Is it bad form to return Arrays in C#? Should I return List?

Question

I have a function which returns a variable number of elements, should I return an array or a List? The \"collection\'s\" size does not change once returned, ie for all purpo

Answer 1

Return IEnumerable unless you need:

1. access Count - then return IReadOnlyCollection.
2. modify elements and Count is less the ArrayList.Capacity - then return array.
3. add new elements - then return list.
4. some microoptimizations - do benchmark. E.g. foreach over array is quicker then over the list

BenchmarkDotNet=v0.10.8, OS=Windows 10 Redstone 1 (10.0.14393) Processor=Intel Core i5-2500K CPU 3.30GHz (Sandy Bridge), ProcessorCount=4 Frequency=3233538 Hz, Resolution=309.2588 ns, Timer=TSC [Host] : Clr 4.0.30319.42000, 64bit RyuJIT-v4.6.1648.0 Clr : Clr 4.0.30319.42000, 64bit RyuJIT-v4.6.1648.0 Core : .NET Core 4.6.25211.01, 64bit RyuJIT

    Method |  Job | Runtime |       Mean |      Error |    StdDev |        Min |        Max |     Median | Rank | Allocated |
---------- |----- |-------- |-----------:|-----------:|----------:|-----------:|-----------:|-----------:|-----:|----------:|
  TestList |  Clr |     Clr | 5,153.3 ns |  34.002 ns | 31.806 ns | 5,119.2 ns | 5,213.4 ns | 5,135.9 ns |    3 |       0 B |
 TestArray |  Clr |     Clr |   730.1 ns |   6.962 ns |  6.512 ns |   722.4 ns |   743.9 ns |   729.5 ns |    2 |       0 B |
  TestList | Core |    Core | 5,188.4 ns | 102.816 ns | 96.174 ns | 5,070.3 ns | 5,342.6 ns | 5,185.6 ns |    3 |       0 B |
 TestArray | Core |    Core |   709.0 ns |   6.126 ns |  5.730 ns |   700.8 ns |   718.6 ns |   708.1 ns |    1 |       0 B |

Code:

[RankColumn, MinColumn, MaxColumn, StdDevColumn, MedianColumn]
[ClrJob, CoreJob]
[HtmlExporter, MarkdownExporter]
[MemoryDiagnoser]
public class BenchmarkForEach
{
    List testData = new List();
    string[] testArray;
    public BenchmarkForEach()
    {
        for(int i=0;i<1000;i++)
        {
            testData.Add(i.ToString());
        }
        testArray = testData.ToArray();
    }
    [Benchmark]
    public int TestList()
    {
        var x = 0;
        foreach(var i in testData)
        {
            x += i.Length;
        }
        return x;
    }
    [Benchmark]
    public int TestArray()
    {
        var x = 0;
        foreach (var i in testArray)
        {
            x += i.Length;
        }
        return x;
    }

}