Fix S3878 FP: When input array is collection expression with spread operator

[mary-georgiou-sonarsource]

Description

S3878 should not raise an issue for collection expression with the spread operator. The [a, .. array] expression below is equivalent to array.Prepend(a).ToArray() and cannot be replaced with a,...,array as it's a compile error.

Repro steps

const int a = 1;
int[] array = [2, 3];
Method([a, .. array]);  // Noncompliant FP

void  Method(params int[] args) { } 

[Guiorgy]

S3878 is also reported when using a different collection type on the caller side with params parameter:

public void MethodWithParams(params string[] args)
{
    // do something
}

public void MethodPassingList()
{
    var list = new List<string>();
    MethodWithParams([.. list]); // Warning	S3878	Remove this array creation and simply pass the elements.
}

Passing list.ToArray() makes Visual Studio advise the use of the collection expression (Use collection expression for fluent (IDE0305)), which refactors to the above.

Edit: Surprisingly, however, the warning is not raised when invoking another constructor:

class MyClass {
    static List<Pattern> _args = new List<Pattern>();

    MyClass(params string[] args) {}

    MyClass() : this([.._args]) {} // No warning raised
}

class MyClass(params string[] args) {
    static List<Pattern> _args = new List<Pattern>();

    MyClass() : this([.._args]) {} // No warning raised
}

Don't know why the inconsistency ¯\_(ツ)_/¯

[mary-georgiou-sonarsource]

Another report for this issue here .

[Guiorgy]

@mary-georgiou-sonarsource BTW, should my case (comment above) be a separate issue?

[mary-georgiou-sonarsource]

@Guiorgy In my opinion no, because the main idea is that we don't raise when the spread operator is used (like we do for ToArray()).