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Although you normally set the properties of your object with a constructor, that s not the only way to do it. C# also offers object initializers, which let you set any accessible members of your object when you create it. Notice that we said accessible, which means you can set public members, but not private ones. Suppose you have this Dog class, with member fields that are public:
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public class Dog { public string name; public int weight; // constructor public Dog(string myName) { name = myName; } }
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Notice here that the constructor takes a string to set the dog s name, but it doesn t take a weight. That s fine, because weight is public, so you could easily set it later, like this:
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Dog milo = new Dog("Milo"); Milo.weight = 5;
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With an object initializer, though, you can set the public weight immediately after you create the object, like this:
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Dog milo = new Dog("Milo") { weight = 5 };
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There s not a whole lot of reason to do this; you could just rewrite the constructor to accept the weight. In fact, it s generally a bad idea to have your member fields be public, as we ve said. However, this technique has some advantages with anonymous types.
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From time to time you find yourself creating a class only so that you can create a single instance of it, never to use that class again. C# allows you to dispense with all that and combine object initializers and implicitly typed variables, which you learned about back in 3, to create a class with no name at all: an anonymous type. You create an instance of an anonymous type with the keyword new, just as you would if you were instantiating an object of a declared class. Instead of passing parameters to a constructor, though, you use braces and define the member fields that you want your anonymous class to contain, like this:
new { Color = "Blue", Size = 13 }
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7: Classes and Objects
The compiler creates a new class with two member fields. Notice that we ve capitalized the names of the fields in this declaration, which is contrary to the naming scheme we ve been using. That s because these aren t fields, but properties, which we ll explain more fully in the next chapter. In brief, properties look like fields to the users of your class, and look like methods to the creator of your class. This new class contains two properties, Color and Size. Just as with implicitly typed variables, the compiler can determine that Color is a String and Size is an int, and it types them accordingly. And as with implicitly typed variables, you can t declare the property without assigning it a value, because the compiler needs that value to determine the property s type. Of course, the compiler does assign your class a name for its own internal purposes; it just doesn t tell you what that name is. So how do you use an anonymous class You can use the var keyword to assign an instance of that class to a variable:
var myShoe = new { Color = "Blue", Size = 13 };
You can now access each of the properties using dot notation:
Stirng whatColor = myShoe.Color; int howBig = myShoe.Size;
These fields are read-only, however. If you try to assign a new value to one of them, you ll get an error:
myShoe.Size = 12; // error
This technique is only really useful when your class contains only read-only data, and no methods. That might seem like a rather limited use, but it s very handy with LINQ, so you ll see anonymous methods again in 21, but not before then.
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