Applied a variation of @radical's idea.

src/mono/wasm/debugger/BrowserDebugProxy/Common/HelperExtensions.cs

@@ -5,27 +5,13 @@
 
 using System;
 using Newtonsoft.Json.Linq;
-using BrowserDebugProxy;
-using System.Collections.Generic;
-using System.Linq;
 
 namespace Microsoft.WebAssembly.Diagnostics;
 
 internal static class HelperExtensions
 {
     private const int MaxLogMessageLineLength = 65536;
     private static readonly bool TruncateLogMessages = string.IsNullOrEmpty(Environment.GetEnvironmentVariable("WASM_DONT_TRUNCATE_LOG_MESSAGES"));
-    private static Dictionary<ProxyInternalUseProperty, string> proxyInternalUsePropNames = new Dictionary<ProxyInternalUseProperty, string>() {
-        { ProxyInternalUseProperty.Hidden, "__hidden" },
-        { ProxyInternalUseProperty.State, "__state" },
-        { ProxyInternalUseProperty.Section, "__section" },
-        { ProxyInternalUseProperty.Owner, "__owner" },
-        { ProxyInternalUseProperty.IsStatic, "__isStatic" },
-        { ProxyInternalUseProperty.IsNewSlot, "__isNewSlot" },
-        { ProxyInternalUseProperty.IsBackingField, "__isBackingField" },
-        { ProxyInternalUseProperty.ParentTypeId, "__parentTypeId" }
-    };
-    private static Dictionary<string, ProxyInternalUseProperty> proxyInternalUsePropNamesInverse = proxyInternalUsePropNames.ToDictionary((i) => i.Value, (i) => i.Key);
 
     public static string Truncate(this string message, int maxLen, string suffix = "")
 
@@ -43,9 +29,6 @@ public static void AddRange(this JArray arr, JArray addedArr)
             arr.Add(item);
     }
 
-    public static string ToUnderscoredString(this ProxyInternalUseProperty key) => proxyInternalUsePropNames[key];
-    public static bool TryConvertToProxyInternalUseProperty(this string key) => proxyInternalUsePropNamesInverse.TryGetValue(key, out _);
-
     public static bool IsNullValuedObject(this JObject obj)
         => obj != null && obj["type"]?.Value<string>() == "object" && obj["subtype"]?.Value<string>() == "null";
 }

src/mono/wasm/debugger/BrowserDebugProxy/Common/InternalUseFieldName.cs

@@ -0,0 +1,35 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System.Collections.Generic;
+
+namespace Microsoft.WebAssembly.Diagnostics;
+
+internal sealed class InternalUseFieldName
+{
+    public static string Hidden => "__hidden__";
+    public static string State => "__state__";
+    public static string Section => "__section__";
+    public static string Owner => "__owner__";
+    public static string IsStatic => "__isStatic__";
+    public static string IsNewSlot => "__isNewSlot__";
+    public static string IsBackingField => "__isBackingField__";
+    public static string ParentTypeId => "__parentTypeId__";
+
+    private static readonly HashSet<string> s_names = new()
+    {
+        Hidden,
+        State,
+        Section,
+        Owner,
+        IsStatic,
+        IsNewSlot,
+        IsBackingField,
+        ParentTypeId
+    };
+
+    public static int Count => s_names.Count;
+    public static bool IsKnown(string name) => !string.IsNullOrEmpty(name) && s_names.Contains(name);
+}

src/mono/wasm/debugger/BrowserDebugProxy/MemberObjectsExplorer.cs

@@ -63,17 +63,17 @@ private static async Task<JObject> ReadFieldValue(
                 // for backing fields, we are getting it from the properties
                 typePropertiesBrowsableInfo.TryGetValue(field.Name, out state);
             }
-            fieldValue[ProxyInternalUseProperty.State.ToUnderscoredString()] = state?.ToString();
-            fieldValue[ProxyInternalUseProperty.Section.ToUnderscoredString()] = field.Attributes.HasFlag(FieldAttributes.Private)
+            fieldValue[InternalUseFieldName.State] = state?.ToString();
+            fieldValue[InternalUseFieldName.Section] = field.Attributes.HasFlag(FieldAttributes.Private)
                 ? "private" : "result";
 
