microsoft/aspire · error · InvalidOperationException

Expected KubernetesManifestResource but got

Error message

Expected KubernetesManifestResource but got {value?.GetType()}.

What it means

WriteYaml of KubernetesManifestResourceYamlConverter expects the value being serialized to be exactly a KubernetesManifestResource. Any other type reaching this converter is an InvalidOperationException, indicating the converter was invoked for a type it does not handle (it only claims typeof(KubernetesManifestResource) in Accepts).

Solutions

Pass the KubernetesManifestResource instance itself to the serializer, not a derived wrapper or unrelated object.
Verify converter registration: KubernetesManifestResourceYamlConverter should be bound only to typeof(KubernetesManifestResource).
If serializing related models, write dedicated converters for those types rather than reusing this one.

Example fix

// before
serializer.Serialize(writer, manifestWrapper); // wrong object type
// after
serializer.Serialize(writer, kubernetesManifestResource);

Defensive patterns

Strategy: type-guard

Validate before calling

if (value is not KubernetesManifestResource) throw new ArgumentException("Serializer expects KubernetesManifestResource.");

Type guard

static bool IsManifestResource(object? v) => v is KubernetesManifestResource;

Try / catch

try { yamlSerializer.Serialize(writer, value); } catch (InvalidOperationException ex) when (ex.Message.Contains("KubernetesManifestResource")) { /* pass the manifest resource itself */ }

Prevention

Serialize the KubernetesManifestResource instance itself, not wrappers
Bind this converter only to typeof(KubernetesManifestResource)
Cover manifest serialization with a snapshot test

When it happens

Trigger: Serializing an object graph where the converter is applied to a non-KubernetesManifestResource value — e.g. registering the converter globally and serializing other types, or passing a subclass/wrapper instead of the manifest resource (src/Aspire.Hosting.Kubernetes/Yaml/KubernetesManifestResourceYamlConverter.cs:26).

Common situations: Custom YamlDotNet pipelines that mis-route types; serializing a wrapper or projection of the manifest instead of the resource itself; reflection-based tooling that pulls the wrong object out of the app model.

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

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/7ee106c1171b10c3. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Kubernetes/Yaml/KubernetesManifestResourceYamlConverter.cs:26

namespace Aspire.Hosting.Kubernetes.Yaml;

internal sealed class KubernetesManifestResourceYamlConverter : IYamlTypeConverter
{
    public bool Accepts(Type type)
    {
        return type == typeof(KubernetesManifestResource);
    }

    public object? ReadYaml(IParser parser, Type type, ObjectDeserializer rootDeserializer)
    {
        throw new NotSupportedException($"{nameof(KubernetesManifestResource)} does not support YAML deserialization.");
    }

    public void WriteYaml(IEmitter emitter, object? value, Type type, ObjectSerializer serializer)
    {
        if (value is not KubernetesManifestResource manifest)
        {
            throw new InvalidOperationException($"Expected {nameof(KubernetesManifestResource)} but got {value?.GetType()}.");
        }

        emitter.Emit(new MappingStart());

        WriteProperty("apiVersion", manifest.ApiVersion, serializer);
        WriteProperty("kind", manifest.Kind, serializer);
        WriteProperty("metadata", manifest.Metadata, serializer);

        foreach (var (key, fieldValue) in manifest.Fields)
        {
            WriteProperty(key, fieldValue, serializer);
        }

        emitter.Emit(new MappingEnd());
    }

    private static void WriteProperty(string name, object? value, ObjectSerializer serializer)
    {

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