C＃Xamarin如何在类中使用现有List？

Question

I am struggling with a problem.

In my MainActivity.cs I made a List like this:

//Define
public List<products> mItems;

Here I created the List:

mItems = new List<products>();

And over here I fill the List:

foreach (var property in Array)
{
    //Convert every value in Array to string
    var propertyList = JsonConvert.DeserializeObject<List<products>>(property.ToString());

    //Add all strings to List
    mItems.AddRange(propertyList);
}

Now I want to use this List in another class, so I use this code in the new class (Lets call it "Class2"):

public List<products> mItems;

But mItems is Null, what am I doing wrong.

Again a little update with an Error of course.

In the MainActivity, I made these changes:

private List<products> mItems = new List<products>();

public List<products> ProductList
{
    get { return mItems; }
}

And in Class2:

List<products> mItems = MainActivity.ProductList;

I don't know if I am doing the good way, but my error is: "An object reference is required for the non-static field, method, or property "

I have no idea what I have to do! Any suggestions?

Answer 1

In your Class2, you have to get it from your Class1, just gave to a second list the same name isn't enough.

Here is an example and you should revise the object programming ( https://msdn.microsoft.com/en-us/library/dd460654(v=vs.120).aspx#Properties )

public class Class1 {
    public List<products> mItems;

    public Class1() {

        //Startup WebClient
        WebClient client = new WebClient();

        //Define URL to download
        string link = @"http://websitelink.todownload.json.php";

        //Download json website content
        string json = new WebClient().DownloadString(link);

        //Parse json content
        var jObject = JObject.Parse(json);

        //Create Array from everything inside Node:"Products"
        var productPropery = jObject["Products"] as JArray;

        mItems = new List<products>();

        foreach (var property in Array)
        {
            //Convert every value in Array to string
            var propertyList = JsonConvert.DeserializeObject<List<products>>(property.ToString());

            //Add all strings to List
            mItems.AddRange(propertyList);
        }
    }
}

public class Class2 {
    public List<products> mItems;

    public Class2() {
        var class1Instance = new Class1();
        mItems = class1Instance. mItems;
    }
}

Answer 2

This answer is too broad to answer definitively, but I shall try help.

Example 1 if you want to pass a list from your main activity to an instance of a class you are creating in the main activity you can pass the list when declaring a new instance of it.

MyListClass listClass = new MyListClass(productList);

then in the class you have something like

public class MyListClass
{
    private List<Products> _productList;
    public MyListClass(List<Products> productList)
    {
        _products = productList
    }
}

Example 2 If you are looking to pass the list from your main activity to lets say a fragment there are a number of ways to do so. you can use the following.

ListFragment listFragment = ListFragment.NewInstance(productList);

then in the fragment you have a static class NewInstance like so.

public class ListFragment :Fragment
{
    private List<Products> _productList;

    public static ListFragment NewInstance(List<Products> productList)
    {
        _productList = productList;
        ListFragment fragment = new ListFragment();
        return fragment;
    }
}

Example 3 You can use a listener that your mainActivity implements and through the listener call a method that will return a list to you, the reason you use a listener is to decouple your fragment from any activity, an example of this would be

public class MainActivity : IListFragmentListener
{
//implement the interface below
    public List<Products> GetProductsList()
    {
        return productsList;
    }
}

then your IListFragmentListener.cs

public interface IListFragmentListener
{
    List<Products> GetProductsList();
}

finally in your fragment

public class ListFragment :Fragment
{
    private List<Products> _productList;
    private IListFragmentListener _listener;
    public override void OnAttach(Context context)
    {
        base.OnAttach(context);
        _listener = (IAddDateFragmentListner)context;
    }
    private void RandomFragmentMethod()
    {
        _productList = _listener.GetProductsList();
    }
}

Activity to Activity Just a quick note on passing a List from one activity to another, as far as I know the cleanest way to do this is to make your list object serializable and serialize it in one activity and Deserialize it in the activity you wish to pass the list too. This can cause performance issues however