web API 异常处理The 'ObjectContent`1' type failed to serialize

web API 错误处理The 'ObjectContent`1' type failed to serialize

just added this three lines in my WebApiConfig.cs file...

var json = config.Formatters.JsonFormatter;

json.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;

config.Formatters.Remove(config.Formatters.XmlFormatter);

After this codes were added.the Return XML will be transferred to Json format