NSPredicate crash on CONTAINS?

Question

I'm trying to do a simple predicate filter on an array of objects. The objects in the array have 2 properties, displayValue and value. I am trying to filter based on a search string and I get a crash. ``` NSPredicate *pred = [NSPredicate predicateWithFormat:@"displayValue CONTAINS[cd] %@", searchString]; NSArray *results = [_data filteredArrayUsingPredicate:pred]; ``` what exactly is incorrect about this format that it causes a `Can't use in/contains operator with collection 100 (not a collection)` crash?

Accepted Answer

I was able to reproduce your problem. This happens if the `displayValue` of one of the objects is not a `NSString`, but some different type.

From your error message, I assume that you assigned an `NSNumber`, for example

```
obj.displayValue = @100;
```

somewhere in your code. The "CONTAINS" predicate works only with strings, so you must assign only string values to the property.

Therefore, you should define the type of the property as `NSString *` instead of `id`, and check the assignments to that property.

If you really need to keep the `id` type and store different kinds of objects in that property, then you cannot use the "CONTAINS" operator in the predicate. A direct comparison with `@"displayValue == %@"` would work, however.

UPDATE: As a workaround, you could use the `description` method, which converts any object to a string, in particular it converts a `NSNumber` to its string representation. So the following could work:

```
[NSPredicate predicateWithFormat:@"displayValue.description CONTAINS[cd] %@", searchString];
```

The drawback is that the exact `description` format is not documented.

Another solution could be to use a block-based predicate, where you can check the type of each object and perform the appropriate comparison.

NSPredicate crash on CONTAINS?

Solution

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