RosMessageTypes.Std source code

Where can I find the source code of std_msgs for the ros-tcp connnector messages implementation.

ex:

Code (CSharp):

1. namespace RosMessageTypes{
2.  
3. class Std{
4. ....
5. }
6.  
7. }

I created a custom ros message something like this;

float32 length
float32 width
std_msgs/String time

then build these message in unity. Then unity gives me these script

Code (CSharp):

1.  
2. namespace RosMessageTypes.AisData
3. {
4.     [Serializable]
5.     public class AisDataMMsg : Message
6.     {
7.         public const string k_RosMessageName = "ais_data/AisDataM";
8.         public override string RosMessageName => k_RosMessageName;
9.  
10.         public float length;
11.         public float width;
12.         public Std.StringMsg time;
13.  
14.         // constructor, toString etc..
15.  
16.  

I want to see the Std.StringMsg implementation How can I find it?