To check the type of an object there are several options:
-
expr is SomeType or expr.GetType() == typeof(SomeType) if the type is known at compile time,
-
typeInstance.IsInstanceOfType(expr) if the type is calculated during runtime.
If runtime calculated Types need to be compared:
-
typeInstance1.IsAssignableFrom(typeInstance2).
Depending on whether the type is returned by a GetType() or typeof() call, the IsAssignableFrom() and
IsInstanceOfType() might be simplified. Similarly, if the type is sealed, the type comparison with == can be
converted to an is call. Simplifying the calls also make null checking unnecessary because both is and
IsInstanceOfType performs it already.
Finally, utilizing the most concise language constructs for type checking makes the code more readable, so
-
expr as T != null checks should be simplified to expr is T, and
-
expr is T should be converted to expr != null, when expr is of type T.
Noncompliant code example
class Fruit { }
sealed class Apple : Fruit { }
class Program
{
static void Main()
{
var apple = new Apple();
var b = apple != null && apple.GetType() == typeof (Apple); // Noncompliant
b = typeof(Apple).IsInstanceOfType(apple); // Noncompliant
if (apple != null)
{
b = typeof(Apple).IsAssignableFrom(apple.GetType()); // Noncompliant
}
var appleType = typeof (Apple);
if (apple != null)
{
b = appleType.IsAssignableFrom(apple.GetType()); // Noncompliant
}
Fruit f = apple;
if (f as Apple != null) // Noncompliant
{
}
if (apple is Apple) // Noncompliant
{
}
}
}
Compliant solution
class Fruit { }
sealed class Apple : Fruit { }
class Program
{
static void Main()
{
var apple = new Apple();
var b = apple is Apple;
b = apple is Apple;
b = apple is Apple;
var appleType = typeof(Apple);
b = appleType.IsInstanceOfType(apple);
Fruit f = apple;
if (f is Apple)
{
}
if (apple != null)
{
}
}
}
Exceptions
Calling GetType on an object of Nullable<T> type returns the underlying generic type parameter T, thus
a comparison with typeof(Nullable<T>) can’t be simplified to use the is operator, which doesn’t make difference
between T and T?.
int? i = 42;
bool condition = i.GetType() == typeof(int?); // false;
condition = i is int?; // true
No issue is reported on the following expressions:
-
expr is T when either operand of the is operator is a value type. In that case CS0183 or CS0184 reports
-
expr is object, as this is a common and efficient pattern to do null checks
