CODEWITHSUNDEEP
c#azureazure-functionsexceptionfilterattribute
Stephen Cleary
Stephen Cleary

Reputation: 456477

Custom IExceptionHandler

I'm trying to get a custom IExceptionHandler to work with my Azure Function (C# class library). The idea is to have my own exception handler for unexpected exceptions that will contain my own in-memory trace log; to clarify, I do want these sent to the client browser and displayed to the user.

Details follow.

With this simple custom exception handler:

public sealed class CustomExceptionHandler : ExceptionHandler
{
  public override void Handle(ExceptionHandlerContext context)
  {
    context.Result = new ResponseMessageResult(
        context.Request.CreateErrorResponse(HttpStatusCode.BadRequest,
            "custom message string"));
  }
}

I've tried to install it as such:

[FunctionName("Function1")]
public static HttpResponseMessage Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "works")]
    HttpRequestMessage req)
{
  req.GetConfiguration().Services.Replace(typeof(IExceptionHandler),
      new CustomExceptionHandler());
  throw new Exception("unexpected exception");
}

but when deployed, I just get the generic "operation failed" error message (e.g., XML-formatted from Chrome):

<ApiErrorModel xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Microsoft.Azure.WebJobs.Script.WebHost.Models">
  <Arguments xmlns:d2p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays" i:nil="true"/>
  <ErrorCode>0</ErrorCode>
  <ErrorDetails i:nil="true"/>
  <Id>dec07825-cf4e-49cc-a80e-bef0dbd01ba0</Id>
  <Message>An error has occurred. For more information, please check the logs for error ID dec07825-cf4e-49cc-a80e-bef0dbd01ba0</Message>
  <RequestId>51df1fec-c1c2-4635-b82c-b00d179d2e50</RequestId>
  <StatusCode>InternalServerError</StatusCode>
</ApiErrorModel>

If I turn on Diagnostic Logs -> Detailed error messages in the AF UI and try to ensure detailed error messages are always written:

[FunctionName("Function1")]
public static HttpResponseMessage Run(
    [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "works")]
    HttpRequestMessage req)
{
  req.GetRequestContext().IncludeErrorDetail = true;
  req.GetRequestContext().IsLocal = true;
  req.GetConfiguration().Services.Replace(typeof(IExceptionHandler),
      new CustomExceptionHandler());
  throw new Exception("unexpected exception");
}

then I do get exception details, but still apparently handled by the builtin exception handler (XML-formatted from Chrome):

<ApiErrorModel xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Microsoft.Azure.WebJobs.Script.WebHost.Models">
  <Arguments xmlns:d2p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays" i:nil="true"/>
  <ErrorCode>0</ErrorCode>
  <ErrorDetails>Microsoft.Azure.WebJobs.Host.FunctionInvocationException : Exception while executing function: Function1 ---> System.Exception : unexpected exception at ...</ErrorDetails>
  <Id>5e752375-002b-4ab7-a348-20edad64a3fe</Id>
  <Message>Exception while executing function: Function1 -> unexpected exception</Message>
  <RequestId>b6611456-f3f1-4591-97e4-920dae59b5ff</RequestId>
  <StatusCode>InternalServerError</StatusCode>
</ApiErrorModel>

so this is exposing the stack trace and exception message, but I don't have my in-memory trace log.

What I want to do is create the response message for unexpected exceptions myself. For example, this is what my custom error response looks like for an expected exception (JSON-formatted, from Postman):

{
  "message": "An error has occurred.",
  "exceptionMessage": "Could not find package `nito.bob`",
  "exceptionType": "Common.ExpectedException",
  "stackTrace": "   at ...",
  "log": [
    "Received request for jsonVersion=4, packageId=`nito.bob`, packageVersion=``, targetFramework=``",
    "Looking up latest package version for `nito.bob`",
    "No non-prerelease package version found for `nito.bob`; looking up latest prerelease package version",
    "No package version found for `nito.bob`",
    "Returning 404: Could not find package `nito.bob`"
  ],
  "requestId": "b2df08cb-1071-4e47-bd24-bf74152e4b2a"
}

This is just the basic HttpError-formatted exception (with details) that I augment with log and requestId. Since this is an expected exception, this is not being done in an IExceptionHandler; I'm just returning the HttpResponseMessage.

The fun part: my custom IExceptionHandler works just fine when hosting the AF locally. It just doesn't work when deployed. If I take the code and run it locally, I get back the response I expect (JSON-formatted from Postman):

{
  "Message": "custom message string"
}

Alternative approaches that won't work for me:

Upvotes: 33

Views: 2019

Answers (1)

Glenn Ferrie
Glenn Ferrie

Reputation: 10390

I think what you are looking for is FunctionExceptionFilterAttribute.

Here is a simple code sample for how to implement the abstract class:

public class YourErrorHandlerAttribute : FunctionExceptionFilterAttribute
{
    public override async Task OnExceptionAsync(FunctionExceptionContext exceptionContext, CancellationToken cancellationToken)
    {
        var fn_name = exceptionContext.FunctionName;
        var fn_instanceId = exceptionContext.FunctionInstanceId;
        var ex = exceptionContext.Exception;

        // TODO: your logic here.
    }
}

Here is how to use it in your function:

[FunctionName("Function1")]
[YourErrorHandler]
public static HttpResponseMessage Run([HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "works")]
HttpRequestMessage req)
{
     // TODO: your Azure Function logic / implementation.
}

It behaves similarly to ActionFilterAttribute in ASP.NET MVC

Upvotes: 1

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