使用格式错误的 Json 调用 ASP.NET WebMethod 时捕获错误 [英] Catching errors from calling ASP.NET WebMethod with malformed Json

问题描述

我们有一个较旧的 ASP.NET WebForms 应用程序，它通过在客户端使用 jQuery $.ajax() 调用来执行 AJAX 请求，在用 [WebMethod] 属性.

We have an older ASP.NET WebForms application which performs AJAX request by using jQuery $.ajax() calls on the client side, calling static methods in the page code-behind decorated with [WebMethod] attributes.

如果 WebMethod 中发生未处理的异常，它不会触发 Application_Error 事件，因此不会被我们的错误记录器 (ELMAH).这是众所周知的，不是问题 - 我们将所有 WebMethod 代码包装在 try-catch 块中，并将异常手动记录到 ELMAH.

If an unhandled exception occurs within the WebMethod, it does not fire the Application_Error event and is thus not picked up by our error logger (ELMAH). This is well known and not a problem - we have all WebMethod code wrapped in try-catch blocks with exceptions being manually logged to ELMAH.

然而，有一个案例让我很难过.如果格式错误的 Json 被发布到 WebMethod URL，它会在输入我们的代码之前抛出异常，我找不到任何方法来捕获它.

However, there is one case that has me stumped. If malformed Json is posted to the WebMethod URL, it throws an exception before entering our code, and I can't find any way to trap this.

例如这个 WebMethod 签名

e.g. this WebMethod signature

[WebMethod]
public static string LeWebMethod(string stringParam, int intParam)

通常使用 Json 负载调用，例如:

Normally called with a Json payload like:

{"stringParam":"oh hai","intParam":37}

我尝试使用 Fiddler 进行测试以将有效负载编辑为格式错误的 Json:

I tried a test using Fiddler to edit the payload to the malformed Json:

{"stringParam":"oh hai","intPara

并从 JavaScriptObjectDeserializer 得到以下 ArgumentException 错误响应发送到客户端(这是在本地运行的简单测试应用程序中，没有自定义错误):

And got the following ArgumentException error response from JavaScriptObjectDeserializer sent to the client (this is in a simple test app running locally with no custom errors):

{"Message":"Unterminated string passed in. (32): {"stringParam":"oh hai","intPara","StackTrace":"   at
System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeString()
   at
System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeMemberName()
   at
System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeDictionary(Int32 depth)
   at 
System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth)
   at 
System.Web.Script.Serialization.JavaScriptObjectDeserializer.BasicDeserialize(String input, Int32 depthLimit, JavaScriptSerializer serializer)
   at 
System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit)
   at 
System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input)
   at 
System.Web.Script.Services.RestHandler.GetRawParamsFromPostRequest(HttpContext context, JavaScriptSerializer serializer)
   at 
System.Web.Script.Services.RestHandler.GetRawParams(WebServiceMethodData methodData, HttpContext context)
   at 
System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)","ExceptionType":"System.ArgumentException"}

它仍然没有触发 Application_Error 事件，它从不进入我们的代码，所以我们不能自己记录错误.

It's still not firing the Application_Error event, and it never enters our code so we can't log the error ourselves.

我发现了一个类似的问题，它指向了博客文章How to create a globalWeb 服务的异常处理程序"，但这似乎只对 SOAP 网络服务有效，对 AJAX GET/POST 无效.

I found a similar question which got a pointer to the blog post "How to create a global exception handler for a Web Service" but that appears to only be valid for SOAP webservices, not AJAX GETs/POSTs.

在我的情况下，是否有一些类似的方法可以附加自定义处理程序?

Is there some similar way to attach a custom handler in my situation?

推荐答案

根据参考源，内部RestHandler.ExecuteWebServiceCall 方法捕获 GetRawParams 抛出的所有异常并将它们简单地写入响应流，这就是为什么 Application_Error 没有被调用:

According to the reference source, the internal RestHandler.ExecuteWebServiceCall method catches all exceptions thrown by GetRawParams and simply writes them to the response stream, which is why Application_Error isn't invoked:

internal static void ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData) {
    try {
        ...
        IDictionary<string, object> rawParams = GetRawParams(methodData, context);
        InvokeMethod(context, methodData, rawParams);
    }
    catch (Exception ex) {
        WriteExceptionJsonString(context, ex);
    }
}

我能想到的唯一解决方法是创建一个输出过滤器来拦截并记录输出:

The only workaround I can think of is to create an output filter that intercepts and logs the output:

public class PageMethodExceptionLogger : Stream
{
    private readonly HttpResponse _response;
    private readonly Stream _baseStream;
    private readonly MemoryStream _capturedStream = new MemoryStream();

    public PageMethodExceptionLogger(HttpResponse response)
    {
        _response = response;
        _baseStream = response.Filter;
    }

    public override void Close()
    {
        if (_response.StatusCode == 500 && _response.Headers["jsonerror"] == "true")
        {
            _capturedStream.Position = 0;
            string responseJson = new StreamReader(_capturedStream).ReadToEnd();
            // TODO: Do the actual logging.
        }

        _baseStream.Close();
        base.Close();
    }

    public override void Flush()
    {
        _baseStream.Flush();
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        return _baseStream.Seek(offset, origin);
    }

    public override void SetLength(long value)
    {
        _baseStream.SetLength(value);
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        return _baseStream.Read(buffer, offset, count);
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        _baseStream.Write(buffer, offset, count);
        _capturedStream.Write(buffer, offset, count);
    }

    public override bool CanRead { get { return _baseStream.CanRead; } }
    public override bool CanSeek { get { return _baseStream.CanSeek; } }
    public override bool CanWrite { get { return _baseStream.CanWrite; } }
    public override long Length { get { return _baseStream.Length; } }

    public override long Position
    {
        get { return _baseStream.Position; }
        set { _baseStream.Position = value; }
    }
}

在 Global.asax.cs(或 HTTP 模块)中，在 Application_PostMapRequestHandler 中安装过滤器:

In Global.asax.cs (or in an HTTP module), install the filter in Application_PostMapRequestHandler:

protected void Application_PostMapRequestHandler(object sender, EventArgs e)
{
    HttpContext context = HttpContext.Current;
    if (context.Handler is Page && !string.IsNullOrEmpty(context.Request.PathInfo))
    {
        string contentType = context.Request.ContentType.Split(';')[0];
        if (contentType.Equals("application/json", StringComparison.OrdinalIgnoreCase))
        {
            context.Response.Filter = new PageMethodExceptionLogger(context.Response);
        }
    }
}

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