C# - DataContractJsonSerializer Generating Ghost String To JSON Keys?

DataContractJsonSerializer this is nice class added in the .net framework which can be used to serialize/desirealize object into JSON.

Now following is the example i am trying

[Serializable] class User { public string name; public string userId; }

Now following is the output generated

Output : Notice structure where only "name" is expected instead of k__BackingField

Now this is the problem after digging so much i am not sure from where <> and _BackingField is coming ?

{
"<name>k__BackingField":"test user",
"<userId>k__BackingField":100001}