Random results with Examine

ExamineExtensions.cs

using System.Collections.Generic;
using System.Linq;
using Examine;
using Examine.LuceneEngine;
using Helpers;

namespace Extensions
{
    public static class ExamineExtensions
    {
         public static IEnumerable<SearchResult> RandomResults(this ISearchResults results, int count)
        {
            var searchResults = results as SearchResults;
            if (searchResults == null) return Enumerable.Empty<SearchResult>();

            var totalResults = searchResults.TopDocs?.ScoreDocs.Length ?? 0;
            if (totalResults == 0) return Enumerable.Empty<SearchResult>();

            var randomIndexes = MathHelper.GenerateUniqueRandomSequence(count, 0, totalResults - 1);
            var docs = randomIndexes.Select(idx => results.Skip(idx).First()).ToList();
            return docs;
        }

        public static SearchResult RandomResult(this ISearchResults results)
        {
            return results.RandomResults(1).FirstOrDefault();
        }
    }
}

Example.cs

public class Example
{
    public void DoSearch()
    {
        var searcher = ExamineManager.Instance.SearchProviderCollection["ExternalSearcher"];
        var criteria = searcher.CreateSearchCriteria()
          .Field("myField", "myValue") // Expand on this for you query
          .Compile();
        var results = searcher.Search(criteria).RandomResults(3);
        // TODO: Convert results to typed content etc
    }
}

MathHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Helpers
{
    public static class MathHelper
    {
        public static IReadOnlyList<int> GenerateUniqueRandomSequence(int count, int min, int max)
        {
            var sample = Enumerable.Range(min, (max - min) + 1).ToList();
            var results = new List<int>();
            var rand = new Random(Guid.NewGuid().GetHashCode());

            for (var i = 0; i < count; i++)
            {
                if (sample.Count > 0)
                {
                    var sampleIdx = rand.Next(1, sample.Count) - 1;
                    results.Add(sample[sampleIdx]);
                    sample.RemoveAt(sampleIdx);
                }
            }

            return results;
        }
    }
}

Using ISearchResults.Skip should be pretty efficient in this context. Internally, the SearchResults implementation does a yield return, so only the results we access get evaluated and fetched from the lucene docs collection so we should only be populating the docs we need.