Cannot convert type IEnumerable to List

Question

I have been using C# with Unity3d for a few years now, but am just starting with .NET programming. I get the error:

Cannot implicitly convert type ' System.Collections.Generic.IEnumerable<URL> ' to ' System.Collections.Generic.List<URL> '. An explicit conversion exists (are you missing a cast?)

Here is my code:

namespace TestBrowserHistory
{
    public class Test1
    {
        public Test1()
        {

        }
          static void Main()
    {
        InternetExplorer myClass = new InternetExplorer();
        List<URL> calledList = myClass.GetHistory();
        Console.WriteLine("Hello!");
        Console.WriteLine(calledList[1]);
        Console.ReadLine();
    }
    }
}

public class InternetExplorer
{
    // List of URL objects
    public List<URL> URLs { get; set; }
    public IEnumerable<URL> GetHistory()
    {
        // Initiate main object
        UrlHistoryWrapperClass urlhistory = new UrlHistoryWrapperClass();

        // Enumerate URLs in History
        UrlHistoryWrapperClass.STATURLEnumerator enumerator =
                                           urlhistory.GetEnumerator();

        // Iterate through the enumeration
        while (enumerator.MoveNext())
        {
            // Obtain URL and Title
            string url = enumerator.Current.URL.Replace('\'', ' ');
            // In the title, eliminate single quotes to avoid confusion
            string title = string.IsNullOrEmpty(enumerator.Current.Title)
                      ? enumerator.Current.Title.Replace('\'', ' ') : "";

            // Create new entry
            URL U = new URL(url, title, "Internet Explorer");

            // Add entry to list
            URLs.Add(U);
        }

        // Optional
        enumerator.Reset();

        // Clear URL History
        urlhistory.ClearHistory();

        return URLs;
    }

}

Thanks for any help!

Answer 1

You get that error because myClass.GetHistory(); returns IEnumerable<URL> , which is not same as List<URL> at compile time, although it is actually List<URL> at runtime. Change method signature to return List<URL> , cause you already do that

public List<URL> GetHistory()

Other workarounds would be to cast method call result to List<URL>

List<URL> calledList = (List<URL>)myClass.GetHistory();

Or construct new list from result

List<URL> calledList = new List<URL>(myClass.GetHistory());

If you do not need List functionality, you could define calledList as IEnumerable

var calledList = myClass.GetHistory();

Answer 2

Your definition of the GetHistory methods returns an IEnumerable, and you are assigning it to an IList. Either change the definition , or the usage.

If you don't need to change the collection I would change the definition of GetHistory to IEnumerable.

Answer 3

A List is IEnumerable, but the reverse is not true.

If you need list operations, you should change your method to return an IList<> instead of IEnumerable. Alternately, you should assign the return value to an IEnumerable variable instead of a List. This will limit you to (without further manipulation) the IEnumerable methods (you can do a foreach and use LINQ things like .First, but you can't reference by specific position, for example). Which might be enough for what you ultimately need it for.

Answer 4

Here is the error

List<URL> calledList = myClass.GetHistory(); 

Since GetHistory method returns IEnumerable<URL>

public IEnumerable<URL> GetHistory()

EDIT:

Solution : just change the return value of GetHistory() method to IList<T>

Answer 5

To get things working you just need to change the return type of your GetHistory() method to List<URL> .

You can typecast a List to an IEnumerable, but not the other way around. The compiler is told that GetHistory returns IEnumerable, and even though it is a list, it doesn't know that.

Answer 6

In alternative to what others have said, you could simply:

GetHistory();
List<URL> calledList = URLs;

Since GetHistory modifies the URLs as its side-effect anyway, there is little purpose of returning any result from it. In addition to that, you might consider whether GetHistory needs to be explicitly called at all - perhaps the equivalent code should be implicitly executed when the URLs getter is first called?

Also, why aren't you using foreach ?

Answer 7

just add

yield return U;

end of while block and remove

 return URLs;

after that function is like this

public IEnumerable<URL> GetHistory()
{
    // Initiate main object
    UrlHistoryWrapperClass urlhistory = new UrlHistoryWrapperClass();

    // Enumerate URLs in History
    UrlHistoryWrapperClass.STATURLEnumerator enumerator =
                                       urlhistory.GetEnumerator();

    // Iterate through the enumeration
    while (enumerator.MoveNext())
    {
        // Obtain URL and Title
        string url = enumerator.Current.URL.Replace('\'', ' ');
        // In the title, eliminate single quotes to avoid confusion
        string title = string.IsNullOrEmpty(enumerator.Current.Title)
                  ? enumerator.Current.Title.Replace('\'', ' ') : "";

        // Create new entry
        URL U = new URL(url, title, "Internet Explorer");

        // Add entry to list
        URLs.Add(U);
        yield return U;
    }

    // Optional
    enumerator.Reset();

    // Clear URL History
    urlhistory.ClearHistory();


}

Answer 8

You can just use tolist() provided by Linq lamda to convert from IEnumerable to List.

using System.Linq;

List<URL> calledList = myClass.GetHistory().ToList();