microsoft/aspire · error · ArgumentException

Expected a string, integer, or

Error message

Expected a string, integer, or {nameof(BicepValueProxy)}.

What it means

BicepValueFactory.ToExpression converts an indexer's value into a Bicep expression. It supports only string literals, int literals, and BicepValueProxy instances; anything else throws ArgumentException naming the offending parameter. This enforces that only Bicep-expressible primitives flow into generated expressions.

Solutions

1. Convert the value to string or int before indexing (e.g. myLong.ToString(), ((int)myEnum).ToString()).
2. Wrap complex values via BicepValueProxy/expression helpers instead of raw objects.
3. If it's an int-like value, ensure it's genuinely int not long (explicit cast).
4. Check the call site against the accepted set: string, int, BicepValueProxy.

Example fix

// before
var value = factory.Index(config.GetSection("replicas").Get<long>()); // long

// after
var value = factory.Index(((int)config.GetValue<long>("replicas")).ToString());

bool supported = value is string or int or BicepValueProxy;

bool IsBicepIndexable(object? v) => v is string or int or BicepValueProxy;

try { expr = factory.Index(raw); }
catch (ArgumentException ex) when (ex.ParamName == "value") { /* convert raw to string/int and retry */ }

When it happens

Trigger: Using the BicepValueProxy Index API with a value of an unsupported type — e.g. long, double, bool, Guid, DateTime, or an enum — as an index/key/argument instead of a string, int, or proxy.

Common situations: Passing a bool or enum from host code into an indexer; using a long from a config value; passing a Guid as a resource key; conversion from JSON config delivering nested objects.

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

• A of type cannot be assigned to a BicepValue< >.
• Expected a literal or .
• An Azure principal parameter was not supplied a value…
• Azure resource ' ' is missing required output ' '. Ensure…
• AzureEnvironmentResource must be present in the application…

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/961a42574ce660b8. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.Provisioning/BicepValueFactory.cs:507

        return new();
    }
#pragma warning restore CA1822

    private static BicepValueProxy FromExpression(BicepExpression expression, bool isSecure)
    {
        return BicepValueProxy.CreateExpression(expression, isSecure);
    }

    private static BicepExpression ToExpression(object value)
    {
        ArgumentNullException.ThrowIfNull(value);

        return value switch
        {
            string literal => literal,
            int literal => literal,
            BicepValueProxy proxy => proxy.ToBicepExpression(),
            _ => throw new ArgumentException($"Expected a string, integer, or {nameof(BicepValueProxy)}.", nameof(value))
        };
    }

    private static bool IsSecure(object value)
    {
        return value is BicepValueProxy proxy && proxy.IsSecure;
    }
}

View on GitHub (pinned to 25830f84bd)