mongodb check if point is in polygon

It seems to be to do with the order. If you are using $geoWithin and you are trying to find points inside a polygon, the thing that is within is the field you are searching on. However, $geoIntersects works in either direction, so you can search for points inside polygons, or polygons containing points, eg:

db.geom.insert({
  "polygons": {
    "type":"Polygon",
    "coordinates": [[
      [ 17.60083012593064, 78.18557739257812],
      [ 17.16834652544664, 78.19381713867188],
      [ 17.17490690610013, 78.739013671875],
      [ 17.613919673106714, 78.73489379882812],
      [ 17.60083012593064, 78.18557739257812]
    ]]
  }
});

db.geom.find({
  polygons: {
    $geoIntersects: {
      $geometry: {
        "type": "Point",
        "coordinates": [17.3734, 78.4738]
      }
    }
  }
});

Also, note that, you need to repeat the first point of the polygon at the end. If you remove the final pair, you will get a $err:

Can't canonicalize query: BadValue bad geo query" error.

It seems that MongoDB allows you to insert invalid geometries and only complains when you try and add a 2dsphere index or do an intersects/within/near query, which, I suppose is reasonable, as GeoJSON can be valid JSON without being a valid geometry.

Thanks for John Powell here is C# driver version of the same query.

 var geometry = new BsonDocument
                            {
                                     { "type", "Point" },
                                     { "coordinates",
                                        new BsonArray(new double[]{ Longitude,
                                        Latitude} ) }
                            };
                    var geometryOperator = new BsonDocument { { "$geometry", geometry } };
                    var geoIntersectsOperator = new BsonDocument { { "$geoIntersects", geometryOperator } };

                    var findField = new BsonDocument { { "geometry", geoIntersectsOperator } };


                    var results = MyCollection.Find(findField).ToList();