Passing List by reference to a method accepting ref ICollection [duplicate]

问题

I've noticed that a List<T> cannot be passed to methods such as:

Foo<T>(ref ICollection<T> bar)
Foo<T>(out ICollection<T> bar)

Taking into account that, on the other side, Foo<T>(ICollection<T> bar) does accept a List<T> as argument, could anyone please explain the described behavior to me?

Complete example:

public class AuxClass { }

public class Test
{
    private void NonRefFoo<T>(ICollection<T> intCollection) {; }
    private void RefFoo<T>(ref ICollection<T> intCollection) {; }
    private void OutFoo<T>(out ICollection<T> intCollection) { intCollection = null; }

    public void Foo()
    {
        {
            List<int> list = new List<int>() { 0, 1, 2, 3, 5, 8, 13 };
            List<AuxClass> classList = new List<AuxClass>() { new AuxClass() };

            NonRefFoo<int>(list);
            RefFoo<int>(ref list);                                  // Invalid
            OutFoo<AuxClass>(out List<int> outListInt);             // Invalid

            NonRefFoo<AuxClass>(classList);
            RefFoo<AuxClass>(ref classList);                        // Invalid
            OutFoo<AuxClass>(out List<AuxClass> outListClass);      // Invalid
        }

        {
            ICollection<int> collection = new List<int>() { 0, 1, 2, 3, 5, 8, 13 };
            ICollection<AuxClass> classCollection = new List<AuxClass>() { new AuxClass() };

            NonRefFoo<int>(collection);
            OutFoo<int>(out ICollection<int> outCollectionInt);
            RefFoo<int>(ref collection);

            NonRefFoo<AuxClass>(classCollection);
            RefFoo<AuxClass>(ref classCollection);
            OutFoo<AuxClass>(out ICollection<AuxClass> outCollectionClass);
        }
    }
}

Thanks in advance

回答1:

With the ref there is a possibility (and with the out a certainty) that you will assign a new object to property you pass in. In this case anything of type ICollection<T>. If you pass in an object of type List<T> you might end up assigning an object of a unrelated type that implements ICollection<T>. The c# type system doesn't allow that and the compiler knows this and so stops you doing it

来源：https://stackoverflow.com/questions/48351097/passing-listt-by-reference-to-a-method-accepting-ref-icollectiont

标签

pass-by-reference

c#-7.0

Passing List<T> by reference to a method accepting ref ICollection<T> [duplicate]

问题

回答1: