Creating a generic array at runtime

Question

Is there a way in c# to do something like this:

public void Foo<T>()
{
    T[] arr = Goo() as T[];
}

Where Goo returns an object[], Using reflection or whatever?

I don't think an object[] could ever be a T[]. An object could be though. — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Sep 24, 2012 at 6:15
@Damien_The_Unbeliever: Covariance and Contravariance in C#, Part Two: Array Covariance — Ani
– Ani, Commented Sep 24, 2012 at 6:17
@Ani - I could be misreading that, but it seems to say that object[] a = new T[] will be valid (where T:class), not the other way around. — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Sep 24, 2012 at 6:19
@Damien_The_Unbeliever it depends on what the object[] actually is ;p Since T[] can be represented as an object[], it is possible that your object[] is actually a T[]. However, if the array is constructed as an object[] but happens to contain all T, then indeed: the cast will fail. — Marc Gravell
– Marc Gravell, Commented Sep 24, 2012 at 6:47

Darin Dimitrov · Accepted Answer · 2012-09-24 06:15:05Z

7

You could use LINQ:

public void Foo<T>()
{
    T[] arr = Goo().OfType<T>().ToArray();
}

In your example you are casting the object[] to T[] which will also work if the two types match exactly:

public void Foo<T>()
{
    T[] arr = Goo() as T[];
    if (arr != null)
    {
        // use the array
    }
}

For example this will work in the following case:

public object[] Goo()
{
    return new string[] { "a", "b" };
}

and then calling Foo like this:

Foo<string>();

answered Sep 24, 2012 at 6:15

Darin Dimitrov

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Comments

Evil Activity · Accepted Answer · 2014-05-19 15:49:39Z

1

Perhaps the simplest and most reliable approach is to copy the array, doing the cast item-by-item:

public void Foo<T>()
{
    T[] arr = Array.ConvertAll(Goo(), x => (T)x);
}

This is an "unbox.any", which means (to the JIT):

do a castclass (i.e. reference-preserving type-check) if T turns out to be reference-type
do an unbox if T turns out to be value-type

Arrays of reference-types are covariant, meaning that a T[] can be represented as an object[]. Because of this, there is a chance that the result of Goo() is actually a T[]. We might also want to test this:

public T[] Foo<T>()
{
    var tmp = Goo();
    T[] arr = tmp as T[];
    if(arr == null && tmp != null) {
        arr = Array.ConvertAll(Goo(), x => (T)x);
    }
    return arr;
}

One annoyance of this though is that we now have different semantics - in some cases it is the same array, in some cases it is copied. This could be problematic if we mutate the array.

edited May 19, 2014 at 15:49

Evil Activity

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answered Sep 24, 2012 at 6:38

Marc Gravell

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2 Comments

Noctis Over a year ago

Hmm ... you have void in the signature, yet you return the array? Shouldn't it have a T[] as a return type?

Marc Gravell Over a year ago

@Nocitus yup; the danger of coding on a 'phone :)

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