XINCGer/Unity3DTraining · error · DescriptorValidationException

Messages can't have default values.

Error message

Messages can't have default values.

What it means

Proto3/message-typed fields cannot carry a default value; defaults only apply to scalar fields (proto2). During CrossLink, after resolving a message-typed field, a non-empty Proto.DefaultValue triggers DescriptorValidationException.

Solutions

1. Remove the DefaultValue from the message-typed field's proto and regenerate.
2. Set message fields via initialization in code after parsing instead of via default_value.
3. Strip/ignore default_value for message fields when importing descriptors from external sources.

Example fix

// before
new FieldDescriptorProto { Name = "addr", Type = TYPE_MESSAGE, TypeName = "Address", DefaultValue = "" }
// after
new FieldDescriptorProto { Name = "addr", Type = TYPE_MESSAGE, TypeName = "Address" }

if (field.Proto.Type == FieldDescriptorProto.Types.Type.Message && field.Proto.DefaultValue != "")
    throw new InvalidDataException("Message field " + field.Name + " must not have a default value");

bool HasNoMessageDefault(FieldDescriptorProto f) =>
    f.Type != FieldDescriptorProto.Types.Type.Message || string.IsNullOrEmpty(f.DefaultValue);

try { descriptor = FileDescriptor.BuildFromByteStrings(data); }
catch (DescriptorValidationException ex) when (ex.Message == "Messages can't have default values.")
{ log.LogError(ex, "Remove default_value from message-typed fields"); }

When it happens

Trigger: A FieldDescriptorProto with Type = TYPE_MESSAGE and a non-empty DefaultValue string — e.g. hand-built descriptors, descriptors edited by tools, or protos written with invalid default_value on a message field.

Common situations: Porting proto2 scalar field configs onto message fields; code generators that emit default_value unconditionally; manual editing of descriptor sets.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• FieldDescriptorProto.oneof_index is out of range for type
• Invalid type specified
• "TypeName" is not a type.
• "TypeName" is not a message type.
• "TypeName" is not an enum type.

AI-assisted analysis of XINCGer/Unity3DTraining@016f98412e (2026-09-12). Data as JSON: /api/errors/87332da8159de264. Report an issue: GitHub.

Appendix: source

Thrown at NetWorkAndResources/Socket_Protobuff/Assets/Plugins/Google.Protobuf/Reflection/FieldDescriptor.cs:331

                        fieldType = FieldType.Enum;
                    }
                    else
                    {
                        throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not a type.");
                    }
                }

                if (fieldType == FieldType.Message)
                {
                    if (!(typeDescriptor is MessageDescriptor))
                    {
                        throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not a message type.");
                    }
                    messageType = (MessageDescriptor)typeDescriptor;

                    if (Proto.DefaultValue != "")
                    {
                        throw new DescriptorValidationException(this, "Messages can't have default values.");
                    }
                }
                else if (fieldType == FieldType.Enum)
                {
                    if (!(typeDescriptor is EnumDescriptor))
                    {
                        throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not an enum type.");
                    }
                    enumType = (EnumDescriptor)typeDescriptor;
                }
                else
                {
                    throw new DescriptorValidationException(this, "Field with primitive type has type_name.");
                }
            }
            else
            {
                if (fieldType == FieldType.Message || fieldType == FieldType.Enum)

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