Reimplement System.Net.Http's ObjectCollection<T> to reduce allocation / interface dispatch

[stephentoub]

The current implementation of ObjectCollection<T> wraps a List<T> and derives from Collection<T>. This means that every ObjectCollection<T> allocated also involves an extra List<T> object (and its array if items are added to it) as well as multiple levels of indirection on each operation.

We can instead just implement ObjectCollection<T> directly. Since most uses end up with just a single object contained (e.g. for a MedaTypeHeaderValue's CharSet), and since it only ever stores Ts that are non-null classes, we can use the items field to be either a T or a T[], optimizing for the case where a single element is stored. In implementing it directly, we then also avoid the extra List<T> object allocation, as well as the interface dispatch that results from going through the IList<T> interface.

This shows up in simple HTTP requests because many requests end up sending down a Content-Type response header, often including a charset, and if headers are enumerated, that results in a MediaTypeHeaderValue being created.

[Benchmark]
public void Create() => new MediaTypeHeaderValue("application/json").CharSet = "utf-8";

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated
Create	\master\corerun.exe	129.38 ns	0.958 ns	0.849 ns	1.00	184 B
Create	\pr\corerun.exe	95.33 ns	0.330 ns	0.258 ns	0.74	104 B

cc: @scalablecory, @davidsh

[ghost]

Tagging @dotnet/ncl as an area owner. If you would like to be tagged for a label, please notify danmosemsft.

[scalablecory]

Code looks good; have you also benchmarked the >1 item case?

[stephentoub]

Code looks good; have you also benchmarked the >1 item case?

Yup:

[Params(1, 2, 4, 8)]
public int Parameters { get; set; }

[Benchmark]
public void Create()
{
    var m = new MediaTypeHeaderValue("application/json");
    for (int i = 0; i < Parameters; i++)
    {
        m.Parameters.Add(new NameValueHeaderValue(i.ToString(), "value"));
    }
}

Method	Toolchain	Parameters	Mean	Error	StdDev	Ratio	Allocated
Create	\master\corerun.exe	1	122.27 ns	0.615 ns	0.513 ns	1.00	184 B
Create	\pr\corerun.exe	1	88.70 ns	0.840 ns	0.744 ns	0.73	104 B
Create	\master\corerun.exe	2	167.39 ns	0.737 ns	0.653 ns	1.00	216 B
Create	\pr\corerun.exe	2	140.42 ns	0.810 ns	0.676 ns	0.84	192 B
Create	\master\corerun.exe	4	265.23 ns	1.504 ns	1.334 ns	1.00	280 B
Create	\pr\corerun.exe	4	237.22 ns	4.193 ns	3.717 ns	0.89	256 B
Create	\master\corerun.exe	8	496.14 ns	8.661 ns	9.974 ns	1.00	496 B
Create	\pr\corerun.exe	8	442.42 ns	8.362 ns	8.213 ns	0.89	472 B