Sitecore 9 have OOTB extention GetMoreLikeThis for IProviderSearchContext. To use it you need to install SolrNet nuget package. You can use it like following:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var parameters = new MoreLikeThisHandlerParameters(new[] {"field1", "field2"})
    {
        MinDocFreq = 1, // minimum document frequency
        MinTermFreq = 1, // minimum term frequency
    };

    var query = SolrMLTQuery.FromQuery(new SolrQuery("search_term"));
    var queryOption = new MoreLikeThisHandlerQueryOptions(parameters);
    var results = context.GetMoreLikeThis<SearchResultItem>(query, queryOption);

    var interestingTerms = results.InterestingTerms;
}

Another way, you can use native SorlNet query:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var results = context.Query<SearchResultItem>(new SolrQuery("search_term"), new QueryOptions
    {
        MoreLikeThis = new MoreLikeThisParameters(new[] { "field1", "field2" })
        {
            MinDocFreq = 1, // minimum document frequency
            MinTermFreq = 1, // minimum term frequency
        },
    });

    var similarResults = results.SimilarResults;
}

If you use Linq-based predicates and they are complex and you can not rewrite your queries to SolrNet-based, you can use this "hack" to convert them. But keep in mind that in this case one additional request will be send to Solr:

var linqQuery = context.GetQueryable<SearchResultItem>().Where(predicate);
var solrNetQuery = new SolrQuery(((IHasNativeQuery)linqQuery).Query.ToString());

Sitecore 9 has an OOTB extension GetMoreLikeThis for the IProviderSearchContext. To use it you need to install the SolrNet nuget package. You can use it like following:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var parameters = new MoreLikeThisHandlerParameters(new[] {"field1", "field2"})
    {
        MinDocFreq = 1, // minimum document frequency
        MinTermFreq = 1, // minimum term frequency
    };

    var query = SolrMLTQuery.FromQuery(new SolrQuery("search_term"));
    var queryOption = new MoreLikeThisHandlerQueryOptions(parameters);
    var results = context.GetMoreLikeThis<SearchResultItem>(query, queryOption);

    var interestingTerms = results.InterestingTerms;
}

Alternatively, you can use a native SorlNet query:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var results = context.Query<SearchResultItem>(new SolrQuery("search_term"), new QueryOptions
    {
        MoreLikeThis = new MoreLikeThisParameters(new[] { "field1", "field2" })
        {
            MinDocFreq = 1, // minimum document frequency
            MinTermFreq = 1, // minimum term frequency
        },
    });

    var similarResults = results.SimilarResults;
}

If you use LINQ-based predicates and they are complex and you can not rewrite your queries to SolrNet-based, you can use this "hack" to convert them. But keep in mind that in this case one additional request will be sent to Solr:

var linqQuery = context.GetQueryable<SearchResultItem>().Where(predicate);
var solrNetQuery = new SolrQuery(((IHasNativeQuery)linqQuery).Query.ToString());

Sitecore 9 have OOTB extention GetMoreLikeThis for IProviderSearchContext. To use it you need to install SolrNet nuget package. You can use it like following:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var parameters = new MoreLikeThisHandlerParameters(new[] {"field1", "field2"})
    {
        MinDocFreq = 1, // minimum document frequency
        MinTermFreq = 1, // minimum term frequency
    };

    var query = SolrMLTQuery.FromQuery(new SolrQuery("search_term"));
    var queryOption = new MoreLikeThisHandlerQueryOptions(parameters);
    var results = context.GetMoreLikeThis<SearchResultItem>(query, queryOption);

    var interestingTerms = results.InterestingTerms;
}

Another way, you can use native SorlNet query:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var results = context.Query<SearchResultItem>(new SolrQuery("search_term"), new QueryOptions
    {
        MoreLikeThis = new MoreLikeThisParameters(new[] { "field1", "field2" })
        {
            MinDocFreq = 1, // minimum document frequency
            MinTermFreq = 1, // minimum term frequency
        },
    });

    var similarResults = results.SimilarResults;
}

Sitecore 9 have OOTB extention GetMoreLikeThis for IProviderSearchContext. To use it you need to install SolrNet nuget package. You can use it like following:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var parameters = new MoreLikeThisHandlerParameters(new[] {"field1", "field2"})
    {
        MinDocFreq = 1, // minimum document frequency
        MinTermFreq = 1, // minimum term frequency
    };

    var query = SolrMLTQuery.FromQuery(new SolrQuery("search_term"));
    var queryOption = new MoreLikeThisHandlerQueryOptions(parameters);
    var results = context.GetMoreLikeThis<SearchResultItem>(query, queryOption);

    var interestingTerms = results.InterestingTerms;
}

Another way, you can use native SorlNet query:

var contextIndex = _contextRepository.GetSearchIndexContext("search_index");
using (var context = contextIndex.CreateSearchContext())
{
    var results = context.Query<SearchResultItem>(new SolrQuery("search_term"), new QueryOptions
    {
        MoreLikeThis = new MoreLikeThisParameters(new[] { "field1", "field2" })
        {
            MinDocFreq = 1, // minimum document frequency
            MinTermFreq = 1, // minimum term frequency
        },
    });

    var similarResults = results.SimilarResults;
}

If you use Linq-based predicates and they are complex and you can not rewrite your queries to SolrNet-based, you can use this "hack" to convert them. But keep in mind that in this case one additional request will be send to Solr:

var linqQuery = context.GetQueryable<SearchResultItem>().Where(predicate);
var solrNetQuery = new SolrQuery(((IHasNativeQuery)linqQuery).Query.ToString());