Custom indexer “cannot be assigned to”?

Question

In a class, CharList, I have a private list (List<List<NewChar>>) and a couple of indexers:

private List<List<NewChar>> _charList;

// ...

public NewChar this[Int32 inner, Int32 outer]
{
    get
    {
       if (inner < 0 || inner >= _charList.Count ||
            outer < 0 || outer >= _charList[inner].Count)
            throw new ArgumentException("NewCharList indexer[,]: Invalid index!");

       return _charList[inner][outer];
    }
}

public List<NewChar> this[Int32 index]
{
    get
    {
        if (index < 0 || index > MaxCharListIndex)
            throw new ArgumentException("NewCharList indexer[]: Invalid index");
         List<NewChar> ret = new List<NewChar>(_charList[index].Count);

        for (int i = 0; i < _charList[index].Count; i++)
            ret.Add(_charList[index][i]);

        return ret;
    }
}

In the testing code (another class), if I call

charList[0] = null;

I get a compiler error "Property or indexer xxx cannot be assigned to - it is read-only", but if I call

charList[0][0] = new NewChar(22,22);

the compiler will allow it, though the value won't change. Why will it let me assign to the second? I can't for the life of me figure it out, and it's driving me mad! (Even though it doesn't change the value)

Answer 1

When you write this:

charList[0][0] = new NewChar(22,22);

You're not actually using your first indexer, but your second. This is more like:

List<NewChar> temp = charList[0];
temp[0] = new NewChar(22,22);

The syntax to use your first indexer would be:

charList[0, 0] = new NewChar(22,22);

However, this will provide the same compiler error you are receiving now, as you do not have a setter on that indexed property.

On a side note, you can simplify your second indexed property implementation by using List<T>.AddRange or even the List<T> constructor that accepts an IEnumerable<T>, ie:

get
{
    if (index < 0 || index > MaxCharListIndex)
        throw new ArgumentException("NewCharList indexer[]: Invalid index");
    return new List<NewChar>(_charList[index]);
}

Answer 2

I think you don't have set for this[int] indexer:

public List<NewChar> this[Int32 index]
{
    get
    {
        //......
        return ret;
    }
   //set { /*YOU DON'T HAVE THIS METHOD*/}
}