More robust data structures - #6

Process completed with exit code 1.

Possible null reference argument for parameter 'key' in 'void NullKeyDictionary.Add(object key, object? value)'.

Nullability of reference types in value of type 'Dictionary<object, object?>.KeyCollection' doesn't match target type 'ICollection<object?>'.

Argument of type 'KeyValuePair<object?, object?>' cannot be used for parameter 'item' of type 'KeyValuePair<object, object?>' in 'bool ICollection<KeyValuePair<object, object?>>.Contains(KeyValuePair<object, object?> item)' due to differences in the nullability of reference types.

Nullability of reference types in type of parameter 'key' of 'void NullKeyDictionary.Add(object key, object? value)' doesn't match implicitly implemented member 'void IDictionary<object?, object?>.Add(object? key, object? value)' (possibly because of nullability attributes).

Possible null reference argument for parameter 'key' in 'void NullKeyDictionary.Add(object key, object? value)'.

Nullability of reference types in value of type 'Dictionary<object, object?>.KeyCollection' doesn't match target type 'ICollection<object?>'.

Argument of type 'KeyValuePair<object?, object?>' cannot be used for parameter 'item' of type 'KeyValuePair<object, object?>' in 'bool ICollection<KeyValuePair<object, object?>>.Contains(KeyValuePair<object, object?> item)' due to differences in the nullability of reference types.