Why is this an issue?

In the interests of readability, code that can be simplified should be simplified. To that end, there are several ways IEnumerable language integrated queries (LINQ) can be simplified. This not only improves readabilty but can also lead to improved performance.

How to fix it

Simplify the LINQ expressions:

• Use OfType instead of Select with the as operator to type cast elements and then null-checking in a query expression to choose elements based on type.
• Use OfType instead of using Where and the is operator, followed by a cast in a Select
• Use an expression in Any instead of Where(element ⇒ [expression]).Any().
• Use the Count or Length properties instead of the Count() method when it’s available (unless you use the predicate parameter of the method for filtering).
• Don’t call ToArray() or ToList() in the middle of a query chain.

Using Entity Framework may require enforcing client evaluations. Such queries should use AsEnumerable() instead of ToArray() or ToList() in the middle of a query chain.

Code examples

Noncompliant code example

public void Foo(IEnumerable<Vehicle> seq, List<int> list)
{
    var result1 = seq.Select(x => x as Car).Any(x => x != null);               // Noncompliant; use OfType
    var result2 = seq.Select(x => x as Car).Any(x => x != null && x.HasOwner); // Noncompliant; use OfType before calling Any
    var result3 = seq.Where(x => x is Car).Select(x => x as Car);              // Noncompliant; use OfType
    var result4 = seq.Where(x => x is Car).Select(x => (Car)x);                // Noncompliant; use OfType
    var result5 = seq.Where(x => x.HasOwner).Any();                            // Noncompliant; use Any([predicate])

    var num = list.Count();                                                    // Noncompliant; use the Count property
    var arr = seq.ToList().ToArray();                                          // Noncompliant; ToList is not needed
    var count = seq.ToList().Count(x => x.HasOwner);                           // Noncompliant; ToList is not needed
}

Compliant solution

public void Foo(IEnumerable<Vehicle> seq, List<int> list)
{
    var result1 = seq.OfType<Car>().Any();
    var result2 = seq.OfType<Car>().Any(x => x.HasOwner);
    var result3 = seq.OfType<Car>();
    var result4 = seq.OfType<Car>();
    var result5 = seq.Any(x => x.HasOwner);

    var num = list.Count;
    var arr = seq.ToArray();
    var count = seq.Count(x => x.HasOwner);
}

Resources

Documentation

• Language Integrated Queries in C#