目录
什么是record？
record是.NET 5中的一种新特性，可以看作是一种概念上不可变的类。records可以帮助我们在C#中更容易地处理数据，同时提供了重要的功能，如对象相等性、hashcode和解构。
与类不同，records具有值语义。也就是说，当比较两个records的实例时，比较的是这些实例的属性而非引用。这意味着，如果两个records的属性值相同，它们就是相等的。
record也可以简化需要类似于Dto的数据结构容器的定义。
使用record
Person p1 =new("小明","南山","11@outlook.com");
Person p2 =new("小明","南山","11@outlook.com");
Console.WriteLine(p1 == p2);publicrecord Person(stringName,stringAddress,stringEmail);
像定义一个类一样,public class Person,只是将class关键字替换成record关键字。然后属性是用括号来定义。
默认的record声明是class，如果想声明一个struct
publicrecordstructPerson(stringName,stringAddress,stringEmail);
record是不可变的类型，括号中声明的属性在构造之后不可变更。可以使用==按属性的值进行比较。可以直接作为hash的key以及结构。
record可以像普通类一样扩展可变更的属性和自定义的方法，语法如下
publicrecord Person(stringName,stringAddress,string Email)
{
publicrequiredstringPhoneNumber {get;set; }
publicstaticIEnumerable GetAll()
{
yieldreturnnewPerson("张三","123 Main St","john@example.com") { PhoneNumber ="123456789"};
yieldreturnnewPerson("李四","456 Elm St","jane@example.com") { PhoneNumber ="123456789" };
yieldreturnnewPerson("王二","789 Oak St","bob@example.com") { PhoneNumber ="123456789" }; ;
}
publicstringGetDisplayName() => $"{Name} ({Email})";
};
record解构
record可以通过解构，将对象解构为元组，方便一次性获取record中的属性值，
Person p1 =new("小明","南山","11@outlook.com") ;var(name,address,email) = p1 ;
record原理
record的原理是编译器提供支持，上述Person定义反编译结果如下
publicclassPerson : IEquatable{
[CompilerGenerated]
protectedvirtual Type EqualityContract
{
[CompilerGenerated]
get {
returntypeof(Person);
}
}
publicstringName {get;set/*init*/; }
publicstringAddress {get;set/*init*/; }
publicstringEmail {get;set/*init*/; }
publicPerson(stringName,stringAddress,string Email)
{
this.Name = Name;
this.Address = Address;
this.Email = Email;
base..ctor();
}
[CompilerGenerated]
publicoverridestring ToString()
{
StringBuilder stringBuilder =new StringBuilder();
stringBuilder.Append("Person");
stringBuilder.Append(" { ");
if (PrintMembers(stringBuilder))
{
stringBuilder.Append('');
}
stringBuilder.Append('}');
return stringBuilder.ToString();
}
[CompilerGenerated]
protectedvirtualbool PrintMembers(StringBuilder builder)
{
RuntimeHelpers.EnsureSufficientExecutionStack();
builder.Append("Name = ");
builder.Append((object?)Name);
builder.Append(", Address = ");
builder.Append((object?)Address);
builder.Append(", Email = ");
builder.Append((object?)Email);
returntrue;
}
[CompilerGenerated]
publicstaticbooloperator!=(Person? left, Person? right)
{
return!(left == right);
}
[CompilerGenerated]
publicstaticbooloperator==(Person? left, Person? right)
{
return(object)left == right || (left?.Equals(right) ??false);
}
[CompilerGenerated]
publicoverrideint GetHashCode()
{
return((EqualityComparer.Default.GetHashCode(EqualityContract) * -1521134295+ EqualityComparer.Default.GetHashCode(Name)) * -1521134295+ EqualityComparer.Default.GetHashCode(Address)) * -1521134295+ EqualityComparer.Default.GetHashCode(Email);
}
[CompilerGenerated]
publicoverrideboolEquals(object? obj)
{
returnEquals(objas Person);
}
[CompilerGenerated]
publicvirtualboolEquals(Person? other)
{
return(object)this== other || ((object)other !=null&& EqualityContract == other.EqualityContract && EqualityComparer.Default.Equals(Name, other.Name) && EqualityComparer.Default.Equals(Address, other.Address) && EqualityComparer.Default.Equals(Email, other.Email));
}
[CompilerGenerated]
protected Person(Person original)
{
Name = original.Name;
Address = original.Address;
Email = original.Email;
}
[CompilerGenerated]
publicvoidDeconstruct(outstringName,outstringAddress,outstring Email)
{
Name =this.Name;
Address =this.Address;
Email =this.Email;
}
}
可以看到，编译器给使用了record关键字的定义生成了对应的属性和构造函数，并且重写了ToString(),GetHashCode,Equals还有一个解构函数和!=和==运算符。其实看到这里就明白了，为什么record可以提供值比较，解构，hash等。
不可变性是因为record的属性是使用了init关键字而不是set，这样子如果对record的对象属性赋值，编译器会报错。
值相等性是重定义了!=和==运算符
hash是因为重写了GetHashCode,Equals
解构是定义了Deconstruct方法
结论
我们介绍了.NET 5中引入的record类型及其优点。但对于许多数据对象的简单情况，如值对象和DTO，推荐使用record类型。虽然record可以定义可变更的属性和添加方法，不过这样子有点违背了record的初衷。
