignoring the format specified in String.Format. But, with .Net framework 4.0 or hgiher , since TimeSpan implements IFormattable, the format specified mm:ss would be passed to the ToString call. Now this format mm:ss is invalid for TimeSpan, it requires colon to be escaped with back slash like: mm\:ss. That is why you are getting exception.

See: Custom TimeSpan Format Strings

In .Net 3.5 or lower you can use:

TimeSpan elapsed = DateTime.Now - renderStartTime; //or DateTime.Now.Subtract(renderStartTime)
string formatted = string.Format("Init took {0}:{1}", elapsed.Minutes, elapsed.Seconds); //returns minutes and seconds components, 
                                        // If you are looking for Total Minutes and Total Seconds then use TotalMinutes/TotalSeconds

In .Net framework 4.0 or higher you can do:

string.Format("Init took {0:mm\\:ss}", elapsed);