Expand property bag definition

[AndriySvyryd]

Look for IDictionary<string, object> implementation or just look for an indexer and ContainsKey() method.

Add Fluent API to configure an ET as a Property bag or not.

[ajcvickers]

I'm finding it hard to use property bags without this; should we pull it into 5.0?

For example, I thought this would work:

public class ProxyableSharedType
{
    private readonly Dictionary<string, object> _keyValueStore = new Dictionary<string, object>();

    public virtual object this[string key]
    {
        get => _keyValueStore.TryGetValue(key, out var value) ? value : default;
        set => _keyValueStore[key] = value;
    }
}

modelBuilder.SharedTypeEntity<ProxyableSharedType>(
    "PST", b =>
    {
        b.IndexerProperty<int>("Id").ValueGeneratedNever();
        b.IndexerProperty<string>("Payload");
    });

But it doesn't because initially "Id" is not in the dictionary, so null is returned by the indexer, but the property doesn't support nulls.

[smitpatel]

Should property getter for indexer property account for the fact that object is returned from indexer (which could be null) and convert it to appropriate CLR default?

[ajcvickers]

@smitpatel Possibly; I can look at making it act like a nulable backing field.