How to use string resource in switch statement

Question

Is there a way to use a string resource with a switch statement? After trying to obtain a string resource programmatically, an error is returned.

Error

'resourceLoader' is variable but is used like a type

C#

public class PageConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        var resourceLoader = Windows.ApplicationModel.Resources.ResourceLoader.GetForCurrentView();

        Type page = null;
        switch (value as string)
        {
            case resourceLoader.GetString("SteveJohnson/Text"):
                page = typeof(FirstPage);
                break;
            case resourceLoader.GetString("PeteDavidson/Text"):
                page = typeof(SecondPage);
                break;
            case resourceLoader.GetString("OneDrive/Text"):
                page = typeof(FistPage);
                break;
            case resourceLoader.GetString("Twitter/Text"):
                page = typeof(SecondPage);
                break;
            default:
                break;
        }

        return page;
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        throw new NotImplementedException();
    }
}

No, they must be compile-time constants. Looks like you could use a Dictionary<string, Type> instead. — CodeCaster
– CodeCaster, Commented Aug 17, 2020 at 15:41
@CodeCaster: "they must be compile-time constants" -- this used to be the case, but no longer. It's unfortunate that the author of this question did not spend time researching before posting, but it is true that existing Q&A on SO about the limitations of switch have largely been invalidated by recent features added to C#, i.e. pattern-matching, which take away the blanket prohibition against non-constant expressions for case statements. — Peter Duniho
– Peter Duniho, Commented Aug 17, 2020 at 20:09
@PeterDuniho I did do research before asking this question but didn’t find the relevant sources. d219 gave an answer but the code returns a warning. — wbk727
– wbk727, Commented Aug 17, 2020 at 21:16
@CodeCaster: yeah, I don't dispute your complaints about that answer. But it does seem to literally accomplish what was asked. And honestly, without having more context from the OP, it's not clear whether a dictionary-based approach would really be more readable. That said, my only point was to correct the notion that in today's C#, case statements are required to use only constant expressions. — Peter Duniho
– Peter Duniho, Commented Aug 18, 2020 at 5:09

d219 · Accepted Answer · 2020-08-17 23:11:50Z

2

Are you looking for pattern matching? It's only something I've found recently (it's a C# 7 feature which I came across in this link) and what you're doing is a little different from the use case I had but I've just run a quick test with pseudo code (calling a method of another class which takes and returns a string from the switch statement) which seemed to work. For what you're doing I think you'd have something like this:

switch (value)
{
    case string _ when (resourceLoader.GetString("SteveJohnson/Text") == value as string):
                    page = typeof(FistPage);
                    break;
    case string _ when (resourceLoader.GetString("PeteDavidson/Text") == value as string):
                    page = typeof(SecondPage);
                    break;

edited Aug 17, 2020 at 23:11

answered Aug 17, 2020 at 16:13

d219

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5 Comments

wbk727 Over a year ago

A warning is returned: Possible unintended reference comparison; to get a value comparison, cast the right hand side type to 'string'

d219 Over a year ago

Hmm, potentially you need to ToString() the value you are using for the switch? (test code I wrote is on other laptop but have made an edit above, see if that works, if not I'll have another look tomorrow - it's assuming the ToString of the object is what you need to do the comparison on of course).

wbk727 Over a year ago

After trying ToString() the warning is still there

d219 Over a year ago

OK fired up the other laptop as couldn't see why there'd been a problem - I skim read your message above and thought it was an error, now I realise it's a warning (as you said) have corrected by moving your original as string into each case statement; that gets rid of the warning :)

d219 Over a year ago

Also replaced the variable name I had before with the discard _ as you don't need to do anything with that return value.

CodeCaster · Accepted Answer · 2020-08-18 05:02:44Z

I think you need to take a step back and look at why you want a switch statement: probably to have a readable block of code. You could easily do the same with a couple of if()s:

var resourceLoader = Windows.ApplicationModel.Resources.ResourceLoader.GetForCurrentView();

Type page = null;
if (value == resourceLoader.GetString("SteveJohnson/Text"))
    page = typeof(FirstPage);
else if (value == resourceLoader.GetString("PeteDavidson/Text"))
    page = typeof(SecondPage);
else if (value == resourceLoader.GetString("OneDrive/Text"))
    page = typeof(FistPage);
else if (value == resourceLoader.GetString("Twitter/Text"))
    page = typeof(SecondPage);      

return page;

Now you could rewrite this using pattern when clauses to shoehorn it into a switch, but I don't think that's the appropriate construct here, as it doesn't really increase readability. You could opt for a Dictionary with options instead:

var pageTypes = new Dictionary<string, Type>
{
    { resourceLoader.GetString("SteveJohnson/Text"), typeof(FirstPage) },
    { resourceLoader.GetString("PeteDavidson/Text"), typeof(SecondPage) },
    { resourceLoader.GetString("OneDrive/Text"), typeof(FirstPage) },
    { resourceLoader.GetString("Twitter/Text"), typeof(SecondPage) },
};

// Defaults to `null` when not mached
pageTypes.TryGetValue((string)value, out Type page);
return page;

You could even cache the resource lookups by initializing this dictionary once, for example in the constructor of the containing class.

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