stride3d/stride · error · YamlException

Cannot instantiate an object for type

Error message

Cannot instantiate an object for type [{objectContext.Descriptor}]

What it means

ObjectSerializer.ReadMembers expects the object instance to have been created before reading its members. If objectContext.Instance is still null after the instantiation step, a YamlException with the node's span is thrown — the library could not construct an object of the target type.

Solutions

1. Check for a prior exception/activation failure — the constructor or ObjectFactory likely threw or returned null
2. Register realignment (abstract type -> concrete type) so the concrete type can be instantiated
3. Ensure the type has an accessible parameterless or matching constructor
4. Verify the YAML contains the concrete type tag when serializing polymorphic members

Example fix

// before
// property typed IFoo serialized as plain mapping, no concrete type
myFoo:
  Bar: 1
// after
myFoo:
  Bar: 1
  ~ConcreteFoo: {}   // or ensure type tag / realignment is emitted

if (targetType.IsAbstract || targetType.IsInterface)
    serializerContext.Services?.GetService<RealOffset>();
// ensure realignment registered:
if (!factory.CanInstantiate(targetType))
    throw new InvalidOperationException($"Register a concrete type for {targetType}");

static bool CanActivate(Type t) => !t.IsAbstract && !t.IsInterface && t.GetConstructor(Type.EmptyTypes) != null;

try
{
    objectSerializer.ReadYaml(ref objectContext);
}
catch (YamlException ex) when (ex.Message.Contains("Cannot instantiate an object for type"))
{
    // register realignment or supply an ObjectFactory, then retry
}

When it happens

Trigger: Deserializing a mapping node whose target type cannot be instantiated: abstract type without a registered concrete mapping, type with no usable constructor, constructor throwing, or null produced by a custom ObjectFactory.

Common situations: Polymorphic YAML lacking the concrete type discriminator; constructor signature changed after refactor/upgrade; serializer for interface-typed members without realignment; activation errors in AOT/trimming environments.

Understand the failure class

Background: "failed to unmarshal" / json.Unmarshal errors: why parsing a response into a Go struct fails and how to fix it — this error's family across 23 libraries.

Related errors

• Cannot add item with key
• Cannot deserialize collection to readonly collection type
• Cannot deserialize collection to type
• Cannot set member [ . ]
• ex.Message

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/f3c375d99d3ed270. Report an issue: GitHub.

Appendix: source

Thrown at sources/core/Stride.Core.Yaml/Serialization/Serializers/ObjectSerializer.cs:174

        /// <summary>
        /// Reads the members from the current stream.
        /// </summary>
        /// <typeparam name="TStart">The type of the t start.</typeparam>
        /// <typeparam name="TEnd">The type of the t end.</typeparam>
        /// <param name="objectContext"></param>
        /// <returns>Return the object being read, by default thisObject passed by argument.</returns>
        protected virtual void ReadMembers<TStart, TEnd>(ref ObjectContext objectContext)
            where TStart : NodeEvent
            where TEnd : ParsingEvent
        {
            var reader = objectContext.Reader;
            var start = reader.Expect<TStart>();

            // throws an exception while deserializing
            if (objectContext.Instance == null)
            {
                throw new YamlException(start.Start, start.End, $"Cannot instantiate an object for type [{objectContext.Descriptor}]");
            }

            while (!reader.Accept<TEnd>())
            {
                ReadMember(ref objectContext);
            }
            reader.Expect<TEnd>();
        }

        /// <summary>
        /// Reads an item of the object from the YAML flow (either a sequence item or mapping key/value item).
        /// </summary>
        /// <param name="objectContext"></param>
        /// <exception cref="YamlException">Unable to deserialize property [{0}] not found in type [{1}].DoFormat(propertyName, typeDescriptor)</exception>
        protected virtual void ReadMember(ref ObjectContext objectContext)
        {
            Scalar memberScalar;
            string memberName;

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