Anonymous type (c Guide Basics)

Anonymous types provide a convenient way , Can be used to encapsulate a set of read-only properties into a single object , Instead of explicitly defining a type first . The type name is generated by the compiler , And it can't be used at the source level . The type of each property is inferred by the compiler .

Combined use new Operators and object initializers create anonymous types . For more information about object initializers , See object and collection initializers .

The following example shows two named Amount and Message An anonymous type that initializes the properties of .

var v = new {
     Amount = 108, Message = "Hello" };

// Rest the mouse pointer over v.Amount and v.Message in the following
// statement to verify that their inferred types are int and string.
Console.WriteLine(v.Amount + v.Message);

Anonymous types are often used in the of query expressions select clause , To return a subset of the attributes of each object in the source sequence . More about queries , see also C# Medium LINQ.

Anonymous types contain one or more public read-only properties . Contains other kinds of class members （ Such as method or event ） Is invalid . The expression used to initialize the property cannot be null、 Anonymous function or pointer type .

The most common solution is to initialize anonymous types with other types of properties . In the following example , Suppose it's called Product Class exists . class Product Include Color and Price attribute , And other attributes you're not interested in . Variable Productproducts yes A collection of objects . Anonymous type declaration with new Keyword start . The declaration initializes a that uses only Product New types of two properties of . Using anonymous types will result in less data returned in the query .

If you don't specify a member name in an anonymous type , The compiler assigns anonymous type members the same name as the properties used to initialize them . You need to provide a name for the property initialized with an expression , As the following example shows . In the following example , The attribute names of anonymous types are PriceColor and .

var productQuery =
    from prod in products
    select new {
     prod.Color, prod.Price };

foreach (var v in productQuery)
{
    
    Console.WriteLine("Color={0}, Price={1}", v.Color, v.Price);
}

Usually , When using anonymous types to initialize variables , By using var Declare variables as implicitly typed local variables . Type name cannot be given in variable declaration , Because only the compiler can access the base name of anonymous types . of var Details of , see also var.

You can create an array of anonymous typed elements by combining implicitly typed local variables with implicitly typed arrays , As the following example shows .

var anonArray = new[] {
     new {
     name = "apple", diam = 4 }, new {
     name = "grape", diam = 1 }};

The anonymous type is class type , They are derived directly from object, And cannot be cast to division object Any type other than . Although your application can't access it , The compiler also provides the name of each anonymous type . From the perspective of the common language runtime , Anonymous types are no different from any other reference type .

If the initial value of two or more anonymous objects in the assembly specifies a sequence of properties , These attributes are in the same order and have the same name and type , The compiler treats the object as an instance of the same type . They share type information generated by the same compiler .

Anonymous types support with expression Non destructive modification of form . This allows you to create new instances of anonymous types , One or more of these properties have new values ：

var apple = new {
     Item = "apples", Price = 1.35 };
var onSale = apple with {
     Price = 0.79 };
Console.WriteLine(apple);
Console.WriteLine(onSale);

Cannot add field 、 attribute 、 The return type of a time or method is declared to have an anonymous type . Again , You can't put the method 、 attribute 、 A formal parameter of a constructor or indexer is declared as having an anonymous type . To pass anonymous types or collections containing anonymous types as parameters to a method , Parameters can be used as types object Make a statement . however , Use... For anonymous types object Against the purpose of strong typing . If the query result must be stored or passed outside the method boundary , Consider using normal naming structures or classes instead of anonymous types .

Because of... On anonymous types Equals and GetHashCode Methods are based on method properties Equals and GetHashCode Defined , Therefore, only if all properties of two instances of the same anonymous type are equal , These two instances are equal .