SchemaBuilder

// Create a data structure, attach it to a wall, populate it with data, and retrieve the data back from the wall
void StoreDataInWall(Wall wall, XYZ dataToStore)
{
    using (Transaction createSchemaAndStoreData = new Transaction(wall.Document, "tCreateAndStore"))
    {
       createSchemaAndStoreData.Start();
       SchemaBuilder schemaBuilder = new SchemaBuilder(new Guid("720080CB-DA99-40DC-9415-E53F280AA1F0"));
       schemaBuilder.SetReadAccessLevel(AccessLevel.Public); // allow anyone to read the object
       schemaBuilder.SetWriteAccessLevel(AccessLevel.Vendor); // restrict writing to this vendor only
       schemaBuilder.SetVendorId("ADSK"); // required because of restricted write-access
       schemaBuilder.SetSchemaName("WireSpliceLocation");
       FieldBuilder fieldBuilder = schemaBuilder.AddSimpleField("WireSpliceLocation", typeof(XYZ)); // create a field to store an XYZ
       fieldBuilder.SetSpec(SpecTypeId.Length);
       fieldBuilder.SetDocumentation("A stored location value representing a wiring splice in a wall.");

       Schema schema = schemaBuilder.Finish(); // register the Schema object
       Entity entity = new Entity(schema); // create an entity (object) for this schema (class)
       Field fieldSpliceLocation = schema.GetField("WireSpliceLocation"); // get the field from the schema
       entity.Set<XYZ>(fieldSpliceLocation, dataToStore, UnitTypeId.Meters); // set the value for this entity

       wall.SetEntity(entity); // store the entity in the element

       // get the data back from the wall
       Entity retrievedEntity = wall.GetEntity(schema);
       XYZ retrievedData = retrievedEntity.Get<XYZ>(schema.GetField("WireSpliceLocation"), UnitTypeId.Meters);
       createSchemaAndStoreData.Commit();  
    }
}