microsoft/aspire · error · ArgumentException

Port range start must be less than or equal to the range…

Error message

Port range start must be less than or equal to the range end.

What it means

Thrown by PortRange.ValidateRange when both endpoints are individually valid ports but rangeStart exceeds rangeEnd, i.e. the range is inverted. Raised as ArgumentException on the start parameter to enforce an ordered, non-empty range.

Solutions

Swap the values so start <= end.
Clamp: use start = Math.Min(a, b), end = Math.Max(a, b) when accepting user input.
Add a startup validation that rejects inverted ranges with a clear config message.

Example fix

// before
PortRange.ValidateRange(9000, 8000, nameof(start), nameof(end));
// after
PortRange.ValidateRange(8000, 9000, nameof(start), nameof(end));

Defensive patterns

Strategy: validation

Validate before calling

(start, end) = (Math.Min(a, b), Math.Max(a, b)); // normalize before ValidateRange

Try / catch

try { PortRange.ValidateRange(start, end, nameof(start), nameof(end)); }
catch (ArgumentException ex) when (ex.Message.Contains("less than or equal")) { /* swap or fix config */ }

Prevention

Accept ranges as ordered (low, high) and normalize with Min/Max.
Check env vars aren't swapped when port ranges come from configuration.
Add a startup assertion that configured ranges are ordered.

When it happens

Trigger: Calling ValidateRange(9000, 8000) — start greater than end — or ranges where config substitution swapped the values (e.g. env vars ASPORT_END set lower than ASPORT_START).

Common situations: Swapped environment variables or config keys for the port range; refactoring that reordered positional arguments to ValidateRange.

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/68de2a9b7c26bf52. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting/Dcp/PortRange.cs:48

    /// <summary>
    /// Validates that <paramref name="rangeStart"/> and <paramref name="rangeEnd"/> form a valid,
    /// non-empty, ordered port range, throwing if not.
    /// </summary>
    public static void ValidateRange(int rangeStart, int rangeEnd, string rangeStartParamName, string rangeEndParamName)
    {
        if (!IsValidPort(rangeStart))
        {
            throw new ArgumentOutOfRangeException(rangeStartParamName, rangeStart, $"Port range start must be between {MinPort} and {MaxPort}.");
        }

        if (!IsValidPort(rangeEnd))
        {
            throw new ArgumentOutOfRangeException(rangeEndParamName, rangeEnd, $"Port range end must be between {MinPort} and {MaxPort}.");
        }

        if (rangeStart > rangeEnd)
        {
            throw new ArgumentException("Port range start must be less than or equal to the range end.", rangeStartParamName);
        }
    }
}

View on GitHub (pinned to 25830f84bd)