Anonymous object creation expressions

An anonymous-object-creation-expression is used to create an object of an anonymous type.

anonymous-object-creation-expression:
new anonymous-object-initializer

anonymous-object-initializer:
{ member-declarator-list_opt }
{ member-declarator-list, }

member-declarator-list:
member-declarator
member-declarator-list, member-declarator

member-declarator:
simple-name
member-access
base-access
identifier = expression

An anonymous object initializer declares an anonymous type and returns an instance of that type. An anonymous type is a nameless class type that inherits directly from object. The members of an anonymous type are a sequence of read-only properties inferred from the anonymous object initializer used to create an instance of the type. Specifically, an anonymous object initializer of the form

new { p₁ = e₁, p₂ = e₂, … p_n = e_n }

declares an anonymous type of the form

class __Anonymous1
{
private readonly T₁ f₁;
private readonly T₂ f₂;
…
private readonly T_n f_n;

public __Anonymous1(T₁ a₁, T₂ a₂,…, T_n a_n) {
f₁ = a₁;
f₂ = a₂;
…
f_n = a_n;
}

public T₁ p₁ { get { return f₁; } }
public T₂ p₂ { get { return f₂; } }
…
public T_n p_n { get { return f_n; } }

public override bool Equals(object __o) { … }
public override int GetHashCode() { … }
}

where each T_x is the type of the corresponding expression e_x. The expression used in a member-declarator must have a type. Thus, it is a compile-time error for an expression in a member-declarator to be null or an anonymous function. It is also a compile-time error for the expression to have an unsafe type.

The names of an anonymous type and of the parameter to its Equals method are automatically generated by the compiler and cannot be referenced in program text.

Within the same program, two anonymous object initializers that specify a sequence of properties of the same names and compile-time types in the same order will produce instances of the same anonymous type.

In the example

var p1 = new { Name = "Lawnmower", Price = 495.00 };
var p2 = new { Name = "Shovel", Price = 26.95 };
p1 = p2;

the assignment on the last line is permitted because p1 and p2 are of the same anonymous type.

The Equals and GetHashcode methods on anonymous types override the methods inherited from object, and are defined in terms of the Equals and GetHashcode of the properties, so that two instances of the same anonymous type are equal if and only if all their properties are equal.

A member declarator can be abbreviated to a simple name (§7.5.2), a member access (§7.5.4) or a base access (§7.6.8). This is called a projection initializer and is shorthand for a declaration of and assignment to a property with the same name. Specifically, member declarators of the forms

identifier expr. identifier

are precisely equivalent to the following, respectively:

identifer = identifier identifier = expr. identifier

Thus, in a projection initializer the identifier selects both the value and the field or property to which the value is assigned. Intuitively, a projection initializer projects not just a value, but also the name of the value.

The typeof operator

The typeofoperator is used to obtain the System.Typeobject for a type.

typeof-expression:
typeof (type)
typeof (unbound-type-name)
typeof (void)

unbound-type-name:
identifier generic-dimension-specifier_opt
identifier:: identifier generic-dimension-specifier_opt
unbound-type-name. identifier generic-dimension-specifier_opt

generic-dimension-specifier:
< commas_opt >

commas:
,
commas,

The first form of typeof-expression consists of a typeof keyword followed by a parenthesized type. The result of an expression of this form is the System.Type object for the indicated type. There is only one System.Type object for any given type. This means that for a type T, typeof(T) == typeof(T) is always true. The type cannot be dynamic.

The second form of typeof-expression consists of a typeof keyword followed by a parenthesized unbound-type-name. An unbound-type-name is very similar to a type-name (§3.8) except that an unbound-type-name contains generic-dimension-specifiers where a type-name contains type-argument-lists. When the operand of a typeof-expression is a sequence of tokens that satisfies the grammars of both unbound-type-name and type-name, namely when it contains neither a generic-dimension-specifier nor a type-argument-list, the sequence of tokens is considered to be a type-name. The meaning of an unbound-type-name is determined as follows:

· Convert the sequence of tokens to a type-name by replacing each generic-dimension-specifier with a type-argument-list having the same number of commas and the keyword object as each type-argument.

· Evaluate the resulting type-name, while ignoring all type parameter constraints.

· The unbound-type-name resolves to the unbound generic type associated with the resulting constructed type (§4.4.3).

The result of the typeof-expression is the System.Type object for the resulting unbound generic type.

The third form of typeof-expression consists of a typeof keyword followed by a parenthesized void keyword. The result of an expression of this form is the System.Type object that represents the absence of a type. The type object returned by typeof(void) is distinct from the type object returned for any type. This special type object is useful in class libraries that allow reflection onto methods in the language, where those methods wish to have a way to represent the return type of any method, including void methods, with an instance of System.Type.

The typeof operator can be used on a type parameter. The result is the System.Type object for the run-time type that was bound to the type parameter. The typeof operator can also be used on a constructed type or an unbound generic type (§4.4.3). The System.Type object for an unbound generic type is not the same as the System.Type object of the instance type. The instance type is always a closed constructed type at run-time so its System.Type object depends on the run-time type arguments in use, while the unbound generic type has no type arguments.

The example

using System;

class X<T>
{
public static void PrintTypes() {
Type[] t = {
typeof(int),
typeof(System.Int32),
typeof(string),
typeof(double[]),
typeof(void),
typeof(T),
typeof(X<T>),
typeof(X<X<T>>),
typeof(X<>)
};
for (int i = 0; i < t.Length; i++) {
Console.WriteLine(t[i]);
}
}
}

class Test
{
static void Main() {
X<int>.PrintTypes();
}
}

produces the following output:

System.Int32
System.Int32
System.String
System.Double[]
System.Void
System.Int32
X`1[System.Int32]
X`1[X`1[System.Int32]]
X`1[T]

Note that int and System.Int32 are the same type.

Also note that the result of typeof(X<>) does not depend on the type argument but the result of typeof(X<T>) does.