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Using the object initializer syntax, on the line that define the Terms, I am getting an error: Cannot convert source type 'System.Collections.Generic.List<int>' to target type 'System.Collections.Generic.IEnumerable<object>'

var query = new TermsQuery
{
 Field = new Field("seq"),
 Terms = new List<int> {1 ,2, 3}
};

This works if define using fluent DSL

var search = new List<int> {1, 2, 3};   

q.Terms(c => c
    .Field(p => p.seq)
    .Terms(search)
)

What are I doing wrong with the object initializer syntax? Thanks for any help in advance.

Edmand Looi
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  • `Terms = new List {1 ,2, 3}` – mjwills Feb 21 '21 at 06:50
  • Despite @mjwills solves the error I would check why `Terms` expects an enumerable of `object` instead of `int`. The conversion between them called boxing and unboxing and may be very costly in terms of performance. – cly Feb 21 '21 at 06:54
  • `The conversion between them called boxing and unboxing and may be very costly in terms of performance.` For three values the performance cost will be miniscule. – mjwills Feb 21 '21 at 08:10

1 Answers1

2

I can attribute the failure in the first case to a fundamental point of C#. int is a value type and trying to assign a List<int> to IEnumerable<object> fails because generic variance is not supported for value types. The same thing works fine if you assign a List<string> to the IEnumerable<object> reference type. Take a look at this answer by Jon Skeet to understand more on this. Additionally, you can look up more on the Covariance and Contravariance in generics from MSDN. One way to get this working is performing an explicit cast on your List<int> type variable.

Further, for the case of fluent DSL approach, there are two overloads that are possible for the Terms method.

TermsQueryDescriptor<T> Terms<TValue>(IEnumerable<TValue> terms)

and

TermsQueryDescriptor<T> Terms<TValue>(params TValue[] terms)

If you take a look at the implementation of these methods from here, you would see that the type T that is being passed in is inferred and the required casting is done internally. That is why you wouldn't face this issue in the case of fluent DSL approach.

Sai Gummaluri
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  • Sai, thanks for the explanation of the issue. I have convert the list of integer into list of string and it seems to work great. Thanks again. – Edmand Looi Feb 21 '21 at 18:04