C# LINQ Strongly type return type for SetWindowFields?

Hi to all.
I have a complex aggregation pipeline, where SetWindowFields is only one of the stages in the middle of it.
Since the method is defined as IAggregateFluent<BsonDocument> it stops being strongly typed and I have to work with BsonDocument in the next stages (or find a way to project it back to the type it is).

Am I missing something or is this a current limitation? I didn’t find many examples using the C# LINQ provider with SetWindowFields. All the examples have it as a final stage returning BsonDocument.

Since the function works more or less like AddFields I checked to see how this is defined but it’s not available through LINQ.

SetWindowFields is defined to return IAggregateFluent<BsonDocument> in order to reduce the number of required overloads and also to not require you to create a custom class to hold the result.

If you do want to define a custom class to hold the result of $setWindowFields you can use the existing As<TNewResult> pseudo-stage to tell the driver that you want to specify a new class to represent the documents in the pipeline at that stage.

For example, if you had the following classes:

public class C
    public int Id { get; set; }
    public int X { get; set; }

public class CWithAverage
    public int Id { get; set; }
    public int X { get; set; }
    public double Average { get; set; }

You could use the CWithAverage class to represent the result of SetWindowFields like this:

// collection is of type IMongoCollection<T>
var aggregate = collection
    .SetWindowFields(output: p => new { Average = p.Average(c => c.X, null) })

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