How do I swap 2 elements in a list [duplicate]

Possible Duplicate:
Swap two items in List<T>

Edit: Maybe this will work for getting the 'b' value?

for (int i = 0; i < inventory.Count; i++)
{
    if (inventory[a].ItemRectangle.Intersects(inventory[i].ItemRectangle))
    {
        itemB = inventory[i];
    }
}

Edit: Here's my progress.

Item itemA;
Item itemB;

int a = -1;
int b = -1;

if (a != -1 && b != -1)
{
    itemA = inventory[a];
    itemB = inventory[b];

    Swap(ref itemA, ref itemB);

    inventory[a] = itemA;
    inventory[b] = itemB;
}

And here's is where I'm getting the 'a' value.

if (item.ItemSelected == true)
{
    a = item.ItemIndex;
}
else
    a = -1;

I haven't figured out how to get the 'b' value because I would have to check for an item colliding with another item that are both in the same list. If anybody know how I can do this, please tell me. It would look something like this I guess:

if (item.ItemRectangle.Intersects(//the other item.ItemRectangle)
{
    b = item.ItemIndex;
}
else
    b = -1;

I've made a List < Item > called inventory. So now I want to implement a swap function, like this:

foreach (Item item in inventory)
{
    if (mouseRectangle.Intersects(item.ItemRectangle))
    {
        if (Input.EdgeDetectLeftMouseDown())
        {
            switch (item.ItemSelected)
            {
                case false:
                    item.ItemSelected = true;
                    break;
                case true:
                    item.ItemSelected = false;
                    break;
            }
        }  
    }
    else if (Input.EdgeDetectLeftMouseDown())
    {
        switch (item.ItemSelected)
        {
            case true:
                item.ItemSelected = false;
                break;
        }
    }
    else if (item.ItemSelected == true)
    {
        item.ItemPosition = new Vector2(mouseRectangle.X, mouseRectangle.Y);
        item.ItemRectangle = new Rectangle(mouseRectangle.X, mouseRectangle.Y, 32, 32);
    }
    else if (item.ItemSelected == false && //a lot of checks to determine it is not intersecting with an equip slot
    {
        item.ItemPosition = item.OriginItemPosition;
        item.ItemRectangle = item.OriginItemRectangle;
    }
    else if (item.ItemRectangle.Intersects(item.ItemRectangle))
    {
        //SwapItem(inventory, item, item);
    }

So that's the part of the code I need help with. I want any item in the list to be able to swap with any other item in the list. My SwapItem method is just a placeholder, I dont actually have a SwapItem method yet.

I want the arguments that you pass in to the method to be related to the items I want to swap. So the first item would be the item that I have selected with my mouse, and the other item should be the item that the first item is intersecting with.

To swap the values of two variables, the easiest method is using references. This is a classic pointer exercise in c++, but it can apply to C# as well.

// Replace int with any data type / class you need
void Swap (ref int a, ref int b)
{
   int c = a; 
   a = b; 
   b = c;
}

The algorithm used is very simple, and the explanation is usually done like this: you have two glasses, one with water, and one with oil. To put the oil in the first glass, you will need to use a third glass, put the water inside, then put the oil in the first glass, and the water in the second one.

Here is what I had in mind. Look for the comments, so you can understand what's going on.:

// Unlike foreach, with for I can change the values in the list
for (int i = 0; i < inventory.Count; i++)
{
    if (mouseRectangle.Intersects(inventory[i].ItemRectangle))
    {
        if (Input.EdgeDetectLeftMouseDown())
        {
            // You can replace the switch with this shorter structure
            // if A is a bool value, !A will have the opposite value
            inventory[i].ItemSelected = !inventory[i].ItemSelected;
        }  
    }
    else if (Input.EdgeDetectLeftMouseDown())
    {
        // You don't need a case-switch for a single condition. An if should suffice
        if (inventory[i].ItemSelected) 
            inventory[i].ItemSelected = false;
    }
    else if (inventory[i].ItemSelected == true)
    {
        inventory[i].ItemPosition = new Vector2(mouseRectangle.X, mouseRectangle.Y);
        inventory[i].ItemRectangle = new Rectangle(mouseRectangle.X, mouseRectangle.Y, 32, 32);
    }
    else if (inventory[i].ItemSelected == false && //a lot of checks to determine it is not intersecting with an equip slot
    {
        inventory[i].ItemPosition = inventory[i].OriginItemPosition;
        inventory[i].ItemRectangle = inventory[i].OriginItemRectangle;
    }

    // Something definitely wrong with this line, a rectangle to instersect with itself??
    else if (inventory[i].ItemRectangle.Intersects(inventory[PROBABLY_SOMETHING_ELSE].ItemRectangle))
    {
        Swap (ref inventory[i], ref inventory[PROBABLY_SOMETHING_ELSE])
    }
}

To swap an element of the list you can write an extension method as.

public static class ExtensionMethods
{
    public static void Swap<T>(this List<T> list, int index1, int index2)
    {
         T temp = list[index1];
         list[index1] = list[index2];
         list[index2] = temp;
    }
}

Remember to put the extension method inside a static class.

then you can do:

yourList.Swap(0,1); // swap element at index 0 with element at index 1

来源：https://stackoverflow.com/questions/11283028/how-do-i-swap-2-elements-in-a-list

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