How to cast object array to generic type array

Question

I am trying to cast an object array to generic object array in CSharp, keep getting this compiler error. Any ideas?

Thanks

 private const int INITIAL_CAPACITY = 20;
    protected int numElements;
    protected E[] elements;

    /*
     * default constructor that creates a new set
     * that is initially empty
     * */
    public ArraySet()
    {
        numElements = 0;
        elements = (E[])(new Object[INITIAL_CAPACITY]);// unchecked
    }

thanks guys, Another related question, I want to set a particular element to null. The following code does not work.

elements[numElements - 1] = null:

What is the correct null value we should use in here then. thank

You should add the whole class and the exact compiler error message. — weismat
– weismat, Commented Jul 4, 2013 at 8:05

Patryk Ćwiek · Accepted Answer · 2013-07-04 08:13:23Z

4

PLB's answer points out why you don't need to do that, I'll just chime in on why you can't do that.

Let's say you have an object array:

var objectArray = new object[] { 3, 5, 3.3, "test" };

Now how would you say I convert that to a uniform, generic type? There's no guarantee that all of the instances inside the object array will be of the same type, the array might be heterogenous, so this:

public static T[] ConvertTo<T>(object[] arr)
{
    return (T[])arr;
}

var intArray = ConvertTo<int>(objectArray);

Can't really work: Cannot convert type 'object[]' to 'T[]' in the ConvertTo method.

What you can do in such situation is test each and every item if it's of appropriate type, especially since LINQ already has fitting method:

var intArray = objectArray.OfType<int>().ToArray();

Which will give you an array: [3, 5]

So a generic version would be: elementsArray = objectArray.OfType<E>().ToArray();

answered Jul 4, 2013 at 8:13

Patryk Ćwiek

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4 Comments

June Over a year ago

Excellent stuff, elementsArray = objectArray.OfType<E>().ToArray(); is what I am looking for.

June Over a year ago

s, Another related question, I want to set a particular element to null. The following code does not work. elements[numElements - 1] = null: How do you do this? thanks

Patryk Ćwiek Over a year ago

@Jeffrey From the example I gave you, if you do OfType<string> and then do stringArray[0] = null it works perfectly fine. You'd have to elaborate on the 'doesn't work' part. Maybe you're trying to set a value type to null...?

June Over a year ago

Hey Patryk yes I was trying to set a value to null, now I am using the following code elements[i] = default(E); thanks

Leri · Accepted Answer · 2013-07-04 08:04:02Z

3

You don't need cast here. Initializing array with capacity is enough:

elements = new E[INITIAL_CAPACITY];

answered Jul 4, 2013 at 8:04

Leri

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Comments

panoskarajohn · Accepted Answer · 2017-11-19 12:49:05Z

0

elements[numElements - 1] = null: What is the correct null value we should use in here then. thank

The c# class has the 'default keyword' for your generics. Because some types might not be nullable.

var number = default(E);

For example an integer cannot be null.

edited Nov 19, 2017 at 12:49

answered Jun 28, 2017 at 10:44

panoskarajohn

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