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.

Possible duplicate of [Iterate through a HashMap](https://stackoverflow.com/questions/1066589/iterate-through-a-hashmap) — Jon Douglas, Aug 13 '17 at 20:55
@JonDouglas I definitely looked at that, but was unable to translate it to C#/Xamarin — aweFalafelApps, Aug 13 '17 at 21:27
if you can't cast it to IMapEntry, what type does the debugger say that it actually is? — Jason, Aug 13 '17 at 22:40
@Jason Java.Util.HashMap.HashMapEntry, which I am unable to figure out how to cast to. I have made some progress using JavaCast, but still working through it. I'll of course post an answer if/when I come up with something complete. — aweFalafelApps, Aug 14 '17 at 00:05

SushiHangover · Accepted Answer · 2017-08-14T01:44:28.820

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);

Xamarin.Android iterate Java.Util.HashMap

1 Answers1

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

Create a JavaDictionary from the Handle of a Hashmap:

Iterate the JavaDictionary:

Use Linq to create your C# Dictionary:

Create a `JavaDictionary` from the `Handle` of a `Hashmap`:

Iterate the `JavaDictionary`: