Convert / Cast Base Type to Derived Type

Question

I am extending the existing class. NET framework by deriving it. How to convert an object of a base type to a derived type?

public class Results { //Framework methods } public class MyResults : Results { //Nothing here } //I call the framework method public static MyResults GetResults() { Results results = new Results(); //Results results = new MyResults(); //tried this as well. results = CallFrameworkMethod(); return (MyResults)results; //Throws runtime exception } 

I understand that this happens when I try to apply a base type to a derived type, and if the derived type has additional properties, then memory is not allocated. When I add additional properties, I don't care if they are initialized to zero.

How can I do this without a manual copy?

Answer 1

You can not. If results does not refer to MyResults (for example, if CallFrameworkMethod returns the base instance of results ), then the casting will not do this: you need to create a new MyResults based on an existing non- MyResults . Casting is a change in the type of the compilation link, not a change in the specific type of the reference object.

You can use tools such as Reflection or AutoMapper to help initialize a new MyResults object, but MyResults should be a new MyResults object because you cannot specify a base results object to become a MyResults object.

Answer 2

What about:

 ... MyResults results = new MyResults(); ... 

And you may also need to create a COnstructor in the MyResults class:

 public class MyResults : Results { public MyResults() : base() {} } 

Strike> What exactly means "nothing here"?

EDIT

  results = (CallFrameworkMethod() as MyResults); 

It does not throw an exception, but if it is useful to you, it depends on what you would like to do next ...

Answer 3

No, you cannot avoid copying content to an instance of a derived type. Well, if you can change CallFrameworkMethod as a generic method, and MyResults has a constructor with a null argument, then CallFrameworkMethod can create a new instance of your derived type directly, and then use only members of the parent type.

But you probably have to finish copying to a new object. Remember that this copy code can certainly be reused in another way, you do not need to rewrite it wherever you need it.

Answer 4

As mentioned earlier, you need to instantiate a new instance of MyResults, and then copy the properties from the Results object. You asked what a “copy constructor” is - it is just a constructor that takes an object and uses it to populate the object that is being created. For example:

  public class Results { public string SampleProperty1 { get; set; } public string SampleProperty2 { get; set; } } public class MyResults : Results { public MyResults(Results results) { SampleProperty1 = results.SampleProperty1; SampleProperty2 = results.SampleProperty2; } } 

The copy constructor is usually more convenient, readable and reusable than when using this code:

 MyResults myResults = new MyResults { SampleProperty1 = results.SampleProperty1, SampleProperty2 = results.SampleProperty2 }; 

If there are many properties and / or you make many changes to the class, you can use reflection (for example, C # Using Reflection to copy properties of a base class ) or a tool like AutoMapper ( http://automapper.codeplex.com ) to copy properties. But often this can be redundant.

Answer 5

 Results results = new MyResults(); ... return (MyResults)results; 

must work. If not, then the problem will be somewhere else.