Array will allocate less and little faster when creating RequestDelegateFactoryOptions in CreateRDFOptions

It uses List<string>, which gets assigned to RouteParameterNames as IEnumerable. https://github.com/dotnet/aspnetcore/blob/432e6a061f28dda696ba16bc5442328d23b25d93/src/Http/Routing/src/RouteEndpointDataSource.cs#L260

Ran a benchmark with 10 strings of length 6. The results are below.

Another run:

The benchmark code used:

public class Benchmarks
    {
        private const int Length = 10;
        private readonly IReadOnlyCollection<string> _readOnlyCollection = new List<string>() { "param1","param2", "param1","param2", "param1","param2", "param1","param2", "param1","param2"}.AsReadOnly();
        [Benchmark]
        public void Add10StringsToList()
        {
            List<string> parameters = new(Length);
            foreach (var s in _readOnlyCollection)
            {
                parameters.Add(s);
            }

            
        }

        [Benchmark]
        public void Add10StringsToArray()
        {
            var parameters = new string[Length];
            var i = 0;
            foreach (var s in _readOnlyCollection)
            {
                parameters[i] = s;
                i++;
            }
            
        }
    }

[marafiq]

After spending a little bit more time with it, it seems like even array allocation is not required since parameter names are sourced from IReadOnlyCollection, and RequestDelegateFactoryOptions only requires IEnumerable<string> so all the allocation can go away. Either using the Linq Select or with the local function that yields.

Benchmark results

Benchmark code

[MemoryDiagnoser]
    public class Benchmarks
    {
        class RoutePatternPart
        {
            public RoutePatternPart(bool isOptional, string name)
            {
                IsOptional = isOptional;
                Name = name;
            }

            public bool IsOptional { get; }
            public string Name { get; }
        }

        private const int Length = 10;

        private readonly IReadOnlyCollection<RoutePatternPart> _readOnlyCollection = new List<RoutePatternPart>()
        {
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1"),
            new(true, "param1")
        };
        private readonly Consumer _consumer = new Consumer();
        [Benchmark]
        public void Add10StringsToList()
        {
            List<string> parameters = new(Length);
            foreach (var s in _readOnlyCollection)
            {
                parameters.Add(s.Name);
            }
            _consumer.Consume(parameters);
        }

        [Benchmark]
        public void Add10StringsToArray()
        {
            var parameters = new string[Length];
            var i = 0;
            foreach (var s in _readOnlyCollection)
            {
                parameters[i] = s.Name;
                i++;
            }
            _consumer.Consume(parameters);
        }
        
        [Benchmark]
        public void UseEnumerableWithLinqSelect()
        {
            var parameters = _readOnlyCollection.Select(x => x.Name);
            _consumer.Consume(parameters);
        }
        [Benchmark]
        public void UseEnumerableWithLocalFuncYield()
        {
            var parameters = LocalFuncEnumerable();

            _consumer.Consume(parameters);

            IEnumerable<string> LocalFuncEnumerable()
            {
                foreach (var routePatternPart in _readOnlyCollection)
                {
                    yield return routePatternPart.Name;
                }
            }
        }
    }

[marafiq]

Thanks, I understand. I opened the draft PR nervously for you to examine if it will make sense.