是

檢查物件是否與給定型別相容，即物件是 BaseInterface 型別的例項，還是從 BaseInterface 派生的型別：

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

如果演員的意圖是使用該物件，最好使用 as 關鍵字'

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，從 C#7 開始， pattern matching 功能擴充套件了 is 運算子以檢查型別並同時宣告一個新變數。與 C#7 相同的程式碼部分：

Version >= 7

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