An editor tool that allows you inspect the contents of Fish-Networking's sync collections. (https://ko-fi.com/winterboltgames)

SyncCollectionViewer.cs

using FishNet.Object;
using System.Collections.Generic;
using System.Reflection;
using System;
using UnityEditor;
using UnityEngine;
using System.Collections;
using System.Linq;
using FishNet.Object.Synchronizing;

internal sealed class SyncCollectionViewer : EditorWindow
{
	private Vector2 _scroll = Vector2.zero;

	private NetworkBehaviour[] _networkBehaviours = Array.Empty<NetworkBehaviour>();

	private NetworkBehaviour _selectedNetworkBehaviour;

	private FieldInfo[] _syncCollectionFields = Array.Empty<FieldInfo>();

	private readonly Dictionary<FieldInfo, bool> _fieldToggles = new Dictionary<FieldInfo, bool>();

	[MenuItem("Fish-Networking/Tools/Sync Collection Viewer")]
	private static void ShowWindow()
	{
		SyncCollectionViewer syncCollectionViewer = GetWindow<SyncCollectionViewer>();

		syncCollectionViewer.titleContent = new GUIContent("Sync Collection Viewer");
	}

	private void OnEnable()
	{
		EditorApplication.update += Repaint;

		RefreshNetworkBehaviours();
	}

	private void OnDisable()
	{
		EditorApplication.update -= Repaint;
	}

	private void OnGUI()
	{
		if (_selectedNetworkBehaviour == null)
		{
			DrawNetworkBehaviourSelectionGui();
		}
		else
		{
			DrawSyncCollectionViewerGui();
		}
	}

	private void RefreshNetworkBehaviours()
	{
		_networkBehaviours = FindObjectsOfType<NetworkBehaviour>();
	}

	private void SelectNetworkBehaviour(NetworkBehaviour networkBehaviour)
	{
		if (networkBehaviour == null) return;

		_selectedNetworkBehaviour = networkBehaviour;

		_syncCollectionFields = _selectedNetworkBehaviour.GetType().GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)
			.Where(f => f.FieldType.IsGenericType && (f.FieldType.GetGenericTypeDefinition() == typeof(SyncList<>) || f.FieldType.GetGenericTypeDefinition() == typeof(SyncHashSet<>) || f.FieldType.GetGenericTypeDefinition() == typeof(SyncDictionary<,>)))
			.ToArray();

		_fieldToggles.Clear();

		foreach (FieldInfo fieldInfo in _syncCollectionFields)
		{
			_fieldToggles.Add(fieldInfo, false);
		}
	}

	private void DrawNetworkBehaviourSelectionGui()
	{
		if (GUILayout.Button("Refresh")) RefreshNetworkBehaviours();

		_scroll = EditorGUILayout.BeginScrollView(_scroll);

		if (_networkBehaviours.Length == 0)
		{
			EditorGUILayout.HelpBox("No NetworkBehaviours found in scene.", MessageType.Info);
		}
		else
		{
			for (int i = 0; i < _networkBehaviours.Length; i++)
			{
				NetworkBehaviour networkBehaviour = _networkBehaviours[i];

				if (networkBehaviour == null) continue;

				if (GUILayout.Button(networkBehaviour.name)) SelectNetworkBehaviour(networkBehaviour);
			}
		}

		EditorGUILayout.EndScrollView();
	}

	private void DeselectNetworkBehaviour()
	{
		_selectedNetworkBehaviour = null;

		_syncCollectionFields = Array.Empty<FieldInfo>();

		_fieldToggles.Clear();
	}

	private void DrawSyncCollectionItem(object item)
	{
		if (item == null) return;

		if (item is UnityEngine.Object unityEngineObject)
		{
			GUI.enabled = false;

			EditorGUILayout.ObjectField(unityEngineObject, unityEngineObject.GetType(), true);

			GUI.enabled = true;
		}
		else
		{
			Type itemType = item.GetType();

			if (itemType.IsGenericType && itemType.GetGenericTypeDefinition() == typeof(KeyValuePair<,>))
			{
				EditorGUILayout.BeginHorizontal();

				EditorGUILayout.LabelField("Key");

				DrawSyncCollectionItem(itemType.GetProperty("Key").GetValue(item));

				EditorGUILayout.EndHorizontal();

				EditorGUILayout.BeginHorizontal();

				EditorGUILayout.LabelField("Value");

				DrawSyncCollectionItem(itemType.GetProperty("Value").GetValue(item));

				EditorGUILayout.EndHorizontal();
			}
			else
			{
				EditorGUILayout.LabelField(item.ToString());
			}
		}
	}

	private void DrawSyncCollectionViewerGui()
	{
		if (GUILayout.Button("Deselect"))
		{
			DeselectNetworkBehaviour();

			return;
		}

		_scroll = EditorGUILayout.BeginScrollView(_scroll);

		foreach (FieldInfo syncCollectionField in _syncCollectionFields)
		{
			IEnumerable syncCollection = syncCollectionField.GetValue(_selectedNetworkBehaviour) as IEnumerable;

			_fieldToggles[syncCollectionField] = EditorGUILayout.Foldout(_fieldToggles[syncCollectionField], syncCollectionField.Name);

			if (_fieldToggles[syncCollectionField])
			{
				EditorGUILayout.BeginVertical(EditorStyles.helpBox);

				foreach (object item in syncCollection)
				{
					EditorGUILayout.BeginVertical(EditorStyles.helpBox);

					DrawSyncCollectionItem(item);

					EditorGUILayout.EndVertical();
				}

				EditorGUILayout.EndVertical();
			}
		}

		EditorGUILayout.EndScrollView();
	}
}