get type of array items?

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boxboy - 15 Feb 2004 02:41 GMT

I am trying to write a utility method/function in c# to resize an array, the
problem I have encountered is how to safely get the Type of the items in the
array, even when the array is empty.

Here is how the code I wrote:

public static Array Redim(Array a, int length)

{
Type type = a.GetValue(0).GetType();
Array copy = Array.CreateInstance(type, length);
Array.Copy(a, 0, copy, 0, Math.Min(a.Length, length));
return copy;
}

This code works fine, but as you might see, the problem arises when an array
of length zero is passed in.

The line:

Type type = a.GetValue(0).GetType();

Will cause an exception as the index 0 is outside the bounds of an empty
array. Also, please note ...

If a is:

string[] a = {};

Then:

a.GetType();

Returns:

System.String[]

Which is not the same as:

System.String

So the question is, how is it possible under all circumstances to safely get
the Type of the items in array?

Thanks in advance for any help.

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Justin Rogers - 15 Feb 2004 03:06 GMT

If the array is empty, since you've already cast it down to an Array, both
Type.IsArray will return false, and Type.GetElementType will return null. You
won't be able to get the type of a zero length array. What you might do is:

public static Array Redim(Array a, Type elementType, int length) {
}

string[] myStrings = new string[] {};
Redim(myStrings, myStrings.GetType().GetElementType(), newLength);

Even the VB Redim statement loses the ability to re-dimension an array once it
has been cast down to an actual System.Array since System.Array is nothing more
than an object. You can probably get the type name based on the type name of
the Array itself. Simply chop off the array-like naming conventions from the
end, but that sounds dangerous to me. Redim will be a lot easier when you get
the ability to use generics.

Signature

Justin Rogers
DigiTec Web Consultants, LLC.
Blog: http://weblogs.asp.net/justin_rogers

> I am trying to write a utility method/function in c# to resize an array, the
> problem I have encountered is how to safely get the Type of the items in the
[quoted text clipped - 41 lines]
>
> Thanks in advance for any help.

Reply to this Message

boxboy - 15 Feb 2004 07:17 GMT

> If the array is empty, since you've already cast it down to an Array, both
> Type.IsArray will return false, and Type.GetElementType will return null. You
[quoted text clipped - 12 lines]
> end, but that sounds dangerous to me. Redim will be a lot easier when you get
> the ability to use generics.

Interesting.

I hadn't noticed the GetElementType method before and decided to try it out
both with filled and empty arrays. The following code works fine:

public static Array Redim(Array a, int length)
{
Type type = a.GetType().GetElementType();
Array copy = Array.CreateInstance(type, length);
Array.Copy(a, 0, copy, 0, Math.Min(a.Length, length));
return copy;
}

Test:

string[] s = (string[])Redim(Array.CreateInstance(typeof(string), 0), 2);
s[0] = "hello";
s[1] = "world";
textBox.Text = s;

Also works with 'new string[] {};' in place of CreateInstance. It would seem
that the GetElementType is returing System.String instead of null as you
suggested.

Much thanks though, as my problem is now solved.

Reply to this Message

Jon Skeet [C# MVP] - 15 Feb 2004 08:07 GMT

> If the array is empty, since you've already cast it down to an Array, both
> Type.IsArray will return false, and Type.GetElementType will return null.

No it won't. Casting a reference to another type doesn't change the
actual object referred to at all. For instance:

using System;

public class Test
{
static void Main()
{
Array foo = new string[0];
Console.WriteLine (foo.GetType().IsArray);
Console.WriteLine (foo.GetType().GetElementType());
}
}

prints out

True
System.String.

> You won't be able to get the type of a zero length array.

Again, not true. See above.

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

Reply to this Message

Justin Rogers - 16 Feb 2004 01:01 GMT

Well, then straight from the documentation on System.Array:

Type.IsArray and Type.GetElementType might not return the expected results with
Array because if an array is cast to the type Array, the result is an object,
not an array. That is, typeof(System.Array).IsArray returns false, and
typeof(System.Array).GetElementType returns a null reference (Nothing in Visual
Basic).

Unlike most classes, Array provides the CreateInstance method, instead of public
constructors, to allow for late bound access.

The Array.Copy method copies elements not only between arrays of the same type
but also between standard arrays of different types; it handles type casting
automatically.

Don't know exactly what to make of that. Possibly a mis-print. However, there
have been instances, were the documentation is correct. However, it may have
been correct pre V1, since that is when I have the error logs demonstrating the
problem.

Signature

Justin Rogers
DigiTec Web Consultants, LLC.
Blog: http://weblogs.asp.net/justin_rogers

> > If the array is empty, since you've already cast it down to an Array, both
> > Type.IsArray will return false, and Type.GetElementType will return null.
[quoted text clipped - 22 lines]
>
> Again, not true. See above.

Reply to this Message

Jon Skeet [C# MVP] - 16 Feb 2004 11:09 GMT

> Well, then straight from the documentation on System.Array:
>
[quoted text clipped - 3 lines]
> typeof(System.Array).GetElementType returns a null reference (Nothing in Visual
> Basic).

<snip>

> Don't know exactly what to make of that. Possibly a mis-print. However, there
> have been instances, were the documentation is correct. However, it may have
> been correct pre V1, since that is when I have the error logs demonstrating the
> problem.

That's very odd. It makes sense to me for typeof(System.Array).IsArray
to return false, and typeof(System.Array).GetElementType to return
null, because Array itself isn't an array, it's just the base type for
all arrays. This is similar to how ValueType itself is a reference
type. That said, "casting" an array to Array doesn't change the object
(the array) itself, so that bit of documentation is just bizarre, IMO.

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

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Kit George (MSFT) - 23 Feb 2004 18:19 GMT

All, we are slightly modifying our docs based on this thread, thanks for
the input. This basically comes down to a standard for what IsArray means.
We require that you have a specific instance of, or a more solid form of an
Array, for IsArray to be true. THis is not a cavernous pit here, we do the
same thing with ValueTypes, and we do the same thing for Enums.

Thanks,
Kit

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get type of array items?

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