何时使用装箱和拆箱

本文关键字：何时使 | 更新日期: 2023-09-27 18:34:09

我知道装箱将值类型转换为对象和取消装箱将对象转换为值类型。例：

int num = 1;
Object obj = num;      //boxing
int num2 = (int)obj    //unboxing

但是当我在现实生活中使用它时（当我编写一些代码时）？我从不将 Int 分配给对象（或者这就是我认为的，我没有明确编写它）

何时使用装箱和拆箱

装箱/拆箱对于您可能拥有不同事物的集合的情况很有用，例如，其中一些是基元，而另一些是对象类型，例如List<object>。事实上，这就是他们在装箱和拆箱文档页面中给出的示例，如果您搜索"msdn boxing："

// List example. 
// Create a list of objects to hold a heterogeneous collection  
// of elements.
List<object> mixedList = new List<object>();
// Add a string element to the list. 
mixedList.Add("First Group:");
// Add some integers to the list.  
for (int j = 1; j < 5; j++)
{
    // Rest the mouse pointer over j to verify that you are adding 
    // an int to a list of objects. Each element j is boxed when  
    // you add j to mixedList.
    mixedList.Add(j);
}
// Add another string and more integers.
mixedList.Add("Second Group:");
for (int j = 5; j < 10; j++)
{
    mixedList.Add(j);
}
// Display the elements in the list. Declare the loop variable by  
// using var, so that the compiler assigns its type. 
foreach (var item in mixedList)
{
    // Rest the mouse pointer over item to verify that the elements 
    // of mixedList are objects.
    Console.WriteLine(item);
}
// The following loop sums the squares of the first group of boxed 
// integers in mixedList. The list elements are objects, and cannot 
// be multiplied or added to the sum until they are unboxed. The 
// unboxing must be done explicitly. 
var sum = 0;
for (var j = 1; j < 5; j++)
{
    // The following statement causes a compiler error: Operator  
    // '*' cannot be applied to operands of type 'object' and 
    // 'object'.  
    //sum += mixedList[j] * mixedList[j]); 
    // After the list elements are unboxed, the computation does  
    // not cause a compiler error.
    sum += (int)mixedList[j] * (int)mixedList[j];
}
// The sum displayed is 30, the sum of 1 + 4 + 9 + 16.
Console.WriteLine("Sum: " + sum);
// Output: 
// Answer42True 
// First Group: 
// 1 
// 2 
// 3 
// 4 
// Second Group: 
// 5 
// 6 
// 7 
// 8 
// 9 
// Sum: 30
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