Generics & Type Casting

Generics & Type Casting

• Googlesyrop

  Thanks for the replies. The struct constraint is what I'm after, but the problem I'm having now is casting from a string back to <T> the original type.

  So, in the test I am trying I take an array of ints, convert to strings to sort, but then I want to cast each string back to int.

  In order to do

  T obj = s as T;

  I need the "class" constraint. But I want to work with base value types only.

  So my options seem to be to work with objects only

  Or do I have implement the IConvertible interface and come up with my own ConvertToT method

  Help is much appreciated.

  
  

• carthegenian

  I'm not aware of "OR"ing constraints, only "AND"ing.

  As a workaround, you could put add to your generic class method a System.Diagnostics.Debug.Assert() to check that T.GetType() is one of the allowed types.

  Brian

  
  

• Liel

  In your sample code try changing the 2nd line in the method to "T temp = (T)s;", it should work. I did a simple test on similar lines by declaring a generic class with struct constraint:

  public class Class2<T> where T : struct< xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

  {

      T secondVar;

      string strVar;

   

      public T Test(T firstVar)

      {

          strVar = firstVar.ToString();

          secondVar = (T)firstVar;

   

          return secondVar;

      }

  }

  Test() method simply did a convesion like you tried with a 2-D array. Finally I tested this code with a button on a form:

  Class2<int> obj = new Class2<int>();

  MessageBox.Show(obj.Test(5).ToString());

   

  
  

• alshami

  You can't cast string to int because they are not compitable type. That's why int, double, DateTime etc have Parse method and you can pass in a string and extract data of the correct type. We can use this knowledge, and invoke the Parse method using the reflection. What I have done in CastTest class is wrote a private method to parse out the value from the string:

  public class CastTest<T> where T : struct

  {

  T newT;

  string s;

  public T Test(T item)

  {

  s = item.ToString();

  newT = Parse(s);

  return newT;

  }

  private T Parse(string s)

  {

  Type t = typeof(T);

  System.Reflection.MethodInfo parseMethod = t.GetMethod("Parse", new Type[] { typeof(string) });

  if (parseMethod != null)

  return (T)parseMethod.Invoke(null, new object[] { s });

  else

  throw new ApplicationException(t.Name + " type does not support Parse() method.");

  }

  }

  This was a very good learning exercise for me too.

  
  

• GuyLowndes

  I only just spotted the error in your code - you're casting firstVar as T, but it is already T.
  

• Sueva

  You are right, I don't know what I was thinking, sorry about that. I will keep thinking about this, in the mean time, instead of using a string type for "s", using an object type seems to let you cast back to the appropriate type.

  public class CastTest<T> where T : struct

  {

  T newT;

  object s;

  public T Test(T item)

  {

  s = item;

  newT = (T)s;

  return newT;

  }

  }

  
  

• haal1

  To limit to only value types you can use struct constraint:

  yourclass<T> where T : struct

  
  

• DOAHunterX

  How was "s" defined Isn't it is also of the generic type T Then you may not need the typecast. Can you please post a complete code snippet
  

• jeremym1234

  Thanks - that's very helpful.

  I had considered the Parse method earlier on, but hadn't a clue where to start with writing a custom version of it. I thought that if it seemed too complicated there must have been an easier way to do it.

  However, this is very useful information.

  Thanks for all your help!

  James

  
  

• bobiav

  No worries! Thanks for your continued attention.

  Of course I can compare objects, the original goal of this was get a 2d Array into an ArrayList (as a delimited string), sort it (with the necessary methods to sort by a certain column) then split out each string and add them back to the original T[,] Array.

  In any case, in the meantime I've written the QuickSort to work with 2d Arrays and sort by selected column - so as a learning exercise (which is all this is) it has certainly been useful.

  I'd be interested to know why you can't cast back to <T> from string. I wonder if it is because string is nullable

  Anyway, thanks for your help so far.

  James

  
  

• clarayeung

  Thanks, but I can't get that to work - still get the Cannot convert "string" to "T" error.

  public class CastTest<T> where T : struct
  {
        T newT;
        string s;
       
        public T Test(T item)
        {
              s = item.ToString();
              newT = (T) s;
              return newT;
        }
  }
  public class Tester
  {
        static void Main()
        {
              int i = 5;
             
              CastTest<int> ct = new CastTest<int>();
             
              int j = ct.Test(i);
             
              Console.WriteLine(j.GetType().ToString());
        }
  }

  
  

• Server Girl

  Thanks for the response. Unfortunately it doesn't work - I think because I know nothing about Generics and am having problems with constraints.

  I want restrict the 2-dimensional array to vase* value types only - so: int, string, double etc.

  It doesn't seem that this is possible. Any thoughts

  Thanks,

  James

  *edit: base, not vase

  
  

• AlexTorone

  Type type = typeof(T);

  makes type a variable not a type.

  Try:

  sortedArray[rowIndex, columnIndex] = (T) delimitedFields[columnIndex];

  or a bit more elegantly:

  sortedArray[rowIndex, columnIndex] = delimitedFields[columnIndex] as T;

  
  

• ArifKh

  Apologies, here you go:

  public void CastTest(T[,] tObject)

  {

  string s = tObject[0,0].ToString();

  T temp = s as T;

  tObject[0,0] = temp;

  }

  Where tObject is a 2d array of value types (so ints, doubles etc...)

  
  
• Generics & Type Casting