上文说到Unmanaged、BufferedBinary和BufferedString是NativeBuffering支持的三个基本数据类型，其实我们也可以说NativeBuffering只支持Unmanaged和IReadOnlyBufferedObject两种类型，BufferedString、NativeBuffering和通过Source Generator生成的BufferedMessage类型，以及下面介绍的几种集合和字典类型，都实现了IReadOnlyBufferedObject接口。

一、IReadOnlyBufferedObject二、集合三、字典四、为什么不直接返回接口?

顾名思义，IReadOnlyBufferedObject表示一个针对缓冲字节序列创建的只读数据类型。如下面的代码片段所示，该接口只定义了一个名为Parse的静态方法，意味着对于任何一个实现了该接口的类型，对应的实例都可以利用一个代表缓冲字节序列的NativeBuffer的对象进行创建。

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public interface IReadOnlyBufferedObject where T: IReadOnlyBufferedObject{    static abstract T Parse(NativeBuffer buffer);}public unsafe readonly struct NativeBuffer{    public byte[] Bytes { get; }    public void* Start { get; }    public NativeBuffer(byte[] bytes, void* start)    {        Bytes = bytes ?? throw new ArgumentNullException(nameof(bytes));        Start = start;    }    public NativeBuffer(byte[] bytes, int index = 0)    {        Bytes = bytes ?? throw new ArgumentNullException(nameof(bytes));        Start = Unsafe.AsPointer(ref bytes[index]);    }}

由于IReadOnlyBufferedObject是NativeBuffering支持的基础类型，而生成的BufferedMessage类型也实现了这个接口。通过这种“无限嵌套”的形式，我们可以定义一个具有任意结构的数据类型。比如我们具有如下这个表示联系人的Contact类型，我们需要利用它作为“源类型”生成对应BufferedMessage类型。

[BufferedMessageSource]public partial class Contact{    public Contact(string id, string name, Address address)    {        Id = id;        Name = name;        ShipAddress = address;    }    public string Id { get; }    public string Name { get; }    public Address ShipAddress { get; }}[BufferedMessageSource]public partial class Address{    public string Province { get; }    public string City { get; }    public string District { get; }    public string Street { get; }    public Address(string province, string city, string district, string street)    {        Province = province ?? throw new ArgumentNullException(nameof(province));        City = city ?? throw new ArgumentNullException(nameof(city));        District = district ?? throw new ArgumentNullException(nameof(district));        Street = street ?? throw new ArgumentNullException(nameof(street));    }}

Contact具有Id、Name和ShipAddress 三个数据成员，ShipAddress 对应的Address又是一个复合类型，具有四个表示省、市、区和介绍的字符串类型成员。现在我们为Contact和Address这两个类型生成对应的ContactBufferedMessage和AddressBufferedMessage。

public unsafe readonly struct ContactBufferedMessage : IReadOnlyBufferedObject{    public NativeBuffer Buffer { get; }    public ContactBufferedMessage(NativeBuffer buffer) => Buffer = buffer;    public static ContactBufferedMessage Parse(NativeBuffer buffer) => new ContactBufferedMessage(buffer);    public BufferedString Id => Buffer.ReadBufferedObjectField(0);    public BufferedString Name => Buffer.ReadBufferedObjectField(1);    public AddressBufferedMessage ShipAddress => Buffer.ReadBufferedObjectField(2);}public unsafe readonly struct AddressBufferedMessage : IReadOnlyBufferedObject{    public NativeBuffer Buffer { get; }    public AddressBufferedMessage(NativeBuffer buffer) => Buffer = buffer;    public static AddressBufferedMessage Parse(NativeBuffer buffer) => new AddressBufferedMessage(buffer);    public BufferedString Province => Buffer.ReadBufferedObjectField(0);    public BufferedString City => Buffer.ReadBufferedObjectField(1);    public BufferedString District => Buffer.ReadBufferedObjectField(2);    public BufferedString Street => Buffer.ReadBufferedObjectField(3);}

