Xamarin.Android iterate Java.Util.HashMap

Question

Anyone know the best way to iterate a Java HashMap in Xamarin.Android? According to the docs it appears doing a foreach on EntrySet is suppose to return an instance of IMapEntry on each iteration, but I get an invalid cast when doing:

foreach(Java.Util.IMapEntry entry in hashMap.EntrySet())

Or if you'd rather tell me how to convert the HashMap to a .Net dictionary that would be find too as it is my end goal.

Note: this is for Xamarin.Android specifically and it doesn't seem to be as straight forward as the suggested duplicate.

Answer 1

HashMap.EntrySet() returns an ICollection and items of that collection that can not be cast to an IMapEntry.... issue? probably, but the root issue is trying to use Java generics in C# and thus a non-generic EntrySet() should return a list of HashMap.SimpleEntry (or SimpleImmutableEntry) but is not mapped that way in Xamarin.Android...

I work around this by using the the Xamarin.Android runtime JavaDictionary (Android.Runtime.JavaDictionary) as you can "cast" a Hashmap to its generic version and act upon it as a normal C# generic class.

Assuming you are creating or receiving a simple HashMap key/value (lots of the Google Android APIs use simple HashMaps to convert to Json for their Rest APIs, i.e. the Google Android Drive API makes use of a lot of HashMaps)

Create a Java HashMap (this is from a POCO or POJO):

var map = new HashMap();
map.Put(nameof(uuid), uuid);
map.Put(nameof(name), name);
map.Put(nameof(size), size);

Create a JavaDictionary from the Handle of a Hashmap:

var jdictionaryFromHashMap = new Android.Runtime.JavaDictionary<string, string>(map.Handle, Android.Runtime.JniHandleOwnership.DoNotRegister);

Iterate the JavaDictionary:

foreach (KeyValuePair<string, string> item in jdictionaryFromHashMap)
{
    Log.Debug("SO", $"{item.Key}:{item.Value}");
}

Use Linq to create your C# Dictionary:

var dictionary = jdictionaryFromHashMap.ToDictionary(t => t.Key, t => t.Value);