C#: How to express contraint that type can be casted from a string?

Question

I have the following (working) code:

List<string> stringList = new List<string> { "foo", "bar" };
var ret = stringList.ConvertAll(item => (MyString)item);

This works well since I provided an explicit cast operator:

public struct MyString
{
    private readonly string _myString;
    public MyString(string myString)
    {
        _myString = myString;
    }
    [...]
    public static explicit operator MyString(string myString)
    {
        return new MyString(myString);
    }
}

Now my issue is that I cannot express this as constraints in the following code:

public List<T> conv<T>(List<string> stringList)
{
    var ret = stringList.ConvertAll(item => (T)item); // <-- CS0030: Cannot convert type 'string' to 'T'
    return ret;
}

What where constraint incantation am I missing here ?

Update:

I specifically used ConvertAll based on the following SO post:

https://stackoverflow.com/a/55624925/136285

As a side note, I get the exact same error using:

public List<T> conv<T>(List<string> stringList)
{
    var ret = stringList.Select(x => (T)x).ToList();
    return ret;
}

there is no way to achieve that. You may use an interface `IStringable` or so, though. — MakePeaceGreatAgain, Sep 01 '21 at 12:09
You can also use Linq `Select` which is likely going to be more readable since it's using standard framework functions. — DavidG, Sep 01 '21 at 12:15
constraint ? no ... you may use convention that such class needs a contructor which has string as parameter and then use reflections ... — Selvin, Sep 01 '21 at 12:18
@Charlieface That's what `ToString` is for. OP is asking the other way round. — MakePeaceGreatAgain, Sep 01 '21 at 12:20
@HimBromBeere Ah sorry, that's what happens when the question title is wrong, have edited it now — Charlieface, Sep 01 '21 at 12:20
You could use a factory function `Func` and pass in `s => (MyString)s`. Not great, but at least it doesn't use reflection — Charlieface, Sep 01 '21 at 12:24
I can´t see any benefit in using your conv-method compared to the mentioned select-statement. You don´t need a constraint at all, as `(MyString)s` won't compile if the cast does not exist. All you need is `myList.Select(x => (MyString)s)`. — MakePeaceGreatAgain, Sep 01 '21 at 14:41

score 0 · Answer 1 · answered Sep 02 '21 at 08:14

0

Based on @Charliefact suggestion, I implemented the following:

public List<T> conv<T>(List<string> stringList, Func<string,T> castFromStringConstraint)
{
    var ret = stringList.ConvertAll(item => castFromStringConstraint(item));
    return ret;
}

answered Sep 02 '21 at 08:14

malat

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C#: How to express contraint that type can be casted from a string?

1 Answers1