CA2021: Don't call Enumerable.Cast<T> or Enumerable.OfType<T> with incompatible types

Property	Value
Rule ID	CA2021
Title	Don't call Enumerable.Cast<T> or Enumerable.OfType<T> with incompatible types
Category	Reliability
Fix is breaking or non-breaking	Breaking
Enabled by default in .NET 9	As warning

Cause

A call to Enumerable.Cast<TResult>(IEnumerable) or Enumerable.OfType<TResult>(IEnumerable) specifies a type parameter that's incompatible with the type of the input collection.

Rule description

Enumerable.Cast<TResult>(IEnumerable) and Enumerable.OfType<TResult>(IEnumerable) require compatible types to produce the expected result:

• The generic cast used by the sequence returned by Cast<TResult>(IEnumerable) throws an InvalidCastException at run time on elements of incompatible types.
• The generic type check used by OfType<TResult>(IEnumerable) won't succeed with elements of incompatible types, resulting in an empty sequence.

Widening and user-defined conversions aren't supported with generic types.

How to fix violations

Use a compatible type for the type parameter of Cast<TResult>(IEnumerable) and OfType<TResult>(IEnumerable).

Example

The following code snippet shows violations:

var foods = new List<Food>();
// Violation - Food is incompatible with Beverages.
var drinks = Enumerable.Cast<Beverages>(foods);
// Violation - Food is incompatible with Beverages.
var drinks2 = Enumerable.OfType<Beverages>(foods);

class Food { }
class Bread : Food { }
class Beverages { }

' Violation - Integer is incompatible with String.
Dim a1 = (Array.Empty(Of Integer)()).Cast(Of String)
' Violation - Integer is incompatible with String.
Dim a1 = (Array.Empty(Of Integer)()).OfType(Of String)

The following code snippet shows the fixes:

var foods = new List<Food>();
// Bread is compatible with Food.
var breads = Enumerable.Cast<Bread>(foods);
// Bread is compatible with Food.
var breads2 = Enumerable.OfType<Bread>(foods);

class Food { }
class Bread : Food { }
class Beverages { }

' Integer is compatible with Object.
Dim a1 = (Array.Empty(Of Integer)()).Cast(Of Object)
' Integer is compatible with Object.
Dim a1 = (Array.Empty(Of Integer)()).OfType(Of Object)

When to suppress warnings

You shouldn't suppress warnings from this rule, as you'll either encounter run-time exceptions or unexpected behavior (empty sequences).