Highlight does not return anything


I am new to ES and am trying to set up a demo to evaluate if ES works for us as search application. In my demo, I can index and search content of a document eg MS Word or PDF successfully. But I did not manage get highlight work. Highlight field always return nothing even though results have been found.

Can I get some help please?

My environment:

ElasticSearch with attachment plugin
NEST client is used to talk to ES server (our application is based on .NET) :

I have created two VO : Attachment and Document

public class Attachment
[ElasticProperty(Name = "_content", Store = true)]
public string Content { get; set; }

    [ElasticProperty(Name = "_content_type", Store = true),]
    public string ContentType { get; set; }

    [ElasticProperty(Name = "_name", Store = true)]
    public string Name { get; set; }

[ElasticType(Name = "document")]
public class Document
public int Id { set; get; }

    [ElasticProperty(Store = true)]
    public string Title { set; get; }

    [ElasticProperty(Type = FieldType.Attachment, TermVector = TermVectorOption.WithPositionsOffsets, Store = true)]
    public Attachment File { set; get; }

The following methods are used to index and search content of a indexed MS word document:
private static void IndexWordDoc()
docElasticClient.CreateIndex(INDEX_NAME, c =>
c.AddMapping(m => m.MapFromAttributes().SearchAnalyzer("standard").IndexAnalyzer("standard")));
var attachment = new Attachment
Content = Convert.ToBase64String(File.ReadAllBytes(path)),
ContentType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
Name = "SeachTest.docx"
var doc = new Document()
Id = 1,
Title = "SeachTest.docx(title)",
File = attachment

And search funciton is:

private static ISearchResponse searchMSWord(string term)
var query = Query.Term("file", term);
var searchResults = docElasticClient.Search(s => s
return searchResults;

searchResults can resturn result(s) when I perform a search against indexed document. But Highlight field in the searchResults is always null.

Any idea which part I got it wrong? Thank you

By the way, If I only index and search VO(s),eg I create a simple VO : Person:

public class Person
public string FirstName { set; get; }
public string LastName { set; get; }
public int Age { set; get; }
public string About { set; get; }

Highlight works fine if I do the following search:
private static ISearchResponse SearchVO(string term)
var searchResults = elasticClient.Search(s => s
.Query(q => q.Term(p => p.About, term))
return searchResults;

But it just does not return anything if I search against indexed content of a MS document