How to resolve: Microsoft.Azure.WebJobs.Host: Exception binding parameter 'data'. Microsoft.Azure.WebJobs.Host: Object reference not set to an instance of an object

Question

I am attempting to locally run my subpub code while testing during development and am running into this error

"System.Private.CoreLib: Exception while executing function: Functions.subpub-trigger-message. Microsoft.Azure.WebJobs.Host: Exception binding parameter 'data'. Microsoft.Azure.WebJobs.Host: Object reference not set to an instance of an object."
when attempting to execute my subpub message trigger with the following code

function.json:
{
"bindings": [
{
"type": "webPubSubTrigger",
"direction": "in",
"name": "data",
"hub": "simplechat",
"eventName": "message",
"eventType": "user"
},
{
"type": "webPubSub",
"name": "actions",
"hub": "simplechat",
"direction": "out"
}
]
}

index.js:

module.exports = async function (context, data) {  
    context.bindings.actions = {  
        "actionName": "sendToAll",  
        "data": `[${context.bindingData.request.connectionContext.userId}] ${data}`,  
        "dataType": context.bindingData.dataType  
    };  
    // UserEventResponse directly return to caller  
    var response = {   
        "data": '[SYSTEM] ack.',  
        "dataType" : "text"  
    };  
    return response;  
};  

The code is actually taken directly from this tutorial: https://learn.microsoft.com/en-us/azure/azure-web-pubsub/quickstart-serverless?tabs=javascript

Answer 1

@Aaron Manill , I couldn't get this quick start to work locally with ngrok. I used the Live Trace Tool and kept hitting AbuseProtectionResponseMissingAllowedOrigin followed by web socket connection closing. Not sure if had anything to do with https://github.com/MicrosoftDocs/azure-docs/issues/86799, but after deploying the application to an Azure Function app, it worked as expected.

See if you get the same error when deployed, that will confirm if it's a configuration error or not. Could be a misconfiguration on the hub event handler settings, or it could be an issue with the reverse proxy you're using. Since you are getting the web pubsub trigger to fire locally, try adding --verbose to func host start or using fiddler to get a closer look at the traffic going across the web socket. Might be a malformation in the payload itself.