Getting NLog to send out JSON with proper headers?

Looking at the HttpNetworkSender source, I don't see an obvious way to pass in a content type to the WebRequest.

I think you will have to create a custom target based on the NetworkTarget which uses a custom HttpNetworkSender, and include config to set the content-type on the WebRequest Appropriately.

NLog ver. 4.5 WebService Target has the ability to configure custom headers.

<target xsi:type="WebService"         name="CouchDB"         url="http://127.0.0.1:5984/logger/"         protocol="JsonPost"         encoding="utf-8">      <parameter layout="FileLayout" />      <header name="Content-Type" layout="application/json" /></target>

See also https://github.com/nlog/NLog/wiki/WebService-target

In case someone still stumble on this, here is simple demo for custom http target (NLog v4.x.x).

public class WebPostTarget : Target{    public string ServerUrl { get; set; }    private readonly HttpClient _client = new HttpClient();    protected override void Write(LogEventInfo logEvent)    {        PostData(new [] { logEvent.AsLogModel() });     }    protected override void Write(AsyncLogEventInfo[] logEvents)    {        var data = logEvents.Select(x => x.LogEvent.AsLogModel()).ToArray();        PostData(data);    }    private void PostData(object data)    {        if (!ServerUrl.IsNullOrEmpty())        {            // add your custom headers to the client if you need to             _client.PostJsonAsync(ServerUrl, data).FireAndForget();        }    }}// HttpClientExtensionspublic static async Task<HttpResponseMessage> PostJsonAsync(this HttpClient client, string url, object data){    return await client.PostAsync(url, new StringContent(data.ToJson(), Encoding.UTF8, "application/json"));}

From configuration code:

var asyncWebTarget = new AsyncTargetWrapper(){    Name = "web_batch_logServer",    BatchSize = 100,    TimeToSleepBetweenBatches = 1000,    OverflowAction = AsyncTargetWrapperOverflowAction.Grow,    WrappedTarget = new WebPostTarget { ServerUrl = logServerUrl }};