microsoft/aspire · error · InvalidOperationException

Failed to get or create property

Error message

Failed to get or create property '{key}'

What it means

This internal JsonExtensions.Prop helper guarantees that indexing a JsonObject by key always returns a non-null JsonNode: it reads the property, and if missing, adds a new empty JsonObject under that key. The InvalidOperationException is thrown only if TryAdd fails (the key was concurrently added or the node exists with a JSON null value) and the subsequent lookup still yields null. In practice this fires when the property exists but holds a JSON literal null rather than an object, so TryAdd cannot replace it and the stored null is returned.

Solutions

1. Inspect the JSON document being navigated: replace any explicit JSON null for the failing key with an object ({}), or remove the property.
2. Guard traversal with jsonObj.ContainsKey/key checks or [key] is JsonObject checks before calling Prop for keys that may be null.
3. Avoid sharing a mutable JsonObject across concurrent tasks; build JSON trees single-threaded or under a lock.
4. If this appears without an obvious null in the input, treat it as an Aspire internal invariant break and report it with the JSON payload being processed.

Example fix

// before
var value = bicepJson.Prop("parameters").Prop("location");

// after
if (bicepJson["parameters"] is not JsonObject paramsObj)
{
    throw new InvalidOperationException("'parameters' is missing or not an object (check for JSON null).");
}
var value = paramsObj.Prop("location");

static bool TryProp(JsonNode? obj, string key, out JsonNode? node)
{
    node = (obj as JsonObject)?[key];
    return node is JsonObject;   // false when the property is JSON null or missing
}

try
{
    var value = json.Prop("parameters").Prop(key);
}
catch (InvalidOperationException ex) when (ex.Message.StartsWith("Failed to get or create property"))
{
    logger.LogWarning(ex, "JSON property was null or unexpectedly absent; skipping.");
}

When it happens

Trigger: Calling Prop() on a JSON property whose stored value is JSON null: jsonObj[key] is not null-check-friendly here only when TryAdd fails — i.e. the key exists with a null JsonNode — so the fallback jsonObj[key] returns null and the exception fires. Also possible with concurrent mutation of the same JsonObject from multiple threads between the read and TryAdd.

Common situations: Parsing Bicep/ARM parameter files or deployment outputs where a nested property is explicitly set to null ("parameters": { "foo": null }); code traversing that shape with obj.Prop("foo").Prop("bar") hits the null node. Concurrent JSON mutation from parallel tasks sharing one JsonObject.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

• Expected a JSON array.
• Expected property name.
• Expected start of chat message object.
• Service should have valid address at this point
• Unexpected end of JSON while reading property value.

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/5d58ed57688a1f0b. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure/Provisioning/JsonExtensions.cs:29

    {
        var jsonObj = obj.AsObject();

        // Try to get the existing node
        var node = jsonObj[key];
        if (node is not null)
        {
            return node;
        }

        // Create a new node and try to add it
        node = new JsonObject();

        if (!jsonObj.TryAdd(key, node))
        {
            node = jsonObj[key];
            if (node is null)
            {
                throw new InvalidOperationException($"Failed to get or create property '{key}'");
            }
        }

        return node;
    }
}

View on GitHub (pinned to 25830f84bd)