<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/" xmlns:search="http://www.sitecore.net/xmlconfig/search/">
    <sitecore>
        <contentSearch search:require="solr">
         <configuration type="Sitecore.ContentSearch.ContentSearchConfiguration, Sitecore.ContentSearch">
                <indexes hint="list:AddIndex">
                    <index id="scempty102_custom_index" type="Sitecore.ContentSearch.SolrProvider.SolrSearchIndex, Sitecore.ContentSearch.SolrProvider">
                        <param desc="name">$(id)</param>
                        <param desc="core">$(id)</param>
                        <param desc="propertyStore" ref="contentSearch/indexConfigurations/databasePropertyStore" param1="$(id)" />
                        <configuration ref="contentSearch/indexConfigurations/defaultSolrIndexConfiguration">
                        </configuration>
                        <locations hint="list:AddCrawler">
                            <crawler type="MyProject.Foundation.Indexing.Crawlers.MyCrawler, MyProject.Foundation.Indexing">
                            </crawler>
                        </locations>
                    </index>
                </indexes>
            </configuration>
        </contentSearch>
    </sitecore>
</configuration>

namespace MyProject.Foundation.Indexing.Indexable
{
  public class IndexableCustomerField : IIndexableDataField
  {
    private readonly Customer _customer;
    private readonly PropertyInfo _propertyInfo;

    public IndexableCustomerField(Customer customer, PropertyInfo fieldInfo)
    {
      _customer = customer;
      _propertyInfo = fieldInfo;
    }

    public string Name
    {
      get { return _propertyInfo.Name; }
    }

    public string TypeKey => string.Empty;
    public Type FieldType => _propertyInfo.PropertyType;
    public object Value => _propertyInfo.GetValue(_customer);
    public object Id => _propertyInfo.Name.ToLower();
  }

  public class Customer
  {
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string Email { get; set; }
  }

  //indexable class
  public class IndexableCustomer : IIndexable
  {
    private Customer _customer;

    public IndexableCustomer(Customer customer)
    {
      _customer = customer;
    }

    public void LoadAllFields()
    {
      Fields = _customer.GetType()
        .GetProperties(BindingFlags.Public
                       | BindingFlags.Instance
                       | BindingFlags.IgnoreCase)
        .Select(fi => new IndexableCustomerField(_customer, fi));
    }

    public IIndexableDataField GetFieldById(object fieldId)
    {
      return Fields.FirstOrDefault(x => x.Id.Equals(fieldId));
    }

    public IIndexableDataField GetFieldByName(string fieldName)
    {
      return Fields.FirstOrDefault(x => x.Name.Equals(fieldName));
    }

    public IIndexableId Id => new IndexableId<string>(_customer.Id.ToString());
    public IIndexableUniqueId UniqueId => new IndexableUniqueId<IIndexableId>(Id);
    public string DataSource => "Customer";
    public string AbsolutePath => "";
    public CultureInfo Culture => CultureInfo.CurrentCulture;
    public IEnumerable<IIndexableDataField> Fields { get; private set; }
  }

}

namespace MyProject.Foundation.Indexing.Crawlers
{
  public class MyCrawler: FlatDataCrawler<IndexableCustomer>
  {
    protected override IndexableCustomer GetIndexableAndCheckDeletes(IIndexableUniqueId indexableUniqueId)
    {
      return null;
    }

    protected override IndexableCustomer GetIndexable(IIndexableUniqueId indexableUniqueId)
    {
      return null;
    }

    protected override bool IndexUpdateNeedDelete(IndexableCustomer indexable)
    {
      return false;
    }

    protected override IEnumerable<IIndexableUniqueId> GetIndexablesToUpdateOnDelete(
      IIndexableUniqueId indexableUniqueId)
    {
      return null;
    }

    //If you get data from external api you could also save it to disk 
    //in a json file then return that list instead depending on the size.
    protected override IEnumerable<IndexableCustomer> GetItemsToIndex()
    {
      var customersToIndex = new List<IndexableCustomer>();

      string connectionString = "Data Source=(local);Initial Catalog=custom_Customer;User ID=<id>;Password=<password>";
      
       //Here i'm just using a simple Sql connection to showcase with some data. 
      //You would not really do this in a real case.
      using (SqlConnection connection = new SqlConnection(connectionString))
      {
        connection.Open();

        SqlCommand command = connection.CreateCommand();
        command.CommandText = "SELECT * FROM Customer_table";
        command.CommandTimeout = 15;
        command.CommandType = CommandType.Text;

        using (SqlDataReader reader = command.ExecuteReader())
        {
          while (reader.Read())
          {
            var customer = new Customer();

            customer.Id = reader.GetInt32(reader.GetOrdinal("Id"));
            customer.Name = reader.GetString(reader.GetOrdinal("Name"));
            customer.Age = reader.GetInt32(reader.GetOrdinal("Age"));
            customer.Email = reader.GetString(reader.GetOrdinal("Email"));

            customersToIndex.Add(new IndexableCustomer(customer));
          }
        }
        connection.Close();
      }
      return customersToIndex;
    }
  }
}

public class ExtendedSearchResultItem : SearchResultItem
  {
    [IndexField("name")]
    public string CustomerName { get; set; }
    [IndexField("age")]
    public int CustomerAge { get; set; }
    [IndexField("email")]
    public string CustomerEmail { get; set; }
  }

public class SearchClass
  {
    public IEnumerable<ExtendedSearchResultItem> Search()
    {
      var searchContext = ContentSearchManager.GetIndex("scempty102_custom_index").CreateSearchContext();
      IQueryable<ExtendedSearchResultItem> queryable = searchContext.GetQueryable<ExtendedSearchResultItem>();

      queryable.Where(x => x.CustomerAge > 30); //insert your own predicate here.
      var searchResults = queryable.GetResults();
      return searchResults.Hits.Select(i => i.Document);
    }
  }