koobas.hobune.stream
Cannot cast from source type to destination type in C#
Hi, I'm trying to use the code this.
public static string CreateOptionTooltip(Item item)
{
if(item.getEquipable())
{
string tooltip = "";
EquipmentItem equipItem = (EquipmentItem)item;
for(int i = 0; i < equipItem.optionList.Count; i++)
{
ItemOption option = equipItem.optionList[i];
tooltip += option.itemOption.ToString();
if(option.optionAmount >= 0)
tooltip += "\t+" + option.optionAmount.ToString();
else
tooltip += "\t-" + option.optionAmount.ToString();
tooltip += "\n";
}
return tooltip;
}
else
{
return "";
}
}
The EquipmentItem Class is child class of Item like this.
public class EquipmentItem : Item
{
public EquipmentItem()
{
//Here is initializing code.
}
}
But when I call the method CreateOptionTooltip(Item item) in gaming there is error InvalidCast: Cannot cast from source type to destination type in line EquipmentItem equipItem = (EquipmentItem)item;
You can only cast up the inheritance tree, you can't cast down the inheritance tree because not everything is guaranteed to exist.
Say you have a class:
class Foo {
int index;
float position;
}
And another class:
class Bar : Foo {
void Do() {
}
}
Now lets say you are given a variable of type Bar. This you can easily cast to a Foo because Bar inherits from Foo, ie. you're casting up the tree. Once you have a Foo from your Bar, everything from Foo still exists in your variable, ie. you have access to index and position.
But what if you have a variable of type Foo. This you cannot cast to a Bar, simply because a Bar can Do(), while a Foo cannot. Therefore your Foo cannot be a Bar, but your Bar can be a Foo.
The same goes for the Finger-Thumb analogy. Think of Foo as a Finger and Bar as a Thumb. Your Thumb is a finger, ie. you can cast your Thumb to a Finger. But not all Fingers are Thumbs, you therefore cannot cast any Finger to a Thumb.
You therefore cannot cast an Item to a EquipmentItem, since not all items are equipment items, but you can cast any EquipmentItem to an Item, since all EquipmentItems are Items.
Okay, I understood clearly. But I have one more question. Then how about using List<>? When I learned Java or other program$$anonymous$$g languages, there is List<> Class for the Array of ClassType. With your example, if I made a List<>Foo(Sorry to write the Foo outside of brackeys if I write inside of it, it is blowed up.) arrayFoo, then we can put the Bar Class into arrayFoo. You know, when we get the thing which was Bar Class from the arrayFoo, the type of the thing is Foo because we use List<>Foo. But if I need the specific variable of Bar then we need to cast thing Foo to Bar so that we can use the specific variable of Bar. In this case, I am making inventory system with List<>Item. And EquipmentItem has specific variables like durability or Equip Job Limit. Then how can I access this kinds of variables when I get the Item from List<>Item?
http://msdn.microsoft.com/en-us/library/cc488006.aspx
You have to make sure you have the right type before casting, ie. using the is operator.
Thank you for your help! I really appreciate about your answer for me.
