Generic method for converting string array into list

Question

I would like to create a function that will return list of type that is specified by me at run time. I tried something along this line:

  public static List<T> GetMyList<T>(string[] itemList)
  {
     List<T>  resultList = new List<T>(itemList.Length);
     return resultList.AddRange(itemList);
  }

But this doesn't work. Obviously I don't fully understand how to pass a type to be converted to. Any help would be appreciated it.

Edit: It looks like that it is not possible, but here is more info. String array will contain numbers and I would like to convert those numbers sometimes into int, sometimes into short. Idea behind is to have a generic function that will attempt to convert items into whatever type list I tell it.

Could you provide more information? What do the strings represent? Are they class names? Is it serialized information from type T? — Nick
– Nick, Commented Mar 5, 2010 at 18:53
How will the strings be converted to type T? Your function will need to get a Converter(of String, T) or Converter(of String(), T()) to make the conversion. — M.A. Hanin
– M.A. Hanin, Commented Mar 5, 2010 at 18:55

Matthew Whited · Accepted Answer · 2010-03-05 19:35:55Z

7

You need to provide a method to convert a string into a T - you can do this using a Func<string, T>:

public static List<T> GetMyList<T>(string[] itemList, Func<string, T> conversionFunc)
{
    return itemList.Select(conversionFunc).ToList();
}

e.g.

List<int> ints = GetMyList(new[] { "1", "2", "3" }, s => int.Parse(s));

edited Mar 5, 2010 at 19:35

Matthew Whited

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answered Mar 5, 2010 at 18:57

Lee

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1 Comment

Mads Ravn Over a year ago

How about List.ConvertAll instead of writing a new function? Eg. string[] itemArray = { "1", "2" }; (itemArray.ToList()).ConvertAll((x) => int.Parse(x));

Chris Bush · Accepted Answer · 2013-02-22 06:45:47Z

A slightly more elegant solution would be to add an extension method to string that automatically calls the parser for type T, like so:

public static class GenericParser {
    public static T Parse<T>(this string input) {
        var converter = TypeDescriptor.GetConverter(typeof(T));
        if ( converter != null ) {
            return ( T )converter.ConvertFromString(input);
        }
        return default(T);
    }
}

Then, your conversion function would look something like:

public static List<T> GetMyList<T>(string[] itemList) {
    List<T> list = new List<T>();
    list.AddRange(Array.ConvertAll<string, T>(itemList, delegate(string s)  {
        return s.Parse<T>();
    }));
    return list;
}

And the usage would be:

List<int> integers = GetMyList<int>(new []{"1", "2", "3", "4"});
List<double> doubles = GetMyList<double>(new []{"1.0", "2.0", "3.0", "4.0"});

and so on.

Al Crowley · Accepted Answer · 2010-03-05 18:57:42Z

1

My first thought is that this won't work because not every object type can be constructed from a string. Perhaps you want something with a signature more like:

public static List<T> GetMyList<T>(T[] itemList)
{
  List resultList = new List(itemList.Length);
  foreach (t in itemList)
  {
    resultList.add(t);
  }
  return resultList;
}

(forgive my syntax. I don't have a compiler handy right now to check it.)

answered Mar 5, 2010 at 18:57

Al Crowley

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Comments

Andrey · Accepted Answer · 2010-03-05 18:53:53Z

0

this doesn't work because system has no idea how to convert string to generic T. Also even if it is known, it will not work, because C# (prior to 4) doesn't have type covariance. So use either foreach to copy and convert elements one by one or use Select from Linq

answered Mar 5, 2010 at 18:53

Andrey

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1 Comment

Andrey Over a year ago

btw covariance works for arrays. but only if CLR can convert elements itseld, without conversion method

Community · Accepted Answer · 2017-05-23 11:45:05Z

0

Similar to Lee's but more generic...

public static class Tools
{
    public static List<TResult> ToList<T, TResult>(
       this IEnumerable<T> input,
            Func<T, TResult> conversion)
    {
        return input.Select(conversion).ToList();
    }
    public static List<TResult> ToList<T, TResult>(
        this IEnumerable<T> input,
             Func<T, int, TResult> conversion)
    {
        return input.Select(conversion).ToList();
    }
}

class Program
{
    static void Main(string[] args)
    {
        var input = new[] { "1", "2", "3" };

        var ret = input.ToList(i => int.Parse(i));
        // 1,2,3

        var ret2 = input.ToList((i,j) => int.Parse(i) + j * 10);
        // 1,12,23
    }
}

edited May 23, 2017 at 11:45

CommunityBot

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answered Mar 5, 2010 at 19:20

Matthew Whited

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1 Comment

Matthew Whited Over a year ago

I was a little surprised that an overload for .ToList<T>() didn't match the above signature.

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