I was getting following error while registering a custom tracer in asp.net web api.The type must 'xx' derive from ITraceWriter.\r\nParameter name: service Solution
The solution of this problem is to specify the ITraceWriter with complete name including namespace. AS there is already a ITraceWriter in the JSON.
config.Services.Replace(typeof(System.Web.Http.Tracing.ITraceWriter), new CustomTracer());
This solves the problem.