stride3d/stride · error · YamlException

Unable to find a type for this element

Error message

Unable to find a type for this element [{node}]

What it means

TagTypeSerializer.ReadYaml throws this when it cannot determine a .NET type for the node being read (type == null) and the node also has no value to infer the type from (value == null). Without a tag, descriptor type, or value, deserialization cannot proceed.

Solutions

1. Add an explicit type tag to the YAML node so the serializer knows the type.
2. Ensure the target property/field has a concrete declared type (not object) in the model.
3. Fix alias anchors so they point to existing nodes with values.
4. Check serialization output of the same model — if it emits untyped nulls it cannot read back, adjust the model or serializer settings.

Example fix

// before (YAML)
items:
  -
// after
items:
  - !MyApp.Item
    id: 1

static bool NodeIsUntypedNull(YamlNode node) => node is YamlScalarNode s && (s.Value == null || s.Value == "~") && string.IsNullOrEmpty(s.Tag);

try { return serializer.Deserialize(reader); } catch (YamlException ex) when (ex.Message.StartsWith("Unable to find a type")) { throw new InvalidOperationException("YAML contains an untyped null element; add a type tag or give the target member a concrete type", ex); }

When it happens

Trigger: Deserializing a node whose type cannot be derived: no tag, no compatible expected-type descriptor, and the produced value is null — typically an aliased/untagged null node in a context where the expected type is unknown.

Common situations: YAML documents with untyped null entries in polymorphic collections; alias anchors resolving to nothing; schema configuration where the property descriptor's type was not resolved; hand-written YAML omitting tags where the library requires them.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Unable to resolve tag
• Cannot add item with key
• Cannot deserialize collection to readonly collection type
• Cannot deserialize collection to type
• Cannot instantiate an object for type

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/53fd327217343c51. Report an issue: GitHub.

Appendix: source

Thrown at sources/core/Stride.Core.Yaml/Serialization/Serializers/TagTypeSerializer.cs:131

            // This allow to load any YAML documents into dictionary/list
            // automatically
            if ((type == null || type == typeof(object)) && value == null)
            {
                // If the node is a sequence start, fallback to a IList<object>
                if (node is SequenceStart)
                {
                    type = typeof(IList<object>);
                }
                else if (node is MappingStart)
                {
                    // If the node is a mapping start, fallback to a IDictionary<object, object>
                    type = typeof(IDictionary<object, object>);
                }
            }

            if (type == null && value == null)
            {
                throw new YamlException(node.Start, node.End, $"Unable to find a type for this element [{node}]");
            }

            if (type == null)
            {
                type = value.GetType();
            }
            else if (typeFromTag != null && value != null && value.GetType() != typeFromTag)
            {
                // Reset the instance if the value loaded from the tag is not of the same type then the value already instantiated
                type = typeFromTag;
                objectContext.Instance = null;
            }

            objectContext.Descriptor = objectContext.SerializerContext.FindTypeDescriptor(type);

            // If this is a nullable descriptor, use its underlying type directly
            if (objectContext.Descriptor is NullableDescriptor)
            {

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