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Object and Collection Initializers (C# Programming Guide)

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 (C# Programming Guide).

class Cat
{
    // Auto-implemented properties. 
    public int Age { get; set; }
    public string Name { get; set; }
}
Cat cat = new Cat { Age = 10, Name = "Fluffy" };

Although object initializers can be used in any context, they are especially useful in LINQ 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 LINQ 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.

var productInfos =
    from p in products
    select new { p.ProductName, p.UnitPrice };

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 class that implements IEnumerable. 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.

List<Cat> cats = new List<Cat>
{
    new Cat(){ Name = "Sylvester", Age=8 },
    new Cat(){ Name = "Whiskers", Age=2 },
    new Cat(){ Name = "Sasha", Age=14 }
};

You can specify null as an element in a collection initializer if the collection's Add method allows it.

List<Cat> moreCats = new List<Cat>
{
    new Cat(){ Name = "Furrytail", Age=5 },
    new Cat(){ Name = "Peaches", Age=4 },
    null
};
// The following code consolidates examples from the topic. 
class ObjInitializers
{
    class Cat
    {
        // Auto-implemented properties. 
        public int Age { get; set; }
        public string Name { get; set; }
    }

    static void Main()
    {
        Cat cat = new Cat { Age = 10, Name = "Fluffy" };

        List<Cat> cats = new List<Cat>
        {
            new Cat(){ Name = "Sylvester", Age=8 },
            new Cat(){ Name = "Whiskers", Age=2 },
            new Cat(){ Name = "Sasha", Age=14 }
        };

        List<Cat> moreCats = new List<Cat>
        {
            new Cat(){ Name = "Furrytail", Age=5 },
            new Cat(){ Name = "Peaches", Age=4 },
            null
        };

        // Display results.
        System.Console.WriteLine(cat.Name);

        foreach (Cat c in cats)
            System.Console.WriteLine(c.Name);

        foreach (Cat c in moreCats)
            if (c != null)
                System.Console.WriteLine(c.Name);
            else
                System.Console.WriteLine("List element has null value.");
    }
    // Output: 
    //Fluffy 
    //Sylvester 
    //Whiskers 
    //Sasha 
    //Furrytail 
    //Peaches 
    //List element has null value.
}
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