Is there a safe way to convert NSMutableDictionary<TKey, TValue> to NSDictionary<TKey, TValue> without allocating a new object?

Question

Is there a safe way to convert NSMutableDictionary<TKey, TValue> to NSDictionary<TKey, TValue> without allocating a new object?

Oliver Brown 1

The iOS types, in Objective-C are as follows

NSMutableDictionary inherits from NSDictionary.

Objective-C has lightweight generics, providing NSMutableDictionary<TKey, TValue> and NSDictionary<TKey, TValue>. These are implemented via type erasure, so at runtime you are free to cast between them and everything will work (at the risk of runtime errors if you get a type you did not expect).

The Xamarin binding has:

NSMutableDictionary inherits from NSDictionary
NSDictionary<TKey, TValue> inherits from NSDictionary
NSMutableDictionary<TKey, TValue> inherits from NSMutableDictionary

This means from .NET's type system's point of view, NSMutableDictionary<TKey, TValue> and NSDictionary<TKey, TValue> are not compatible.

Is there a way to get an NSDictionary<TKey, TValue> from an NSMutableDictionary<TKey, TValue> without creating a new dictionary? The best I could see is Unsafe.As, but I don't actually know if that does what I want.

Oliver

Kyle Wang 5,531 Reputation points Microsoft External Staff

2021-08-12T08:48:16.717+00:00
@Oliver Brown I am afraid that if you want to convert it to "NSDictionary<TKey, TValue>" allocating new memory space is required. A new object will be generated anyway.

NSMutableDictionary<NSString, NSObject> mutablekeyValuePairs = new NSMutableDictionary<NSString, NSObject>(keys, objects); NSDictionary<NSString, NSObject> keyValuePairs = new NSDictionary<NSString, NSObject>(mutablekeyValuePairs.Keys, mutablekeyValuePairs.Values);

As to Unsafe.As, in my test, it doesn't convert correctly.
Oliver Brown 1 Reputation point

2021-08-16T09:00:59.327+00:00

Thanks, that's sadly what I expected.

Your answer

Kyle Wang 5,531 Reputation points Microsoft External Staff

2021-08-12T08:48:16.717+00:00

@Oliver Brown I am afraid that if you want to convert it to "NSDictionary<TKey, TValue>" allocating new memory space is required. A new object will be generated anyway.

NSMutableDictionary<NSString, NSObject> mutablekeyValuePairs = new NSMutableDictionary<NSString, NSObject>(keys, objects); NSDictionary<NSString, NSObject> keyValuePairs = new NSDictionary<NSString, NSObject>(mutablekeyValuePairs.Keys, mutablekeyValuePairs.Values);

As to Unsafe.As, in my test, it doesn't convert correctly.
Oliver Brown 1 Reputation point

2021-08-16T09:00:59.327+00:00

Thanks, that's sadly what I expected.

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Is there a safe way to convert NSMutableDictionary<TKey, TValue> to NSDictionary<TKey, TValue> without allocating a new object?

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