| title | Object and Collection Initializers (C# Programming Guide) | ||
|---|---|---|---|
| ms.date | 07/20/2015 | ||
| ms.prod | .net | ||
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| ms.topic | article | ||
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| ms.assetid | c58f3db5-d7d4-4651-bd2d-5a3a97357f61 | ||
| caps.latest.revision | 27 | ||
| author | BillWagner | ||
| ms.author | wiwagn |
Object initializers let you assign values to any accessible fields or properties of an object at creation time without having to invoke a constructor followed by lines of assignment statements. The object initializer syntax enables you to specify arguments for a constructor or omit the arguments (and parentheses syntax). The following example shows how to use an object initializer with a named type, Cat and how to invoke the default constructor. Note the use of auto-implemented properties in the Cat class. For more information, see Auto-Implemented Properties.
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The object initializers syntax allows you to create an instance, and after that it assigns the newly created object, with its assigned properties, to the variable in the assignment.
Although object initializers can be used in any context, they are especially useful in [!INCLUDEvbteclinq] query expressions. Query expressions make frequent use of anonymous types, which can only be initialized by using an object initializer, as shown in the following declaration.
var pet = new { Age = 10, Name = "Fluffy" }; Anonymous types enable the select clause in a [!INCLUDEvbteclinq] query expression to transform objects of the original sequence into objects whose value and shape may differ from the original. This is useful if you want to store only a part of the information from each object in a sequence. In the following example, assume that a product object (p) contains many fields and methods, and that you are only interested in creating a sequence of objects that contain the product name and the unit price.
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When this query is executed, the productInfos variable will contain a sequence of objects that can be accessed in a foreach statement as shown in this example:
foreach(var p in productInfos){...} Each object in the new anonymous type has two public properties which receive the same names as the properties or fields in the original object. You can also rename a field when you are creating an anonymous type; the following example renames the UnitPrice field to Price.
select new {p.ProductName, Price = p.UnitPrice}; It is a compile-time error to use an object initializer with a nullable struct.
Collection initializers let you specify one or more element initializers when you initialize a collection type that implements xref:System.Collections.IEnumerable and has Add with the appropriate signature as an instance method or an extension method. The element initializers can be a simple value, an expression or an object initializer. By using a collection initializer you do not have to specify multiple calls to the Add method of the class in your source code; the compiler adds the calls.
The following examples shows two simple collection initializers:
List<int> digits = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
List<int> digits2 = new List<int> { 0 + 1, 12 % 3, MakeInt() }; The following collection initializer uses object initializers to initialize objects of the Cat class defined in a previous example. Note that the individual object initializers are enclosed in braces and separated by commas.
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You can specify null as an element in a collection initializer if the collection's Add method allows it.
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You can specify indexed elements if the collection supports indexing.
var numbers = new Dictionary<int, string> {
[7] = "seven",
[9] = "nine",
[13] = "thirteen"
}; The following example combines the concepts of object and collection initializers.
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Shown in the following example, an object which implements xref:System.Collections.IEnumerable containing an Add method with multiple parameters allows for collection initializers with multiple elements per item in the list corresponding to the signature of the Add method.
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Add methods can use the params keyword to take a variable number of arguments as shown in the following example. This example demonstrates the custom implementation of an indexer as well to initialize a collection using indexes.
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