             if (field.IsBackingField)
             {
-                fieldValue[ProxyInternalUseProperty.IsBackingField.ToUnderscoredString()] = true;
-                fieldValue[ProxyInternalUseProperty.ParentTypeId.ToUnderscoredString()] = parentTypeId;
+                fieldValue[InternalUseFieldName.IsBackingField] = true;
+                fieldValue[InternalUseFieldName.ParentTypeId] = parentTypeId;
             }
             if (field.Attributes.HasFlag(FieldAttributes.Static))
-                fieldValue[ProxyInternalUseProperty.IsStatic.ToUnderscoredString()] = true;
+                fieldValue[InternalUseFieldName.IsStatic] = true;
 
             if (getObjectOptions.HasFlag(GetObjectCommandOptions.WithSetter))
             {
@@ -241,7 +241,7 @@ public static async Task<JArray> ExpandFieldValues(
                 if (typeInfo.Info.IsNonUserCode && getCommandOptions.HasFlag(GetObjectCommandOptions.JustMyCode) && field.Attributes.HasFlag(FieldAttributes.Private))
                     continue;
 
-                if (!Enum.TryParse(fieldValue["__state"].Value<string>(), out DebuggerBrowsableState fieldState)
+                if (!Enum.TryParse(fieldValue[InternalUseFieldName.State].Value<string>(), out DebuggerBrowsableState fieldState)
                     || fieldState == DebuggerBrowsableState.Collapsed)
                 {
                     fieldValues.Add(fieldValue);
@@ -303,11 +303,9 @@ public static async Task<JArray> GetExpandedMemberValues(
 
             JArray GetHiddenElement()
             {
-                return new JArray(JObject.FromObject(new
-                {
-                    name = namePrefix,
-                    __hidden = true
-                }));
+                var emptyHidden = JObject.FromObject(new { name = namePrefix });
+                emptyHidden.Add(InternalUseFieldName.Hidden, true);
+                return new JArray(emptyHidden);
             }
         }
 
@@ -368,14 +366,14 @@ public static async Task<Dictionary<string, JObject>> ExpandPropertyValues(
                     continue;
                 }
 
-                bool isExistingMemberABackingField = existingMember[ProxyInternalUseProperty.IsBackingField.ToUnderscoredString()]?.Value<bool>() == true;
+                bool isExistingMemberABackingField = existingMember[InternalUseFieldName.IsBackingField]?.Value<bool>() == true;
                 if (isOwn && !isExistingMemberABackingField)
                 {
                     // repeated propname on the same type! cannot happen
                     throw new Exception($"Internal Error: should not happen. propName: {propName}. Existing all members: {string.Join(",", allMembers.Keys)}");
                 }
 
-                bool isExistingMemberABackingFieldOwnedByThisType = isExistingMemberABackingField && existingMember[ProxyInternalUseProperty.Owner.ToUnderscoredString()]?.Value<string>() == typeName;
+                bool isExistingMemberABackingFieldOwnedByThisType = isExistingMemberABackingField && existingMember[InternalUseFieldName.Owner]?.Value<string>() == typeName;
                 if (isExistingMemberABackingField && (isOwn || isExistingMemberABackingFieldOwnedByThisType))
                 {
                     // this is the property corresponding to the backing field in *this* type
@@ -389,8 +387,8 @@ public static async Task<Dictionary<string, JObject>> ExpandPropertyValues(
                 {
                     // this has `new` keyword if it is newSlot but direct child was not a newSlot:
                     var child = allMembers.FirstOrDefault(
-                        kvp => (kvp.Key == propName || kvp.Key.StartsWith($"{propName} (")) && kvp.Value[ProxyInternalUseProperty.ParentTypeId.ToUnderscoredString()]?.Value<int>() == typeId).Value;
-                    bool wasOverriddenByDerivedType = child != null && child[ProxyInternalUseProperty.IsNewSlot.ToUnderscoredString()]?.Value<bool>() != true;
+                        kvp => (kvp.Key == propName || kvp.Key.StartsWith($"{propName} (")) && kvp.Value[InternalUseFieldName.ParentTypeId]?.Value<int>() == typeId).Value;
+                    bool wasOverriddenByDerivedType = child != null && child[InternalUseFieldName.IsNewSlot]?.Value<bool>() != true;
                     if (wasOverriddenByDerivedType)
                     {
                         /*
@@ -416,7 +414,7 @@ public static async Task<Dictionary<string, JObject>> ExpandPropertyValues(
                  */
 
