CustomSearchResultItemExample2.cs

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"));
}