new gotcha

Using the new keyword to generate a generic object has a gotcha, the key value must be a property and not a string.

var query = new
{
	// Won't work, must be $in without quotes, however, using a $ is not
	// valid in a property definition.
	"$in" = new List<string> { "123" }
};

Instead you can just use a Dictionary<string, object> to achieve the same effect, although it is more verbose.

var query = new Dictionary<string, object>
{
	{ "$in", new List<string> { "123" } }
};