如下的程序演示了如何将一个Contact对象转换成字节数组，然后利用这这段字节序列生成一个ContactBufferedMessage对象。给出的调试断言验证了Contact和ContactBufferedMessage对象承载了一样的数据，fixed关键字是为了将字节数组“固定住”。（源代码从这里下载）

using NativeBuffering;using System.Diagnostics;var address = new Address("Jiangsu", "Suzhou", "Industory Park", "#328, Xinghu St");var contact = new Contact("123456789", "John Doe", address);var size = contact.CalculateSize();var bytes = new byte[size];var context = new BufferedObjectWriteContext(bytes);contact.Write(context);unsafe{    fixed (byte* _ = bytes)    {        var contactMessage = ContactBufferedMessage.Parse(new NativeBuffer(bytes));        Debug.Assert(contactMessage.Id == "123456789");        Debug.Assert(contactMessage.Name == "John Doe");        Debug.Assert(contactMessage.ShipAddress.Province == "Jiangsu");        Debug.Assert(contactMessage.ShipAddress.City == "Suzhou");        Debug.Assert(contactMessage.ShipAddress.District == "Industory Park");        Debug.Assert(contactMessage.ShipAddress.Street == "#328, Xinghu St");    }}

NativeBuffering同样支持集合。由于Unmanaged和IReadOnlyBufferedObject是两种基本的数据类型，它们的根据区别在于：前者的长度有类型本身决定，是固定长度类型，后者则是可变长度类型。元素类型为Unmanaged和IReadOnlyBufferedObject的集合分别通过ReadOnlyFixedLengthTypedList和ReadOnlyVaraibleLengthTypedList类型（结构体）表示，它们同样实现了IReadOnlyBufferedObject接口。ReadOnlyFixedLengthTypedList采用如下的字节布局：集合元素数量（4字节整数）+所有元素的字节内容（下图-上）。对于ReadOnlyVaraibleLengthTypedList类型，我们会在前面为每个元素添加一个索引（4字节的整数），该索引指向目标元素在整个缓冲区的偏移量（下图-下）。

以如下所示的Entity为例，它具有两个数组类型的属性成员Collection1和Collection2，数组元素类型分别为Foobar和double，它们分别代表了上述的两种集合类型。

[BufferedMessageSource]public partial class Entity{    public Foobar[] Collection1 { get; }    public double[] Collection2 { get; }    public Entity(Foobar[] collection1, double[] collection2)    {        Collection1 = collection1;        Collection2 = collection2;    }}[BufferedMessageSource]public partial class Foobar{    public int Foo { get; }    public string Bar { get; }    public Foobar(int foo, string bar)    {        Foo = foo;        Bar = bar;    }}

NativeBuffering.Generator会将作为“源类型”的Entity和Foobar类型的生成对应的BufferedMessage类型（EntityBufferredMessage和FoobarBufferedMessage）。从EntityBufferredMessage类型的定义可以看出，两个集合属性的分别是ReadOnlyVariableLengthTypeList和ReadOnlyFixedLengthTypedList。

public unsafe readonly struct EntityBufferedMessage : IReadOnlyBufferedObject{    public NativeBuffer Buffer { get; }    public EntityBufferedMessage(NativeBuffer buffer) => Buffer = buffer;    public static EntityBufferedMessage Parse(NativeBuffer buffer) => new EntityBufferedMessage(buffer);    public ReadOnlyVariableLengthTypeListCollection1 => Buffer.ReadBufferedObjectCollectionField(0);    public ReadOnlyFixedLengthTypedListCollection2 => Buffer.ReadUnmanagedCollectionField(1);}public unsafe readonly struct FoobarBufferedMessage : IReadOnlyBufferedObject{    public NativeBuffer Buffer { get; }    public FoobarBufferedMessage(NativeBuffer buffer) => Buffer = buffer;    public static FoobarBufferedMessage Parse(NativeBuffer buffer) => new FoobarBufferedMessage(buffer);    public System.Int32 Foo => Buffer.ReadUnmanagedField(0);    public BufferedString Bar => Buffer.ReadBufferedObjectField(1);}

两个集合类型都实现了IEnumerable接口，还提供了索引。下面的代码演示了以索引的形式提取集合元素（源代码从这里下载）。

using NativeBuffering;using System.Diagnostics;var entity = new Entity(    collection1: new Foobar[] { new Foobar(1, "foo"), new Foobar(2, "bar") },    collection2: new double[] { 1.1, 2.2 });var bytes = new byte[entity.CalculateSize()];var context = new BufferedObjectWriteContext(bytes);entity.Write(context);unsafe{    fixed (byte* p = bytes)    {        var entityMessage = EntityBufferedMessage.Parse(new NativeBuffer(bytes));        var foobar = entityMessage.Collection1[0];        Debug.Assert(foobar.Foo == 1);        Debug.Assert(foobar.Bar == "foo");        foobar = entityMessage.Collection1[1];        Debug.Assert(foobar.Foo == 2);        Debug.Assert(foobar.Bar == "bar");        Debug.Assert(entityMessage.Collection2[0] == 1.1);        Debug.Assert(entityMessage.Collection2[1] == 2.2);    }}

