Using the generic type 'System.Collections.Generic.List' requires '1' type argument(s)

Question

I have an assignment to create a "Tweeter" app in c#, and for some reason I can't get it to do the most simple thing, add items to a list and display them.

I keep getting the error:

Using the generic type 'System.Collections.Generic.List' requires '1' type argument(s)

on the following line of code:

List tweets = new List<Tweet>();

I tried doing

Tweet tweets = new List<Tweet>();

first, but it gave me over a dozen errors.

Here's my Tweet.cs:

namespace Assign3_Twitter
{
    public class Tweet
    {
        public string HashTag { get; private set; }
        public string Message { get; private set; }
        public string Sender { get; private set; }
        public string Recipient { get; private set; }
        public Tweet (string sender, string message, string hashtag, string reciepient)
        {
            this.Sender = sender;
            this.HashTag = hashtag;
            this.Message = message;
            this.Recipient = reciepient;
        }

        public override string ToString ()
        {
            return string.Format ("[Tweet: HashTag={0}, Message={1}, Sender={2}, Recipient={3}]", HashTag, Message, Sender, Recipient);
        }
    }
}

and here's where I'm having all the issues:

TweetManager.cs

namespace Assign3_Twitter
{
    public class TweetManager
    {
        private List tweets = new List<Tweet>();

        public TweetManager()
        {
            tweets = new List<Tweet>();

            Tweet tw1 = new Tweet ("Austen", "Hello World!", "#Hey", "Twitter");
            tweets.Add (tw1);

            Tweet tw2 = new Tweet ("Test1", "Hello World! x2", "#Howdy", "Tweeter");
            tweets.Add (tw2);

            Tweet tw3 = new Tweet ("Test2", "Hello World! x3", "#Hey", "Twitter");
            tweets.Add (tw3);

            Tweet tw4 = new Tweet ("Test3", "Hello World! x4", "#Howdy", "Tweeter");
            tweets.Add (tw4);

            Tweet tw5 = new Tweet ("Test4", "Hey there!", "#Hey", "Twitter");
            tweets.Add (tw5);

            Tweet tw6 = new Tweet ("Test5", "Woah this is cool!", "#Howdy", "Tweeter");
            tweets.Add (tw6);
        }

        public void SeeAllTweeters()
        {
            foreach (Tweet Tweets in tweets)
            {
                Console.WriteLine (Tweets);
            }

        }

Answer 1

List tweets = new List<Tweet>();

should be

List<Tweet> tweets = new List<Tweet>();

In the first line you're trying to assign a typed list to an untyped list.

Also, as Kirk points out , simply using var instead of declaring the type yourself would automatically use the type that you intend:

var tweets = new List<Tweet>();

Answer 2

As others have pointed out.... you need to declare it as List<Tweet> or use var . var will automatically be turned into the correct type at compile time.

Now I am not sure if this is a simple mistake in forgetting to declare the generic or if you thought you just needed to declare List .

List doesn't actually exist. If you were looking to use something more generic, you could choose any of the interfaces on the List<T> that has the functionality you wanted. For instance, you could have used IList , IList<T> , ICollection , etc.

Setting your new List<Tweet>() to any of the interfaces above would have worked out as well.

http://msdn.microsoft.com/en-us/library/6sh2ey19(v=vs.110).aspx

Answer 3

你必须做

List<Tweet> tweets = new List<Tweet>();

Answer 4

You need to either specify the full type (including the generics):

 List<Tweet> tweets = new List<Tweet>();

Or use implicit typing:

 var tweets = new List<Tweet>();