microsoft/aspire · error · ArgumentException

Partition key paths must be a string or a string collection.

Error message

Partition key paths must be a string or a string collection.

What it means

AddContainerForPolyglot accepts a loosely-typed partitionKeyPaths parameter to support polyglot callers. At runtime it must be either a string (single path) or an IEnumerable<string>; any other type throws ArgumentException. This is a runtime type check because static typing cannot constrain the polyglot input.

Solutions

1. Pass a single string path like "/id" or an IEnumerable<string> such as string[] or List<string>.
2. Convert generic collections with .Cast<string>().ToArray() before calling.
3. Normalize any dynamic input to IEnumerable<string> first and reject other types with your own validation.

Example fix

// before
object partitionKeys = new object[] { "/id", "/tenant" }; // object[] is not IEnumerable<string>
db.AddContainerForPolyglot("orders", partitionKeys, null);
// after
object partitionKeys = new[] { "/id", "/tenant" }; // string[]
db.AddContainerForPolyglot("orders", partitionKeys, null);

if (partitionKeyPaths is not string and not IEnumerable<string>) throw new ArgumentException("Partition key paths must be a string or a string collection.");

static bool IsValidPartitionKeyArg(object? o) => o is string or IEnumerable<string>;

try { db.AddContainerForPolyglot("orders", rawArg, null); } catch (ArgumentException ex) when (ex.ParamName == "partitionKeyPaths") { /* coerce rawArg to string[] and retry */ }

When it happens

Trigger: Calling AddContainerForPolyglot with partitionKeyPaths that is neither string nor IEnumerable<string>, e.g. an object[], List<object>, char, or a boxed non-string value.

Common situations: Invocation from other-language or generated code that builds the argument dynamically; passing a JSON-derived array typed as object[] instead of string[].

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

• Could not invoke ' ' because parameter ' ' expects , but…
• Endpoint must be a string, endpoint reference, or reference…
• URL must be a string, Uri, or parameter resource builder.
• A of type cannot be assigned to a BicepValue< >.
• Address prefix must be a string or a parameter resource…

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/cac288552276f1bb. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.CosmosDB/AzureCosmosDBExtensions.cs:422

    /// <summary>
    /// Adds an Azure Cosmos DB container resource
    /// </summary>
    [AspireExport("addContainer")]
    internal static IResourceBuilder<AzureCosmosDBContainerResource> AddContainerForPolyglot(
        this IResourceBuilder<AzureCosmosDBDatabaseResource> builder,
        [ResourceName] string name,
        [AspireUnion(typeof(string), typeof(IEnumerable<string>))] object partitionKeyPaths,
        string? containerName = null)
    {
        ArgumentNullException.ThrowIfNull(builder);
        ArgumentException.ThrowIfNullOrEmpty(name);
        ArgumentNullException.ThrowIfNull(partitionKeyPaths);

        return partitionKeyPaths switch
        {
            string partitionKeyPath => builder.AddContainer(name, partitionKeyPath, containerName),
            IEnumerable<string> partitionKeyPathCollection => builder.AddContainer(name, partitionKeyPathCollection, containerName),
            _ => throw new ArgumentException("Partition key paths must be a string or a string collection.", nameof(partitionKeyPaths))
        };
    }

    /// <summary>
    /// Adds a container to the associated Cosmos DB database resource with hierarchical partition keys.
    /// </summary>
    /// <param name="builder">CosmosDBDatabase resource builder.</param>
    /// <param name="name">Name of container resource.</param>
    /// <param name="partitionKeyPaths">Hierarchical partition key paths for the container.</param>
    /// <param name="containerName">The name of the container. If not provided, this defaults to the same value as <paramref name="name"/>.</param>
    /// <returns>A reference to the <see cref="IResourceBuilder{T}"/>.</returns>
    [AspireExportIgnore(Reason = "Polyglot AppHosts use the internal addContainer dispatcher export.")]
    public static IResourceBuilder<AzureCosmosDBContainerResource> AddContainer(this IResourceBuilder<AzureCosmosDBDatabaseResource> builder, [ResourceName] string name, IEnumerable<string> partitionKeyPaths, string? containerName = null)
    {
        ArgumentNullException.ThrowIfNull(builder);
        ArgumentException.ThrowIfNullOrEmpty(name);
        ArgumentNullException.ThrowIfNull(partitionKeyPaths);
        var partitionKeyPathsArray = partitionKeyPaths.ToArray();

View on GitHub (pinned to 25830f84bd)