CODEWITHSUNDEEP
c#arrayslist
Ankush Madankar
Ankush Madankar

Reputation: 3834

Why should I convert List into Array

Look into following line of code:

public string[] GetStringList(params object[] values)
{
    List<string> _stringList = new List<string>();

    foreach (object value in values)
    {
        _stringList.Add(Convert.ToString(value));

        //Do something
    }

    //Why this invalid?
    return _stringList;

    //Why do required to convert list collection in array
    return _stringList.ToArray();
}

Since List and Array both are colletions then why should I need to convert List into Array?

Upvotes: 3

Views: 205

Answers (5)

Arin Ghazarian
Arin Ghazarian

Reputation: 5305

As you mentioned they are both Collections, that means, they both implemented ICollection, but like inheritance, the most specific objects are different things but have some characteristics that are the same. For example, suppose that we have tow classes:

class Plant : ILivingOrganism {...}
class Human: ILivingOrganism {...}

they have both implemented ILivingOrganism interface, that they have some characteristics which are the same, like void Grow() behavior which means they both can grow because they are living organism, but the thing is they are two different things even though they share some characteristics which are the same. Now when I write a method that returns Plant obviously you cannot expect it to return a Human, but with a method that returns ILivingOrganism, like ILivingOrganism Clone(), you can expect any living organism (more general type), like Human or Plant, technically everything that implements ILivingOrganism.

Upvotes: 1

MarcinJuraszek
MarcinJuraszek

Reputation: 125630

Since List and Array both are collections then why should I need to convert List into Array?

Yes, they are both collections, but your method signature specifies exactly what type of collection it returns, and that's why you have to return exactly that one collection type.

You can change your method declaration to return collection of strings using one of collection interfaces:

public IEnumerable<string> GetStringList(params object[] values)

or

public ICollection<string> GetStringList(params object[] values)

It will allow you to return both List<string> and string[], because they both implement the interface. You could also return HashSet<string> or even Queue<string> (for IEnumerable<string> version only. Or write your own class, implement the interface you're interested in and return it within your method.

But as soon as you declare the method using string[], you have to return something that is an array of strings, and List<string> does not fulfill that requirement.

OFT advice

You can make your code a little faster passing values.Length as List<string> constructor parameter:

List<string> _stringList = new List<string>(values.Length);

it will initialize list internal storage with necessary amount of memory and let you add all your items without reallocating any memory.

Upvotes: 6

MAV
MAV

Reputation: 7457

It is invalid to return the list directly because the specified return type is an array

          ↓
public string[] GetStringList(params object[] values)

If you want to return a list instead, it should be

public List<string> GetStringList(params object[] values)

If you want both to be valid, you could make the method return an IEnumerable (or another interface both types implement, like ICollection) like this:

public IEnumerable<string> GetStringList(params object[] values)

Upvotes: 4

Ringo
Ringo

Reputation: 3965

Your function return string[] instead of List<string> that's as you want. 

public List<string> GetStringList(params object[] values)
    {
        List<string> _stringList = new List<string>();

        foreach (object value in values)
        {
            _stringList.Add(Convert.ToString(value));

            //Do something
        }
        //Why do required to convert list collection in array
        return _stringList;
    }

Upvotes: 1

Adil
Adil

Reputation: 148120

Array and List are two different things, Dictionary is also a collection then why dictionary and array are the same?. There are so many types of collections and they all are different from each other. If you do not want to convert then you shall return list instead of Array. 

public List<string> GetStringList(params object[] values)
{
    List<string> _stringList = new List<string>(); 
    foreach (object value in values)        
    {
        _stringList.Add(Convert.ToString(value));
        //Do something
    }
    return _stringList;         
}

Upvotes: 3

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