Mocking an ElasticSearch client using C# Moq
The NullReferenceException occured because you didn't specify a behavior on search method. Your search method returns null and then you calls .Document on the null.
The way to specify a behavior is as the following:
elasticClientMock.Setup(x => x.Search<SegmentRecord>( It.IsAny</* put here the right Func */>)) .Returns( /* put here the instance you want to return */);you have to replace my comments with the correct types.