A big topic.    AKA  Object-to-Relational Mapping.

If you want a general solution, there are third-party tools and runtimes out
there.
http://eurohost.webmatrixhosting.net/madgeek/SharpToolbox/Default.aspx?Page=Sear
ch&Query=ToolName%3d%26Category%3d74089b0a-1105-4389-b1db-eedf27e20cfb
(beware line breaks)

If you want to do something quick and dirty, then ....
 I can imagine building a utility class that employs .NET Reflection to
interrogate a type, retrieve the values of the public fields, then  build a
SQL statement that updates a SQL database.

eg,
public static string UpdateSqlWithFields(object o) {
 System.Type t= o.GetType();
 System.Text.StringBuilder sb1= new System.Text.StringBuilder();
 System.Reflection.FieldInfo[] fia= t.GetFields();

 // use a db table name derived from the type of the object
 string strTable= "[ObjectState " + t.GetName() + "]";

 if ((fia != null) && (fia.Length != 0)) {
   sb1.Append("UPDATE " + strTable + " (");
   int j=0;
   foreach (System.Reflection.FieldInfo fi in fia) {
     if (j > 0) sb1.Append(", ");
     sb1.Append(fi.Name);
     j++;
   }
   sb1.Append(") VALUES (");

   j=0;
   foreach (System.Reflection.FieldInfo fi in fia) {
     try{
       object result = t.InvokeMember (fi.Name,

System.Reflection.BindingFlags.Public |

System.Reflection.BindingFlags.Instance  |

System.Reflection.BindingFlags.GetField ,
                                       null, o, new object [] {});

       if (j > 0) sb1.Append(", ");
       sb1.Append(result.ToString()); // FIX THIS:  quote strings if
necessary (those that include commas)
       j++;
     }
     catch(System.Exception e){
       Console.WriteLine("Exception in InvokeMember(Field) : " + e.Message
+ "\n");
       continue;
     }
   }
   sb1.Append(")");
 }

 string strSql=  sb1.ToString();

 // now you have the SQL string, just need to execute it on the db
connection...

}

BUT,
1. this example doesn't deal with public fields that might be arrays or
structures (or other objects).  the algorithm assumes that all public fields
are primitive types.  Getting into arrays, and objects that belong to other
objects, you need to deal with relationships and aggregation, and that is
the domain of a more serious ORM tool.
2. this example doesn't deal with public PROPERTIES, as distinct from
fields.
3. this simple example doesn't deal with the concept of object identity.
You may have 30,000,000 different instances of a particular object, you'll
want some concept of identity.

---------------------

I don't see where the XML web services part comes in;  you could certainly
add it for a remoting layer, but it is not necessary to address the core
issue of "store public fields into a database".