                 JObject backingFieldForHiddenProp = allMembers.GetValueOrDefault(overriddenOrHiddenPropName);
-                if (backingFieldForHiddenProp is null || backingFieldForHiddenProp[ProxyInternalUseProperty.IsBackingField.ToUnderscoredString()]?.Value<bool>() != true)
+                if (backingFieldForHiddenProp is null || backingFieldForHiddenProp[InternalUseFieldName.IsBackingField]?.Value<bool>() != true)
                 {
                     // hiding with a non-auto property, so nothing to adjust
                     // add the new property
@@ -431,12 +429,12 @@ public static async Task<Dictionary<string, JObject>> ExpandPropertyValues(
 
             async Task UpdateBackingFieldWithPropertyAttributes(JObject backingField, string autoPropName, MethodAttributes getterMemberAccessAttrs, DebuggerBrowsableState? state)
             {
-                backingField[ProxyInternalUseProperty.Section.ToUnderscoredString()] = getterMemberAccessAttrs switch
+                backingField[InternalUseFieldName.Section] = getterMemberAccessAttrs switch
                 {
                     MethodAttributes.Private => "private",
                     _ => "result"
                 };
-                backingField[ProxyInternalUseProperty.State.ToUnderscoredString()] = state?.ToString();
+                backingField[InternalUseFieldName.State] = state?.ToString();
 
                 if (state is null)
                     return;
@@ -479,16 +477,16 @@ async Task AddProperty(
                 }
 
                 propRet["isOwn"] = isOwn;
-                propRet[ProxyInternalUseProperty.Section.ToUnderscoredString()] = getterAttrs switch
+                propRet[InternalUseFieldName.Section] = getterAttrs switch
                 {
                     MethodAttributes.Private => "private",
                     _ => "result"
                 };
-                propRet[ProxyInternalUseProperty.State.ToUnderscoredString()] = state?.ToString();
+                propRet[InternalUseFieldName.State] = state?.ToString();
                 if (parentTypeId != -1)
                 {
-                    propRet[ProxyInternalUseProperty.ParentTypeId.ToUnderscoredString()] = parentTypeId;
-                    propRet[ProxyInternalUseProperty.IsNewSlot.ToUnderscoredString()] = isNewSlot;
+                    propRet[InternalUseFieldName.ParentTypeId] = parentTypeId;
+                    propRet[InternalUseFieldName.IsNewSlot] = isNewSlot;
                 }
 
                 string namePrefix = GetNamePrefixForValues(propNameWithSufix, typeName, isOwn, state);
@@ -593,7 +591,7 @@ public static async Task<GetMembersResult> GetObjectMemberValues(
                     if (getCommandType.HasFlag(GetObjectCommandOptions.AccessorPropertiesOnly))
                     {
                         foreach (var f in allFields)
-                            f[ProxyInternalUseProperty.Hidden.ToUnderscoredString()] = true;
+                            f[InternalUseFieldName.Hidden] = true;
                     }
                     AddOnlyNewFieldValuesByNameTo(allFields, allMembers, typeName, isOwn);
                 }
@@ -639,8 +637,8 @@ static void AddOnlyNewFieldValuesByNameTo(JArray namedValues, IDictionary<string
                     if (valuesDict.TryAdd(name, item as JObject))
                     {
                         // new member
-                        if (item[ProxyInternalUseProperty.IsBackingField.ToUnderscoredString()]?.Value<bool>() == true)
-                            item[ProxyInternalUseProperty.Owner.ToUnderscoredString()] = typeName;
+                        if (item[InternalUseFieldName.IsBackingField]?.Value<bool>() == true)
+                            item[InternalUseFieldName.Owner] = typeName;
                         continue;
                     }
 
@@ -657,18 +655,6 @@ static void AddOnlyNewFieldValuesByNameTo(JArray namedValues, IDictionary<string
 
     }
 
-    public enum ProxyInternalUseProperty
-    {
-        Hidden,
-        State,
-        Section,
-        Owner,
-        IsStatic,
-        IsNewSlot,
-        IsBackingField,
-        ParentTypeId
-    }
-
     internal sealed class GetMembersResult
     {
         // public / protected / internal:
@@ -697,17 +683,27 @@ public GetMembersResult(JArray value, bool sortByAccessLevel)
 
         public void CleanUp()
         {
+            JProperty[] toRemoveInObject = new JProperty[InternalUseFieldName.Count];
+
             CleanUpJArray(Result);
             CleanUpJArray(PrivateMembers);
-            static void CleanUpJArray(JArray arr)
+
+            void CleanUpJArray(JArray arr)
             {
-                foreach(var item in arr)
+                foreach (JToken item in arr)
                 {
-                    item.Children().Where(x =>
-                        x is JProperty p &&
-                        p.Name.TryConvertToProxyInternalUseProperty())
-                    .ToList()
-                    .ForEach(x => x.Remove());
+                    if (item is not JObject jobj || jobj.Count == 0)
+                        continue;
+
+                    int removeCount = 0;
+                    foreach (JProperty jp in jobj.Properties())
+                    {
+                        if (InternalUseFieldName.IsKnown(jp.Name))
+                            toRemoveInObject[removeCount++] = jp;
+                    }
+
+                    for (int i = 0; i < removeCount; i++)
+                        toRemoveInObject[i].Remove();
                 }
             }
         }
@@ -730,10 +726,10 @@ public static GetMembersResult FromValues(JArray values, bool splitMembersByAcce
 
