What causes "Array params contains empty item: [{values}]" in microsoft/aspire?

Inspect the args list in the message and remove or replace the empty string entry.

microsoft/aspire · error · ArgumentException

Array params contains empty item

Error message

Array params contains empty item: [{values}]

What it means

AddPythonApp validates its scriptArgs collection via ThrowIfNullOrContainsIsNullOrEmpty. If any element of scriptArgs is an empty string, it throws ArgumentException naming scriptArgs and including the full argument list in the message.

Solutions

Inspect the args list in the message and remove or replace the empty string entry.
Provide the actual argument value the empty string was meant to represent.
Filter empties before calling: args = candidateArgs.Where(a => !string.IsNullOrEmpty(a)).ToArray().
If a bare flag was intended, pass the flag token itself (e.g. "--verbose") rather than an empty string.

Example fix

// before
builder.AddPythonApp("py", "main.py", "--flag", "");
// after
builder.AddPythonApp("py", "main.py", "--flag", "--other");

Defensive patterns

Strategy: validation

Validate before calling

if (args.Any(a => string.IsNullOrEmpty(a)))
    throw new InvalidOperationException("scriptArgs contains an empty entry.");

Try / catch

try
{
    builder.AddPythonApp("py", "main.py", args);
}
catch (ArgumentException ex)
{
    logger.LogError(ex, "Empty entry in python args: {Args}", ex.Message);
}

Prevention

Filter with Where(a => !string.IsNullOrEmpty(a)) before passing script args.
Provide default values for arguments sourced from empty env vars or config.
Pass bare flags as their own token instead of empty strings.

When it happens

Trigger: Passing an array containing an empty string element to AddPythonApp, e.g. new[] { "--flag", "" } or an argument computed from an empty environment variable.

Common situations: Arguments read from unset environment variables or empty config values; string interpolation producing "" when a value is missing; users intending a bare flag by passing an empty string.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/dd43596f3c0b3eed. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs:745

            case EntrypointType.Executable:
                stage.Entrypoint([entrypoint]);
                break;
        }
    }

    private static void ThrowIfNullOrContainsIsNullOrEmpty(string[] scriptArgs)
    {
        ArgumentNullException.ThrowIfNull(scriptArgs);
        foreach (var scriptArg in scriptArgs)
        {
            if (string.IsNullOrEmpty(scriptArg))
            {
                var values = string.Join(", ", scriptArgs);
                if (scriptArg is null)
                {
                    throw new ArgumentNullException(nameof(scriptArgs), $"Array params contains null item: [{values}]");
                }
                throw new ArgumentException($"Array params contains empty item: [{values}]", nameof(scriptArgs));
            }
        }
    }

    /// <summary>
    /// Resolves the default virtual environment path by checking multiple candidate locations.
    /// </summary>
    /// <param name="builder">The distributed application builder.</param>
    /// <param name="appDirectory">The Python app directory (relative to AppHost).</param>
    /// <param name="virtualEnvironmentPath">The relative virtual environment path (e.g., ".venv").</param>
    /// <returns>The resolved virtual environment path.</returns>
    private static string ResolveDefaultVirtualEnvironmentPath(IDistributedApplicationBuilder builder, string appDirectory, string virtualEnvironmentPath)
    {
        var appDirectoryFullPath = Path.GetFullPath(appDirectory, builder.AppHostDirectory);

        // Walk up from the Python app directory looking for the virtual environment
        // Stop at the AppHost's parent directory to avoid picking up unrelated venvs
        var appHostParentDirectory = Path.GetDirectoryName(builder.AppHostDirectory);

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