Serializing a Dictionary in a VisualElement (Unity 2023.2.0a9)

SerializedDictionaryExample.cs

using System;
using System.Collections.Generic;
using UnityEngine.UIElements;

[Serializable]
class DictItem
{
    public int key;
    public string value;
}

[Serializable]
class MyDictionary : Dictionary<int, string>
{
    // Shown in the inspector
    public List<DictItem> dictItems = new List<DictItem>();
}

// Use the dictionary in an element
[UxmlElement]
public partial class DictionaryExample : VisualElement
{
    [UxmlAttribute]
    internal MyDictionary myDictionary
    {
        get;
        set;
    }
}

SerializedDictionaryExampleEditor.cs

using System;
using System.Collections.Generic;
using System.Text;
using UnityEditor;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;
using Random = UnityEngine.Random;

// Convert to and from UXML
class DictConverter : UxmlAttributeConverter<MyDictionary>
{
    public override MyDictionary FromString(string value)
    {
        var d = new MyDictionary();
        var items = value.Split(',', StringSplitOptions.RemoveEmptyEntries);
        foreach (var item in items)
        {
            var fields = item.Split('|');
            var fieldKey = int.Parse(fields[0]);
            var fieldValue = fields[1];

            // For serializing in the inspector
            d.dictItems.Add(new DictItem
            {
                key = fieldKey,
                value = fieldValue
            });

            // For the actual dictionary
            d[fieldKey] = fieldValue;
        }
        return d;
    }

    public override string ToString(MyDictionary value)
    {
        var sb = new StringBuilder();
        foreach(var kvp in value.dictItems)
        {
            sb.Append(kvp.key);
            sb.Append("|");
            sb.Append(kvp.value);
            sb.Append(',');
        }
        return sb.ToString();
    }
}

[CustomPropertyDrawer(typeof(MyDictionary))]
class MyCustomDrawerPropertyDrawer : PropertyDrawer
{
    public override VisualElement CreatePropertyGUI(SerializedProperty property)
    {
        var items = property.FindPropertyRelative("dictItems");

        var list = new ListView();
        list.headerTitle = "My Dict";
        list.showFoldoutHeader = true;
        list.showAddRemoveFooter = true;
        list.virtualizationMethod = CollectionVirtualizationMethod.DynamicHeight;

        list.makeItem = () =>
        {
            var row = new VisualElement { style = { flexDirection = FlexDirection.Row } };
            row.Add(new IntegerField { name = "key", style = { flexGrow = 1 } });
            row.Add(new TextField { style = { flexGrow = 1 } });

            row.Q<IntegerField>().RegisterValueChangedCallback(evt =>
            {
                var keys = list.Query<IntegerField>("key").ToList();

                // Reset and collect duplicates
                var allKeys = new Dictionary<int, List<VisualElement>>();
                foreach (var key in keys)
                {
                    key.parent.style.backgroundColor = StyleKeyword.Initial;
                    if (!allKeys.TryGetValue(key.value, out var list))
                    {
                        list = new List<VisualElement>();
                        allKeys.Add(key.value, list);
                    }
                    list.Add(key.parent);
                }

                // Mark duplicates
                foreach (var kvp in allKeys)
                {
                    if (kvp.Value.Count > 1)
                    {
                        Random.InitState(kvp.Key);
                        var color = new Color(UnityEngine.Random.value, UnityEngine.Random.value, UnityEngine.Random.value);
                        foreach (var key in kvp.Value)
                        {
                            key.style.backgroundColor = color;
                        }
                    }
                }
            });

            return row;
        };

        list.bindItem += (ve, i) =>
        {
            var prop = items.GetArrayElementAtIndex(i);
            ve.Q<IntegerField>()?.BindProperty(prop.FindPropertyRelative("key"));
            ve.Q<TextField>()?.BindProperty(prop.FindPropertyRelative("value"));
        };

        list.BindProperty(items);
        return list;
    }
}

This example shows how a dictionary can be serialized into a UxmlAttribute.
Note this requires Unity 2023.2.0a9+