ArrayList.IndexOf( wild card? )

I'm trying to find the index of the first element of an ArrayList whose 'tag' property does not 开发者_JAVA技巧equal null.

I thought I could do something to the effect of

ArrayList.IndexOf(p => p.tag != null);

or

ArrayList.IndexOf(*.tag != null);

but neither of these work. Is there a way to use IndexOf with just a property of an object?

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Try Array.FindIndex:

Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the first occurrence within the entire Array.

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If you switch to using a generic List instead of ArrayList, you can do this:

int index = list.FindIndex(p => p.tag != null);

Otherwise, you're stuck having to manually (gasp) loop through the array list.

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The problem with IndexOf is that you have to use as a parameter an Object in the collection. It is not a LINQ extension. That is why it fails because your lambda is not in the collection.

You could also use the following LINQ query

ArrayList.IndexOf(ArrayList.First( x => p.tag != null ))

But regarding the performance it'll be a bit poor (looking through the collection twice)

You should rather refactor your code as suggested by the smart answers around mine.

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If you are wanting to do this while still using the ArrayList you can simply call ToArray() then do your FindIndex

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if you know the type of the objects you could do something like this

ArrayList list = ...;
var elem = list.Cast<T>().Select((item,i) => new {item,Index = i}).FirstOrDefault(p => p.item.tag == null);
var index = elem != null ? elem.Index : -1;

will work if you know there's at least one (not that effecient mind you

Cast turns an IEnumerable into a IEnumerable<T> opening up the door to the rest of LINQ

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int index = a.Cast<T>()
             .Select((p, q) => p != null && p.tag != null ? q + 1 : 0)
             .FirstOrDefault(p => p > 0) - 1;

Returns -1 if no element found.