为什么我会从Azure收到错误消息“不支持ITraceWriter服务类型。”

Question

I am trying to get my Web API 2 up and running on an Azure App Service. It works fine on my machine, on IIS 10, with the following tracing added, from the package Microsoft.AspNet.WebApi.Tracing :

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.EnableSystemDiagnosticsTracing();
        SystemDiagnosticsTraceWriter traceWriter = config.EnableSystemDiagnosticsTracing();
        traceWriter.IsVerbose = true;
        traceWriter.MinimumLevel = TraceLevel.Debug;
        config.Services.Add(typeof(ITraceWriter), new TextFileTracer());

This works fine on my dev machine, but I get the error

The service type ITraceWriter is not supported.

as soon as I publish to Azure. I thought a modern ASP.NET app was supposed to be self contained, and not rely on services being present or configured on the host, and a very up to date host at that. Why is this happening?

Answer 1

According to your description, I tested your code on my Web API application and I got the server error as follows on my local side.

The service type ITraceWriter is not supported. Parameter name: serviceType

As mentioned in this official document about setting the Trace Writer:

To enable tracing, you must configure Web API to use your ITraceWriter implementation. You do this through the HttpConfiguration object, as shown in the following code:

config.Services.Replace(typeof(ITraceWriter), new TextFileTracer());

Only one trace writer can be active. By default, Web API sets a "no-op" tracer that does nothing. (The "no-op" tracer exists so that tracing code does not have to check whether the trace writer is null before writing a trace.)

After replacing ITraceWriter service with the TextFileTracer instance, I could make it work as expected both on my side and Azure.