rspec/rules/S2971/csharp/rule.adoc

== 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 https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1[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 https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.oftype[OfType] instead of https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.select[Select] with the https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#as-operator[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 https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.where[Where] and the https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#is-operator[is operator], followed by a cast in a `Select`
* Use an expression in `Any` instead of `Where(element => [expression]).Any()`.
* Use the https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.count[Count] or https://learn.microsoft.com/en-us/dotnet/api/system.array.length[Length] properties instead of the https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.count[Count() method] when it's available (unless you use the predicate parameter of the method for filtering).
* Don't call https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.toarray[ToArray()] or https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tolist[ToList()] in the middle of a query chain.

Using https://learn.microsoft.com/en-us/ef/[Entity Framework] may require enforcing client evaluations. Such queries should use https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.asenumerable[AsEnumerable()] instead of `ToArray()` or `ToList()` in the middle of a query chain.

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
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

[source,csharp,diff-id=1,diff-type=compliant]
----
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

* https://learn.microsoft.com/en-us/dotnet/csharp/linq[Language Integrated Queries in C#]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* Use "OfType<T>()" here instead.
* Drop "Where" and move the condition into the "Any".
* Drop "ToArray" from the middle of the call chain.
* Replace "ToArray" with "AsEnumerable" in the middle of the call chain.

'''
== Comments And Links
(visible only on this page)

endif::env-github,rspecator-view[]