Cast object to T

Question

I m parsing an XML file with the XmlReader class in  NET and I thought it would be smart to write a generic parse function to read different attributes generically  I came up with the following function   private static T ReadData lt T gt  XmlReader reader  string value        reader MoveToAttribute value       object readData   reader ReadContentAsObject        return  T readData      As I came to realise  this does not work entirely as I have planned  it throws an error with primitive types such as int or double  since a cast cannot convert from a string to a numeric type  Is there any way for my function to prevail in modified form

User · Answer

You can presumably pass-in  as a parameter  a delegate which will convert from string to T

User · Answer

First check to see if it can be cast   if  readData is T        return  T readData     try      return  T Convert ChangeType readData  typeof T       catch  InvalidCastException       return default T

User · Answer

Actually  the responses bring up an interesting question  which is what you want your function to do in the case of error   Maybe it would make more sense to construct it in the form of a TryParse method that attempts to read into T  but returns false if it can t be done       private static bool ReadData lt T gt  XmlReader reader  string value  out T data                bool result   false          try                       reader MoveToAttribute value               object readData   reader ReadContentAsObject                data   readData as T              if  data    null                                   see if we can convert to the requested type                 data    T Convert ChangeType readData  typeof T                              result    data    null                     catch  InvalidCastException              catch  Exception ex                           add in any other exception handling here  invalid xml or whatnot                      make sure data is set to a default value         data    result    data   default T           return result          edit  now that I think about it  do I really need to do the convert changetype test   doesn t the as line already try to do that   I m not sure that doing that additional changetype call actually accomplishes anything   Actually  it might just increase the processing overhead by generating exception   If anyone knows of a difference that makes it worth doing  please post

User · Answer

try  if  readData is T      return  T  object readData

User · Answer

Actually  the problem here is the use of ReadContentAsObject  Unfortunately  this method does not live up to its expectations  while it should detect the most appropirate type for the value  it actually returns a string  no matter what this can be verified using Reflector     However  in your specific case  you already know the type you want to cast to  therefore i would say you are using the wrong method   Try using ReadContentAs instead  it s exactly what you need   private static T ReadData lt T gt  XmlReader reader  string value        reader MoveToAttribute value       object readData   reader ReadContentAs typeof T   null       return  T readData

User · Answer

Add a  class  constraint  or more detailed  like a base class or interface of your exepected T objects    private static T ReadData lt T gt  XmlReader reader  string value  where T   class       reader MoveToAttribute value       object readData   reader ReadContentAsObject        return  T readData      or where T   IMyInterface or where T   new    etc

User · Answer

Have you tried Convert ChangeType   If the method always returns a string  which I find odd  but that s besides the point  then perhaps this changed code would do what you want   private static T ReadData lt T gt  XmlReader reader  string value        reader MoveToAttribute value       object readData   reader ReadContentAsObject        return  T Convert ChangeType readData  typeof T

User · Answer

You could require the type to be a reference type     private static T ReadData lt T gt  XmlReader reader  string value  where T   class         reader MoveToAttribute value        object readData   reader ReadContentAsObject         return  T readData       And then do another that uses value types and TryParse      private static T ReadDataV lt T gt  XmlReader reader  string value  where T   struct         reader MoveToAttribute value        object readData   reader ReadContentAsObject         int outInt       if int TryParse readData  out outInt           return outInt

[c#] Cast object to T

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