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# value(forUndefinedKey:)

Invoked by [`value(forKey:)`](/documentation/ObjectiveC/NSObject-swift.class/value(forKey:)) when it finds no property corresponding to a given key.

```
func value(forUndefinedKey key: String) -> Any?
```

## Parameters

`key`

A string that is not equal to the name of any of the receiver’s properties.

## Discussion

Subclasses can override this method to return an alternate value for undefined keys. The default implementation raises an `NSUndefinedKeyException`.

## See Also

[`func setValue(Any?, forUndefinedKey: String)`](/documentation/ObjectiveC/NSObject-swift.class/setValue(_:forUndefinedKey:))

Invoked by [`setValue(_:forKey:)`](/documentation/ObjectiveC/NSObject-swift.class/setValue(_:forKey:)) when it finds no property for a given key.


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