从数据的存储来看，字典就是键值对的集合，所以我们采用与集合一致的存储形式。NativeBuffering对集合的Key作了限制，要求其类型只能是Unmanaged和字符串（String/BufferredString）。按照Key和Value的类型组合，我们一共定义了四种类型的字典类型，它们分别是：

[BufferedMessageSource]public partial class Entity{    public Dictionary Dictionary1 { get; set; }    public Dictionary Dictionary2 { get; set; }    public Dictionary Dictionary3 { get; set; }    public Dictionary Dictionary4 { get; set; }}public unsafe readonly struct EntityBufferedMessage : IReadOnlyBufferedObject{    public NativeBuffer Buffer { get; }    public EntityBufferedMessage(NativeBuffer buffer) => Buffer = buffer;    public static EntityBufferedMessage Parse(NativeBuffer buffer) => new EntityBufferedMessage(buffer);    public ReadOnlyUnmanagedUnmanagedDictionary Dictionary1 => Buffer.ReadUnmanagedUnmanagedDictionaryField(0);    public ReadOnlyUnmanagedBufferedObjectDictionary Dictionary2 => Buffer.ReadUnmanagedBufferedObjectDictionaryField(1);    public ReadOnlyStringUnmanagedDictionary Dictionary3 => Buffer.ReadStringUnmanagedDictionaryField(2);    public ReadOnlyStringBufferedObjectDictionary Dictionary4 => Buffer.ReadStringBufferedObjectDictionaryField(3);}

using NativeBuffering;using System.Diagnostics;var entity = new Entity{    Dictionary1 = new Dictionary { { 1, 1 }, { 2, 2 }, { 3, 3 } },    Dictionary2 = new Dictionary { { 1, "foo" }, { 2, "bar" }, { 3, "baz" } },    Dictionary3 = new Dictionary { { "foo", 1 }, { "bar", 2 }, { "baz", 3 } },    Dictionary4 = new Dictionary { { "a", "foo" }, { "b", "bar" }, { "c", "baz" } }};var bytes = new byte[entity.CalculateSize()];var context = new BufferedObjectWriteContext(bytes);entity.Write(context);unsafe{    fixed (void* _ = bytes)    {        var bufferedMessage = EntityBufferedMessage.Parse(new NativeBuffer(bytes));        ref var value1 = ref bufferedMessage.Dictionary1.AsRef(1);        Debug.Assert(value1 == 1);        ref var value2 = ref bufferedMessage.Dictionary3.AsRef("baz");        Debug.Assert(value2 == 3);        var dictionary1 = bufferedMessage.Dictionary1;        Debug.Assert(dictionary1[1] == 1);        Debug.Assert(dictionary1[2] == 2);        Debug.Assert(dictionary1[3] == 3);        var dictionary2 = bufferedMessage.Dictionary2;        Debug.Assert(dictionary2[1] == "foo");        Debug.Assert(dictionary2[2] == "bar");        Debug.Assert(dictionary2[3] == "baz");        var dictionary3 = bufferedMessage.Dictionary3;        Debug.Assert(dictionary3["foo"] == 1);        Debug.Assert(dictionary3["bar"] == 2);        Debug.Assert(dictionary3["baz"] == 3);        var dictionary4 = bufferedMessage.Dictionary4;        Debug.Assert(dictionary4["a"] == "foo");        Debug.Assert(dictionary4["b"] == "bar");        Debug.Assert(dictionary4["c"] == "baz");    }}

public readonly unsafe struct ReadOnlyFixedLengthTypedList : IReadOnlyList, IReadOnlyBufferedObject>    where T: unmanaged{    public readonly ref T AsRef(int index);    ...}public unsafe readonly struct ReadOnlyUnmanagedUnmanagedDictionary : IReadOnlyDictionary, IReadOnlyBufferedObject>    where TKey : unmanaged, IComparable    where TValue : unmanaged{    public readonly ref TValue AsRef(TKey index) ;    ...}