soen

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <configuration>
        <DefaultIndexConfiguration>
          <fields hint="raw:AddComputedIndexField">
            <field fieldName="myCustomComputedIndexField" returnType="string">
              <!-- Fully qualified assembly path to the class -->
              YourAssembly.ComputedFields.MyComputedIndexField,YourAssembly 
            </field>

soen

public class MyComputedIndexField : IComputedIndexField
{
  public object ComputeFieldValue(IIndexable indexable)
  {
    // Check that we have something to index
    if (indexable == null)
      return null;

    // Check that it is a Sitecore indexable item
    var indexableItem = indexable as SitecoreIndexableItem;

soen

using System;
using Sitecore.ContentSearch;
using Sitecore.ContentSearch.ComputedFields;
using Sitecore.ContentSearch.Utilities;
using Sitecore.Data;
using Sitecore.Data.Items;

namespace Examples.ComputedIndexFields
{
  public class CombinedDateTime : IComputedIndexField

soen

ISearchIndex index = ContentSearchManager.GetIndex("sitecore_master_index")  
using (IProviderSearchContext context = index.CreateSearchContext())  
{
  // Notice that I've swapped out the 'SearchItemResult' with 'MySearchItemResult'
  // in the method call GetQueryable<T>, and now I'm able to use the property 
  // 'MyTextField' in the filtering
  var results = context.GetQueryable<MySearchItemResult>()
                       .Where(x => x.MyTextField.Contains("Some value"));
}

soen

using System.Collections.Generic;
using System.Linq;
using Sitecore.ContentSearch;
using Sitecore.ContentSearch.ComputedFields;
using Sitecore.ContentSearch.Utilities;
using Sitecore.Data;
using Sitecore.Data.Items;

namespace Examples.ComputedIndexFields
{

soen

// A custom implementation of the SearchResultItem
public class MySearchResultItem : SearchResultItem
{
  // I've added a new property, that will be mapped to the Solr field named 'mytextfield'
  [IndexField("mytextfield")]
  public string MyTextField { get; set; }
}

soen

using (var context = ContentSearchManager.GetIndex("sitecore_master_index").CreateSearchContext())
{
  var facets = new FacetResults();

  facets = context.GetQueryable<SearchResultItem>()                        
                  .FacetOn(x => x.Language)
                  .GetFacets();

  foreach (var facetCategories in facets.Categories)
  {

soen

public class ProductSearcher  
{
  public ProductSearchResult Search(SearchCriteria criteria)  
  {
    using (var context = ContentSearchManager.GetIndex("sitecore_master_index").CreateSearchContext())  
    {
      var filterPredicate = PredicateBuilder.True<SearchResultItem>();
  
      // Only take products created over the past year
      filterPredicate = filterPredicate

soen

// Lower and upper dates are still the same as in the previous example
var results = context.GetQueryable<SearchResultItem>()
                     .Where(x => x.CreatedDate.Between(lowerDate, upperDate, Inclusion.Both));

soen

var lowerDate = new DateTime(2015, 10, 1);
var upperDate = new DateTime(2015, 10, 14);

var results = context.GetQueryable<SearchResultItem>()
                     .Where(x => x.CreatedDate >= lowerDate && x.CreatedDate <= upperDate);