CODEWITHSUNDEEP
c#linq
raghava arr
raghava arr

Reputation: 183

Generic List firstOrDefault

I have a generic list and trying to get item based on value i.e.,

list.FirstOrDefault(u=>u.Key == key) // error 'T' does not contain definition for 'Key'

The generic type T having different base classes.

Upvotes: 0

Views: 3212

Answers (3)

Sefe
Sefe

Reputation: 14007

When using generic methods, you have to be sure your T has a property Key. That can be achieved with generic constraints. That can be either a base class or an interface:

interface IKeyedObject {
    string Key { get; };
}

class BaseWithKey : IKeyedObject {
    public string Key { get; set; };
}

class DerivedA : BaseWithKey {
}

class DerivedB : BaseWithKey {
}

class OtherWithKey : IKeyedObject {
    public string Key { get; set; };
}

//Solution with base class (will work with BaseWithKey, DerivedA, DerivedB)

T GetItemBaseClass<T>(List<T> list, string key)
    where T : BaseWithKey {

    return list.FirstOrDefault(u=>u.Key == key);
}

//Solution with interface (will work with all classes)

T GetItemInterface<T>(List<T> list, string key)
    where T : IKeyedObject {

    return list.FirstOrDefault(u=>u.Key == key);
}

Upvotes: 2

Armand P.
Armand P.

Reputation: 41

You are trying to use Key while T is not specified and thus we do not know if T class contains any field/property Key. On thing you can do is to use abstract class/interface or to try to cast u into the class that contains "Key" (assuming you are expecting some classes specificly). Some more details about your list and its items are needed for a more precise answer. Hope it can help !

Upvotes: 1

Daniel A. White
Daniel A. White

Reputation: 190915

Depending on where your T is getting filled into List<T>, you could specify generic constraints on it to have an interface (or base class).

Interface:

interface IHasKey { string Key { get; } } // or some other type for `Key`

Generic constraint:

where T : IHasKey

Upvotes: 3

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