c#关键字is和as

原创已于 2022-11-03 00:21:43 修改 · 1.7k 阅读

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于 2022-11-02 09:38:38 首次发布

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本文详细介绍了C#中is和as运算符的功能及用法。is用于检查对象是否为指定类型，as则尝试将对象转换为指定类型。文章通过实例展示了这两种运算符的具体应用。

关键点

is 运算符用于检查对象的运行时类型是否与给定类型兼容，而 as 运算符用于在兼容的引用类型或 Nullable 类型之间执行转换.
如果给定对象的类型相同，is 运算符返回 true，而 as 运算符在与给定类型兼容时返回对象.
如果给定对象不是同一类型，is 运算符返回 false，而 as 运算符在无法转换时返回 null.
is 运算符仅用于引用、装箱和拆箱转换，而 as 运算符仅用于可空、引用和装箱转换.
is 运算符关键字仅考虑引用、装箱和拆箱转换
is 运算符不考虑用户定义的转换或使用隐式和显式定义的转换。对于在编译时已知或由隐式运算符处理的转换，is 运算符将为此发出警告
as 运算符关键字仅用于可空、引用和装箱转换。它不能执行只能通过使用强制转换表达式执行的用户定义的转换。

样例

is operator:

// C# program to illustrate the
// use of is operator keyword
using System;
  
// taking a class
public class P { }
  
// taking a class
// derived from P
public class P1 : P { }
  
// taking a class
public class P2 { }
  
// Driver Class
public class GFG {
  
    // Main Method
    public static void Main()
    {
  
        // creating an instance
        // of class P
        P o1 = new P();
  
        // creating an instance
        // of class P1
        P1 o2 = new P1();
  
        // checking whether 'o1'
        // is of type 'P'
        Console.WriteLine(o1 is P);
  
        // checking whether 'o1' is
        // of type Object class
        // (Base class for all classes)
        Console.WriteLine(o1 is Object);
  
        // checking whether 'o2'
        // is of type 'P1'
        Console.WriteLine(o2 is P1);
  
        // checking whether 'o2' is
        // of type Object class
        // (Base class for all classes)
        Console.WriteLine(o2 is Object);
  
        // checking whether 'o2'
        // is of type 'P'
        // it will return true as P1
        // is derived from P
        Console.WriteLine(o2 is P1);
  
        // checking whether o1
        // is of type P2
        // it will return false
        Console.WriteLine(o1 is P2);
  
        // checking whether o2
        // is of type P2
        // it will return false
        Console.WriteLine(o2 is P2);
    }
}
Output:

True
True
True
True
True
False
False

as operator:

// C# program to illustrate the
// concept of 'as' operator
using System;
  
// Classes
class Y { }
class Z { }
  
class GFG {
  
    // Main method
    static void Main()
    {
  
        // creating and initializing object array
        object[] o = new object[5];
        o[0] = new Y();
        o[1] = new Z();
        o[2] = "Hello";
        o[3] = 4759.0;
        o[4] = null;
  
        for (int q = 0; q < o.Length; ++q) {
  
            // using as operator
            string str1 = o[q] as string;
  
            Console.Write("{0}:", q);
  
            // checking for the result
            if (str1 != null) {
                Console.WriteLine("'" + str1 + "'");
            }
            else {
                Console.WriteLine("Is is not a string");
            }
        }
    }
}
Output:

0:Is is not a string
1:Is is not a string
2:'Hello'
3:Is is not a string
4:Is is not a string