是

检查对象是否与给定类型兼容，即对象是 BaseInterface 类型的实例，还是从 BaseInterface 派生的类型：

interface BaseInterface {}
class BaseClass : BaseInterface {}
class DerivedClass : BaseClass {}

var d = new DerivedClass();
Console.WriteLine(d is DerivedClass);  // True
Console.WriteLine(d is BaseClass);     // True
Console.WriteLine(d is BaseInterface); // True
Console.WriteLine(d is object);        // True
Console.WriteLine(d is string);        // False

var b = new BaseClass();
Console.WriteLine(b is DerivedClass);  // False
Console.WriteLine(b is BaseClass);     // True
Console.WriteLine(b is BaseInterface); // True
Console.WriteLine(b is object);        // True
Console.WriteLine(b is string);        // False

如果演员的意图是使用该对象，最好使用 as 关键字'

interface BaseInterface {}
class BaseClass : BaseInterface {}
class DerivedClass : BaseClass {}

var d = new DerivedClass();
Console.WriteLine(d is DerivedClass);  // True - valid use of 'is'
Console.WriteLine(d is BaseClass);     // True - valid use of 'is'

if(d is BaseClass){
    var castedD = (BaseClass)d;
    castedD.Method(); // valid, but not best practice
}

var asD = d as BaseClass;

if(asD!=null){
    asD.Method(); //prefered method since you incur only one unboxing penalty
}

但是，从 C#7 开始， pattern matching 功能扩展了 is 运算符以检查类型并同时声明一个新变量。与 C#7 相同的代码部分：

Version >= 7

if(d is BaseClass asD ){
    asD.Method();
}