Convert List≪Derivedclass≫ to List≪Baseclass≫

Convert ListDerivedClass to ListBaseClass

The way to make this work is to iterate over the list and cast the elements. This can be done using ConvertAll:

List<A> listOfA = new List<C>().ConvertAll(x => (A)x);

You could also use Linq:

List<A> listOfA = new List<C>().Cast<A>().ToList();

Convert List of Base Class to a List of Derived Class - possible?

Cast<T> method applies cast operation to all elements of the input sequence. It works only if you can do the following to each element of the sequence without causing an exception:

ParentClass p = ...
ChildClass c = (ChildClass)p;

This will not work unless p is assigned an instance of ChildClass or one of its subclasses. It appears that in your case the data returned from the server API contains objects of ParentClass or one of its subclasses other than ChildClass.

You can fix this problem by constructing ChildClass instances, assuming that you have enough information from the server:

List<ChildClass> childList = parentList
    .Select(parent => new ChildClass(parent.Name, ... /* the remaining fields */))
    .ToList();

Copying a ListBaseClass to ListDerivedClass

List<DerivedClass> result = 
    listBaseClass.ConvertAll(instance => (DerivedClass)instance);

Actually ConvertAll is good when you need to create new objects based on the original, when you just need to cast you can use the following

List<DerivedClass> result = 
    listBaseClass.Cast<DerivedClass>().ToList();

If not all of the items in your list can be cast to DerivedClass then use OfType instead

List<DerivedClass> result =
    listBaseClass.OfType<DerivedClass>().ToList();

Cannot convert from ListDerivedClass to ListBaseClass

It is because List<T> is in-variant, not co-variant, so you should change to IEnumerable<T> which supports co-variant, it should work:

IEnumerable<BaseClass> bcl = new List<DerivedClass>();
public void doSomething(IEnumerable<BaseClass> bc)
{
    // do something with bc
}

Information about co-variant in generic

Casting List of Derived class to List of base class

The reason you cannot do this is because a list is writable. Suppose it were legal, and see what goes wrong:

List<Cat> cats = new List<Cat>();
List<Animal> animals = cats; // Trouble brewing...
animals.Add(new Dog()); // hey, we just added a dog to a list of cats...
cats[0].Speak(); // Woof!

Well dog my cats, that is badness.

The feature you want is called "generic covariance" and it is supported in C# 4 for interfaces that are known to be safe. IEnumerable<T> does not have any way to write to the sequence, so it is safe.

class Animal    
{    
    public virtual void Play(IEnumerable<Animal> animals) { }    
}    
class Cat : Animal    
{    
    public override void Play(IEnumerable<Animal> animals) { }    
}    
class Program    
{    
    static void Main()    
    {    
        Cat cat = new Cat();    
        cat.Play(new List<Cat>());    
    }    
}  

That will work in C# 4 because List<Cat> is convertible to IEnumerable<Cat>, which is convertible to IEnumerable<Animal>. There is no way that Play can use IEnumerable<Animal> to add a dog to something that is actually a list of cats.

C# convert Listbaseclass to ListDerivedClass

In your GetSensors method, you are creating instances of the base class Sensor. Trying to implicitly cast them to some other derived class is not possible. You should use a generic type parameter in that method and create instances of the actual derived type.

Like so: (notice the where clause and the new constraint of the T parameter)

public static List<T> GetSensors<T>(JToken jToken) where T : Sensor, new()
{
    var sensors = new List<T>();

    try
    {
        foreach (var item in jToken)
        {
            var s = new T();
            s.Label = item["lab"].ToString();
            s.ActualTemp = item["tf"] != null ? item["tf"].ToString() : "";
            s.HighTemp = item["hf"] != null ? item["hf"].ToString() : "";
            s.LowTemp = item["lf"] != null ? item["lf"].ToString() : "";

            sensors.Add(s);
        }
    }
    catch (Exception)
    {
    }            

    return sensors;
}

Now call it like this:

List<IntSensor> iSensors = ParseJsonData.GetSensors<IntSensor>(internalSensorTree);

convert listbaseclass in Listderivedclass

Why do you inherit the Product class? I think you want to add 2 more fields to the Product class, right?

Because the auto-generated class is a partial class, instead of inheriting the Product class as you did, just add one more file to the partial class like this:

public partial class Product
{
    public bool Restock { get; set; }
    public DateTime? DateSold { get; set; }        
}

C#, How to pass a List of a Derived class to a method that receives a List of the Base class?

The reason for such behaviour is explained here. In short - classes do not support variance in C# and List<AppleBox> is not List<FruitBox<Apple>>.

What you can do:

• "convert" collection (actually create a new one):

with OfType<>().ToList()

AppleBox.ChooseFirst(appleBoxes.OfType<FruitBox<Apple>>().ToList())

or just ToList

AppleBox.ChooseFirst(appleBoxes.ToList<FruitBox<Apple>>())

• change ChooseFirst signature to work with covariant IEnumerable<out T> interface:

public abstract class FruitBox<T>
{
    public T item;

    public static T ChooseFirst(IEnumerable<FruitBox<T>> fruitBoxes)
    {
        return fruitBoxes.First().item;
    }
}

C# Moq - Cannot convert ListDerivedClass to ListBaseClass

Because List<DerivedClass> does not derive from List<BaseClass> and the fact that the T inside does is irrelevent when looking at it from that perspective.

What you're looking for is Covariance. There's MSDN documentation on it here: https://docs.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance

Most efficient way to cast ListSubClass to ListBaseClass

The syntax for this sort of assignment uses a wildcard:

List<SubClass> subs = ...;
List<? extends BaseClass> bases = subs;

It's important to realize that a List<SubClass> is not interchangeable with a List<BaseClass>. Code that retains a reference to the List<SubClass> will expect every item in the list to be a SubClass. If another part of code referred to the list as a List<BaseClass>, the compiler will not complain when a BaseClass or AnotherSubClass is inserted. But this will cause a ClassCastException for the first piece of code, which assumes that everything in the list is a SubClass.

Generic collections do not behave the same as arrays in Java. Arrays are covariant; that is, it is allowed to do this:

SubClass[] subs = ...;
BaseClass[] bases = subs;

This is allowed, because the array "knows" the type of its elements. If someone attempts to store something that isn't an instance of SubClass in the array (via the bases reference), a runtime exception will be thrown.

Generic collections do not "know" their component type; this information is "erased" at compile time. Therefore, they can't raise a runtime exception when an invalid store occurs. Instead, a ClassCastException will be raised at some far distant, hard-to-associate point in code when a value is read from the collection. If you heed compiler warnings about type safety, you will avoid these type errors at runtime.

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