         private void Split(JToken member)
         {
-            if (member[ProxyInternalUseProperty.Hidden.ToUnderscoredString()]?.Value<bool>() == true)
+            if (member[InternalUseFieldName.Hidden]?.Value<bool>() == true)
                 return;
 
-            if (member[ProxyInternalUseProperty.Section.ToUnderscoredString()]?.Value<string>() is not string section)
+            if (member[InternalUseFieldName.Section]?.Value<string>() is not string section)
             {
                 Result.Add(member);
                 return;

src/mono/wasm/debugger/BrowserDebugProxy/ValueTypeClass.cs

@@ -98,15 +98,15 @@ JObject GetFieldWithMetadata(FieldTypeClass field, JObject fieldValue, bool isSt
                 if (isStatic)
                     fieldValue["name"] = field.Name;
                 FieldAttributes attr = field.Attributes & FieldAttributes.FieldAccessMask;
-                fieldValue[ProxyInternalUseProperty.Section.ToUnderscoredString()] = attr == FieldAttributes.Private ? "private" : "result";
+                fieldValue[InternalUseFieldName.Section] = attr == FieldAttributes.Private ? "private" : "result";
 
                 if (field.IsBackingField)
                 {
-                    fieldValue[ProxyInternalUseProperty.IsBackingField.ToUnderscoredString()] = true;
+                    fieldValue[InternalUseFieldName.IsBackingField] = true;
                     return fieldValue;
                 }
                 typeFieldsBrowsableInfo.TryGetValue(field.Name, out DebuggerBrowsableState? state);
-                fieldValue[ProxyInternalUseProperty.State.ToUnderscoredString()] = state?.ToString();
+                fieldValue[InternalUseFieldName.State] = state?.ToString();
                 return fieldValue;
             }
         }
@@ -244,7 +244,7 @@ public async Task ExpandedFieldValues(MonoSDBHelper sdbHelper, bool includeStati
             JArray visibleFields = new();
             foreach (JObject field in fields)
             {
-                if (!Enum.TryParse(field["__state"]?.Value<string>(), out DebuggerBrowsableState state))
+                if (!Enum.TryParse(field[InternalUseFieldName.State]?.Value<string>(), out DebuggerBrowsableState state))
                 {
                     visibleFields.Add(field);
                     continue;

src/mono/wasm/debugger/DebuggerTestSuite/DebuggerTestBase.cs

@@ -925,6 +925,18 @@ internal async Task<JToken> GetObjectOnLocals(JToken locals, string name)
             return await GetProperties(objectId);
         }
 
+        internal void AssertInternalUseFieldsAreRemoved(JToken item)
+        {
+            if (item is JObject jobj && jobj.Count != 0)
+            {
+                foreach (JProperty jp in jobj.Properties())
+                {
+                    Assert.False(InternalUseFieldName.IsKnown(jp.Name),
+                     $"Property {jp.Name} of object: {jobj} is for internal proxy use and should not be exposed externally.");
+                }
+            }
+        }
+
         /* @fn_args is for use with `Runtime.callFunctionOn` only */
         internal virtual async Task<JToken> GetProperties(string id, JToken fn_args = null, bool? own_properties = null, bool? accessors_only = null, bool expect_ok = true)
         {
@@ -978,6 +990,7 @@ internal virtual async Task<JToken> GetProperties(string id, JToken fn_args = nu
             {
                 foreach (var p in locals)
                 {
+                    AssertInternalUseFieldsAreRemoved(p);
                     if (p["name"]?.Value<string>() == "length" && p["enumerable"]?.Value<bool>() != true)
                     {
                         p.Remove();
@@ -1035,6 +1048,7 @@ internal virtual async Task<JToken> GetProperties(string id, JToken fn_args = nu
             {
                 foreach (var p in locals)
                 {
+                    AssertInternalUseFieldsAreRemoved(p);
                     if (p["name"]?.Value<string>() == "length" && p["enumerable"]?.Value<bool>() != true)
                     {
                         p.Remove();