CODEWITHSUNDEEP
c#list
Rafael Pietsch
Rafael Pietsch

Reputation: 63

Why is the .Clear() function clearing the wrong list?

public void ConvertMoves()
{
    for (int i = 0; i < maxDirections; i++)
    {
        Debug.Log("gimme tsMoves "+tSpossibleMoves[i].Count +  " from " + this);
        possibleAttacks[i] = tSpossibleAttacks[i];
        possibleAttacksInactive[i] = tSpossibleAttacksInactive[i];
        possibleAttackIndicators[i] = tSpossibleAttackIndicators[i];
        possibleMoves[i] = tSpossibleMoves[i];
        Debug.Log("Gimme moves(1) " + possibleMoves[i].Count + " from " + this);
    }
    for (int i = 0; i < maxDirections; i++)
    {
        tSpossibleAttacks[i].Clear();
        tSpossibleAttacksInactive[i].Clear();
        tSpossibleAttackIndicators[i].Clear();
        tSpossibleMoves[i].Clear();
        Debug.Log("Gimme moves(2) " + possibleMoves[i].Count + " from " + this);
    }
}

so the Debug Log reports the following:

gimme tsMoves 2 from JeanArc(Clone) (JeanArc)
Gimme moves(1) 2 from JeanArc(Clone) (JeanArc)

sofar everything is doing fine but then...

Gimme moves(2) 0 from JeanArc(Clone) (JeanArc)

why does it clear the moves of whole different List variable ?

Upvotes: 3

Views: 114

Answers (2)

tmaj
tmaj

Reputation: 34967

What @John said. You need to copy the lists. 

for (int i = 0; i < maxDirections; i++)
{
    Debug.Log("gimme tsMoves "+tSpossibleMoves[i].Count +  " from " + this);
    possibleAttacks[i] = tSpossibleAttacks[i];
    tSpossibleAttacks[i] = new List<T>;

    possibleAttacksInactive[i] = tSpossibleAttacksInactive[i];
    tSpossibleAttacksInactive[i] = new List<U>();

    possibleAttackIndicators[i] = tSpossibleAttackIndicators[i];
    tSpossibleAttackIndicators[i] = new List<V>();

    possibleMoves[i] = tSpossibleMoves[i];
    tSpossibleMoves[i] = new List<Z>();

    Debug.Log($"Gimme moves(1), i={i}: {possibleMoves[i].Count} from {this}");
    Debug.Log($"Gimme moves(2)  i={i}: {tpossibleMoves[i].Count} from {this}");
}

Example: 

var l1 = new List<string>();
List<string> l2;

l1.Add("One");
l1.Add("Two");

l2 = l1;
l1 = new List<string>();
l1.Add("Three");

Console.WriteLine("L1:");
foreach (var elem in l1)
{
    Console.WriteLine(elem);
}
Console.WriteLine("L2:");
foreach (var elem in l2)
{
    Console.WriteLine(elem);
}

This prints:

L1:
Three
L2:
One
Two

Upvotes: 0

ProgrammingLlama
ProgrammingLlama

Reputation: 38737

This doesn't create a copy of the list item:

possibleAttacks[i] = tSpossibleAttacks[i]

It simply copies the reference to the same object into a second variable, so possibleAttacks[i] and tSpossibleAttacks[i] now both point to the same item in memory. Think of it like having two credit cards to access one bank account.

You can read more about reference types here in Microsoft's docs.

As Heinzi pointed out in the comment below, you can copy your item (as it's a list) by calling:

possibleAttacks[i] = tSpossibleAttacks[i].ToList();

By the way, if you just want to assign tSpossibleAttacks[i] and then reset it, you could also just do this:

possibleAttacks[i] = tSpossibleAttacks[i];
tSpossibleAttacks[i] = new List<your_type_name_here>(); // this will overwrite the reference held by `tSpossibleAttacks[i]`.

Note that if your list contains reference types, you have a similar problem within the list, for example:

public class Test
{
    public string Name { get; set; }
}

List<Test> list1 = new List<Test>();
list1.Add(new Test() { Name = "John" });
List<Test> list2 = list1.ToList();
Console.WriteLine(list1[0].Name); // John
Console.WriteLine(list2[0].Name); // John
list2[0].Name = "Fred";
Console.WriteLine(list1[0].Name); // Fred
Console.WriteLine(list2[0].Name); // Fred

So I'd recommend reading up on value types vs reference types and how references work in C#.

Upvotes: 6

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