Parsing Non-Standard Date and Time Formats [Ron Petrusha]

[アーティクル]
04/13/2011

Frequently, particularly when dealing with remote data collection devices, an application receives string data containing date and time information that must be converted to either DateTime or DateTimeOffset values. In these cases, the most commonly used parsing methods, the overloads of the DateTime.Parse or DateTimeOffset.Parse methods, all throw a FormatException when they attempt to perform the conversion, and their corresponding TryParse overloads all return false and assign a data and time of MinValue to the method’s date and time argument. However, the .NET Framework provides two alternative overloaded methods, ParseExact and TryParseExact, that can be used to parse the string representations of date and time values.

In parsing date and time strings, both Parse and TryParse use a NumberFormatInfo object that corresponds either to the current culture or, if one of the overloads with an IFormatProvider parameter is called, to the culture that corresponds to the IFormatProvider argument. The properties of this NumberFormatInfo object determine the acceptable date and time formats that a string can have in order to parse successfully. Even though the number of supported formats is typically sizable, the Parse and TryParse methods cannot parse strings in non-standard formats successfully.

On the other hand, the ParseExact or TryParseExact methods are ideal, since they require that a string exactly conform to a particular definable pattern if the parse operation is to succeed. They are useful when either of the following conditions holds true:

The precise format of a string containing date and time information is known in advance, as it is when the data is gathered by some data collection object or device.
User input must have a particular format.

The overloads of the ParseExact and TryParseExact methods of both the DateTime and DateTimeOffset objects allow you to specify either a single pattern or an array of patterns. You to define a pattern by specifying a format string that consists of either a standard format specifier or one or more custom format specifiers. The input string must then conform to that single pattern or to any one of the patterns in the array. Two of the overloads also include a DateTimeStyles parameter that allows you to define style elements (such as white-space characters) that may be included in the input, or to determine how the input string is to be interpreted (for example, as local or universal time).

To see how the ParseExact and TryParseExact methods can help with parsing date and time strings, let’s take three examples. First, in some cases, date and time data is provided to an application as a set of numbers without any separators (for example, “20100501061245500”). The following code uses a custom date and time format string to do this.

  [Visual Basic] 
Module Example
   Public Sub Main()
      Dim fmt As String = "yyyyMMddhhmmssFFFFFFF"
      Dim value As String = "20100501061245500"
      Dim dt As DateTime = DateTime.ParseExact(value, fmt, Nothing)
      Console.WriteLine("{0}--> {1}", value, dt)
   End Sub
End Module

 [C#] 
using System;

public class  Example
{
   public static void Main()
   {
      string fmt = "yyyyMMddhhmmssFFFFFFF";
      string value = "20100501061245500";
      DateTime dt = DateTime.ParseExact(value, fmt, null);
      Console.WriteLine("{0} --> {1}", value, dt);
   }
}