How to switch between "possible" type of an object?

Question

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C# - Is there a better alternative than this to ‘switch on type’?

My company legacy code has something as follow

public override Uri GetUri(object obj)
{
    if ((obj is Entry) || (obj is EntryComment))
    {
        //
    }
    if (obj is Blog)
    {
        //
    }

    // if obj is blah blah blah
}

This method is just ugly. I want to refactor it, but I don't know a technique to iterate over "possible" types that obj can be.

How can I refactor this?

Thank you.

I added the region directive to demonstrate how ugly the code is :-s — Delta76, May 10 '11 at 08:36
Could you move the code inside the if-statement blocks into the types themselves? That way your method would just be: `return obj.Uri;`. — Lasse V. Karlsen, May 10 '11 at 08:36
see also http://stackoverflow.com/questions/156467/switch-pattern-matching-idea http://stackoverflow.com/questions/7252186/switch-case-on-type-c-sharp http://stackoverflow.com/questions/7542793/how-to-use-switch-case-on-a-type http://stackoverflow.com/questions/4478464/c-sharp-switch-on-type http://stackoverflow.com/questions/298976/c-sharp-is-there-a-better-alternative-than-this-to-switch-on-type — Mikhail Poda, May 01 '12 at 11:56
and http://stackoverflow.com/questions/94305/what-is-quicker-switch-on-string-or-elseif-on-type http://stackoverflow.com/questions/6304815/why-is-this-switch-on-type-case-considered-confusing http://stackoverflow.com/questions/10115028/best-way-to-switch-behavior-based-on-type http://stackoverflow.com/questions/2551773/c-sharp-which-is-the-best-alternative-to-switch-on-type — Mikhail Poda, May 01 '12 at 11:56

Jon Skeet · Answer 1 · 2015-07-21T05:08:33.093

A couple of options:

If you're using C# 4, you could use dynamic typing, and split the method into a number of overloads:
```
public Uri GetUri(dynamic obj)
{
    return GetUriImpl(obj);
}

private Uri GetUriImpl(Entry entry)
{
    ...
}

private Uri GetUriImpl(EntryComment comment)
{
    ...
}
```
In this case you'd probably want some sort of "backstop" method in case it's not a known type.

You could create a Dictionary<Type, Func<object, Uri>>:

private static Dictionary<Type, Func<object, Uri>> UriProviders = 
    new Dictionary<Type, Func<object, Uri>> {   
    { typeof(Entry), value => ... },
    { typeof(EntryComment), value => ... },
    { typeof(Blog), value => ... },
};

and then:

public Uri GetUri(object obj)
{
    Func<object, Uri> provider;
    if (!UriProviders.TryGetValue(obj.GetType(), out provider))
    {
        // Handle unknown type case
    }
    return provider(obj);
}

Note that this won't cover the case where you receive a subtype of Entry etc.

Neither of these are particularly nice, mind you... I suspect a higher level redesign may be preferable.

+1, Only answer that actually shows how to do it and doesn't break Open/Closed principle — jgauffin, May 10 '11 at 08:40

score 4 · Accepted Answer · answered May 10 '11 at 08:41

You can also refactor this with interfaces

public interface IHasUri
{
   public Uri GetUri();
}

and implement this interface in all you hierarchy. Than you can use

public Uri GetUri(object obj)
{
    IHasUri hasUri = obj as IHasUri;
    if(hasUri != null)
      Uri uri = hasUri.GetUri();

    // bla bla bla 
}

score 1 · Answer 3 · answered May 10 '11 at 08:51

I think Lasse V. Karlsen has the right of it in his comments, a redesign is more appropriate perhaps

You could create an interface like Stecya suggests

public interface IUriAccessible { //or some sort of a more descriptive name
    Uri Url {get;}
}

and then for each object that you care for, eg Entry, you could

public class Entry:IUriAccessible {
    //... other code here
    public Uri Url {
        get {
            //return uri here
        }
    }
}

and now you can just call it on the object

var entry = new Entry();
var uri = entry.Url;

score 0 · Answer 4 · answered May 10 '11 at 08:35

0

You can have a List<Type>() with all types you want to check for and iterate that list. but I don't think that makes the code faster.

answered May 10 '11 at 08:35

DanielB

19,910
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I don't want "Faster" code, but "more readable" code :). The problem is, the processing code for each type is not the same, so I don't think List is suitable. Switch is more suitable choice – Delta76 May 10 '11 at 08:37

score 0 · Answer 5 · answered May 10 '11 at 08:36

0

You can't switch-case types.

A switch expression or case label must be a bool, char, string, integral, enum, or corresponding nullable type

answered May 10 '11 at 08:36

abatishchev

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How to switch between "possible" type of an object?

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