Array.ForEach cannot infer array element type from usage

Question

public class MyType
{
    public int SomeProperty { get; set; }
}

public class CustomArray
{
    public MyType this[int index]
    {
        //some code
    }
}

var intList = new List<int>();
var arr = new CustomArray();
Array.ForEach(arr, x => intList.Add(x.MyProperty));

This will not compile saying:

The type arguments for method void System.Array.ForEach(T[], Action) cannot be inferred from the usage.

How can that be?

Answer 1

The CustomArray type is not an array, it's a custom type with an indexed property.

The ForEach<T> method expects an array with elements of type T , and it didn't get one.

Just because you can use the same operator ( [] ) on both and array and an indexed property, does not mean they are the same thing. For example, arrays are sequential by nature, meaning that if a[0] and a[2] exist, that implies the existence of a[1] . Indexed properties, on the other hand, are more like methods that get a parameter, and they are free to do with it whatever they want.

---

If you want to declare an array of MyType objects, you do not need to do any special declarations, you can just do:

var arr = new MyType[10];

and now you have an array that can house 10 MyType objects. Initially the values in any of the array's slots is null (assuming MyType is a class).

Also I would steer clear of the Array.ForEach method, I'd prefer using a simple for or foreach loop, or maybe, in this particular case, the List.AddRange method:

intList.AddRange(arr.Select(x=> x.MyProperty));