Generic extension method for an array does not compile

Question

Populating a request object for a web-service, I need to dynamically add items to some arrays.

I hoped to simplify it by implementing an extension method:

public static class ArrayExtensions<T> where T : class
{
    public static T[] Extend<T>(T[] originalArray, T addItem)
    {
        if (addItem == null)
        {
            throw new ArgumentNullException("addItem");
        }
        var arr = new[] { addItem };
        if (originalArray == null)
        {
            return arr;
        }
        return originalArray.Concat(arr).ToArray();
    }
}

So that this old code:

if (foo.bazArr == null)
{
    foo.bazArr  = new[] { baz };
}
else
{
    foo.bazArr = new[] { baz }.Concat(foo.bazArr).ToArray(); // (i know this inserts the new item at the beginning, but that's irrelevant, order doesn't matter)
}

could be rewritten as:

foo.bazArr = foo.bazArr.Extend(baz); // won't compile

The error is: 'System.Array' does not contain a definition for 'Extend' and no extension method 'Extend' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

Whereas calling the extension method directly like so:

foo.bazArr = ArrayExtensions<someService.bazType>.Extend(foo.bazArr, baz);

compiles fine.

Why is that so? Why can't the compiler infer the type on its own here, if the array is strongly-typed?

---

EDIT - correct code below:

public static class ArrayExtensions
{
    public static T[] Extend<T>(this T[] originalArray, T addItem) where T : class
    {
        if (addItem == null)
        {
            throw new ArgumentNullException("addItem");
        }
        var arr = new[] { addItem };
        if (originalArray == null)
        {
            return arr;
        }
        return originalArray.Concat(arr).ToArray(); // although Concat is not recommended for performance reasons, see the accepted answer
    }
}

---

For this popular question, here's another good simple example:

public static class Extns
    {
    // here's an unbelievably useful array handling extension for games!

    public static T AnyOne<T>(this T[] ra) where T:class
        {
        int k = ra.Length;
        int r = Random.Range(0,k);
        return ra[r];
        // (add your own check, alerts, etc, to this example code)
        }
    }

and in use ..

someArrayOfSoundEffects.AnyOne().Play();
someArrayOfAnimations.AnyOne().BlendLeft();
winningDisplay.text = successStringsArray.AnyOne() +", " +playerName;
SpawnEnormousRobotAt( possibleSafeLocations.AnyOne() );

and so on. For any array it will give you one random item. Used constantly in games to randomise effects etc. The array can be any type.

Answer 1

Missing this:

public static T[] Extend<T>(this T[] originalArray, T addItem)

Without the this it is not an extension method.

Additional note: extending an array one item at a time is expensive. A List<T> would be far preferable. Check to see if your web-service tools offer lists as an option.

Even with arrays, using Enumerable.Concat is probably overkill here; I would simply measure the two arrays, allocate a new one, and use the CopyTo method of each to write into place in the new array.