Cannot convert lambda expression to type 'PostData' because it is not a delegate type


I have this code in my .net core app

var searchResponse = client.Search<Person>(s => s
.Query(q => q
     .Match(m => m
        .Field(f => f.FirstName)


which I got from this page

I'm getting an error at design time "Cannot convert lambda expression to type 'PostData' because it is not a delegate type". I have referenced Elasticsearch and System.Linq.

What am I missing?


I've also tried this syntax from

QueryContainer query = new TermQuery

Field = "firstName",
Value = "martijn"

var searchRequest = new SearchRequest
From = 0,
Size = 10,
Query = query

And now I get an error "cannot convert from 'Nest.SearchRequest' to 'Elasticsearch.Net.PostData"

I'm not sure why it is so hard to make a simple API call here.

(Russ Cam) #3

First thing to check, are the NEST and Elasticsearch.Net NuGet packages referenced both the same version number?


Yes, they are both Version 6.2.0.

I've managed to use the ElasticLowLevelClient successfully to Index, but if I use that to Count or Search since it seems to just do a post, I can't map the response object to actually use the data I'm trying to get.

(Russ Cam) #5

Ok. What is the type of client above? Would you be able to provide a short, but complete, example of the issue?

Also, would you be able to provide more details of the environment

  • Windows, MacOS, Linux?
  • framework TFM e.g. netcoreapp2.0


Hi, so this is embarrassing. I was wanting to use the Low Level Client, but that was obviously not right. I then tried adding the Nest reference and changing to High Level Client but that didn't work either.
Then I closed VS Code, reopened it and all of a sudden it works. Frustrating!

For info, in case anyone else has this -
Running on Windows
Elasticsearch v6.3.0 running locally (just for dev purposes)
Developing in C# (dotnet core), using VSCode.

Code now looks like:

var node = new Uri(Config.Configuration["elasticsearch:0:uri"]);
var settings = new ConnectionSettings( node );
Client = new ElasticClient(settings);
QueryContainer query = new MatchQuery
	Field = keyValue.Key,
    Query = keyValue.Value.ToString()
var searchRequest = new SearchRequest(index.IndexName, index.IndexType)
		Query = query
var searchResults = Client.Search<object>(searchRequest);

Thanks again!

(Russ Cam) #7

Glad you figured it out @StephM :+1:

(system) #8

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