使用对象调用 foreach 中的方法时出错

发布于 2024-10-18 06:42:22 字数 829 浏览 2 评论 0原文

我有包含字符串和双精度对象的列表，我尝试根据项目类型及其值调用不同的方法。在调试器中，我可以看到第一次迭代工作正常，但在调用方法后第二次输入时显示错误。
如果我注释掉这些方法并放入简单的方法，它就会起作用，所以我知道这与我调用这些方法的方式有关。

我做错了什么，我该怎么做才能让它发挥作用？
如果有更简单的方法来完成我正在尝试的事情，请告诉我。

public double evaluateExpressionUsingVariableValues(List<Object> anExpression, Dictionary<String, double> variables)  
{  
    foreach (object element in anExpression)  
    {   
        if(element.GetType()!=typeof(string))  
        {  
            setOperand((double)element);  
        }  
        else if (element.GetType() == typeof(string))  
        {  
            if (!element.ToString().StartsWith("%"))  
            performOperation((string)element);  
        else  
            setOperand(variables[element.ToString()]);  
         }  
     }  

        return this.operand;  
}

原文

I have List with objects of strings and doubles, and I try to call different methods based on the itemtype and their value. In the debugger I can see that the first iteration works fine, but an error shows when entering the second time after a method is called.
If i comment out the methods and put in simple methods it works, so I understand that it something with how I call the methods.

What do I do wrong, and what can I do to make it work?
If there is easier ways to do what I'm trying, please let me know.

public double evaluateExpressionUsingVariableValues(List<Object> anExpression, Dictionary<String, double> variables)  
{  
    foreach (object element in anExpression)  
    {   
        if(element.GetType()!=typeof(string))  
        {  
            setOperand((double)element);  
        }  
        else if (element.GetType() == typeof(string))  
        {  
            if (!element.ToString().StartsWith("%"))  
            performOperation((string)element);  
        else  
            setOperand(variables[element.ToString()]);  
         }  
     }  

        return this.operand;  
}

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皓月长歌 2024-10-25 06:42:23

如果您的方法（setOperand、performOperation）完全修改了集合，您将收到异常。在迭代集合时无法修改集合。一种方法是创建结果集合并在更改项目时向其中添加项目，而不是尝试就地修改集合。

private void Foo() {
  foreach(var item in items) {
    if (item.IsBad) {
      DeleteItem(item); // will throw an exception as it tries to modify items
    }
  }
}

private void DeleteItem(Item item) {
  items.Remove(item);
}

相反，请尝试：

private void Foo() {
  List<Item> result = new List<Item>();
  foreach(var item in items) {
    if (!item.IsBad) {
      result.Add(item); // we are adding to a collection other
                        // than the one we are iterating through
    }
  }
  items = result; // we are no longer iterating, so we can modify
                  // this collection
}

If your methods (setOperand, performOperation) modify the collection at all, you will get an exception. You can't modify the collection while you are iterating over it. One method is to create a result collection and add items to it as you change them, rather than trying to modify the collection in-place.

private void Foo() {
  foreach(var item in items) {
    if (item.IsBad) {
      DeleteItem(item); // will throw an exception as it tries to modify items
    }
  }
}

private void DeleteItem(Item item) {
  items.Remove(item);
}

Instead, try:

private void Foo() {
  List<Item> result = new List<Item>();
  foreach(var item in items) {
    if (!item.IsBad) {
      result.Add(item); // we are adding to a collection other
                        // than the one we are iterating through
    }
  }
  items = result; // we are no longer iterating, so we can modify
                  // this collection
}

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