手动填写C＃数据类型集合列表

Question

I am working on a C# example to learning the Lists and how to initialize them. So I have a classes with some properties:

public class C1
{
    public string p1 { get; set; }
    public string p2 { get; set; }
    public List<StatusChoices> ChoiceList { get; set; }

    ...

    public string FillList(string v1, int v2, bool v3)
    {
        // How can I fill this.ChoiceList?

        this.ChoiceList.val1 = v1; //Is this possible?

        return this.ChoiceList.val1;
    }
}

public class StatusChoices
{
    public string val1 { get; set; }
    public int val2    { get; set; }
    public bool val3   { get; set; }
}

This is probably easy, but I wasn't able to achieve it so far. How can I add some values " manually " to the list?

Answer 1

The ChoiceList is initialized to null when the class C1 is created. (In fact every class field, including hidden property backing fields, is initialized to its default value.) You must create the list object explicitly.

You can do so in the class constructor

public class C1
{
    ...
    public List<StatusChoices> ChoiceList { get; set; }

    public C1() // Constructor. Is run when a new C1 object is created with `new`.
    {
        ChoiceList = new List<StatusChoices>();
    }

    ...
}

After creation, this list is empty. You must add elements to it:

public string FillList(string v1, int v2, bool v3){
    ChoiceList.Add(new StatusChoices{ val1 = v1, val2 = v2, val3 = v3 });
    return v1;
}

Note that you have to create a StatusChoices object as well. You can think of a class as being a template that can be used to create objects.

When you have added objects to the list, you can access them by enumeration

foreach (StatusChoices sc in ChoiceList) {
    Console.WriteLine($"val 1 = {sc.val1}, val 2 = {sc.val2}, val 3 = {sc.val3}")
}

or through indexing

StatusChoices first = ChoiceList[0];
StatusChoices second = ChoiceList[1];
string v1_of_first  = first.val1;
string v1_of_second  = second.val1;

You can also access properties of the elements directly

string v1_of_first  = ChoiceList[0].val1;
string v1_of_second  = ChoiceList[1].val1;

If you have an object C1 c1 = new C1(); , you can write

string v = c1.ChoiceList[0].val1;

You could even construct a list of C1 objects

var mainList = new List<C1>(); // The var-keyword lets C# infer the type.
                               // This saves you from writing List<C1> again.
mainList.Add(new C1());
mainList.Add(new C1());
mainList.Add(new C1());
mainList[0].FillList("a", 12, true);
mainList[0].FillList("b", 33, false);
mainList[1].FillList("x", 77, true);
...
string v = mainList[1].ChoiceList[2].val1;

---

I used ChoiceList.Add(new StatusChoices{ val1 = v1, val2 = v2, val3 = v3 }); to add a new element to the list. This is a short version of

StatusChoices sc = new StatusChoices();
sc.val1 = v1;
sc.val2 = v2;
sc.val3 = v3;
ChoiceList.Add(sc);

The short version uses an object initializer { val1 = v1, ... } .

Answer 2

Something like this

public string FillList(string v1, int v2, bool v3){
    this.ChoiceList = new List<StatusChoices> {
     new StatusChoices { val1 = v1, val2 = v2, val3 = v3, } };
    return this.ChoiceList[0].val1;
}

Answer 3

Simply create an instance of the StatusChoices class, initialize it and add it to the list.

(I added lazy initialization of the List)

public class C1{
    public string p1 {get; set; }
    public string p2 {get; set; }
    public List<StatusChoices> ChoiceList { get; set; }
    ...
    public string FillList(string v1, int v2, bool v3){

        if(ChoiceList == null)
        {
            this.ChoiceList = new List<StatusChoices>();
        }

        var newItem = new StatusChoices {val1 = v1, val2 = v2, val3 = v3};
        this.ChoiceList.Add(newItem);

        return newItem.val1;
    }
}