Checks if an object is compatible with a given type, i.e. if an object is an instance of the BaseInterface type, or a type that derives from BaseInterface:

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

If the intent of the cast is to use the object, it is best practice to use the [as](<http://stackoverflow.com/documentation/c%23/26/keywords/138/as>) keyword’

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
}

But, from C# 7 [pattern matching](<https://stackoverflow.com/documentation/c%23/1936/c-sharp-7-0-features/13323/pattern-matching#t=201707130940065371224>) feature extends the is operator to check for a type and declare a new variable at the same time. Same code part with C# 7 :

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