Value type that represents a potentially missing value. This is similar to System.Nullable<T>, but does not restrict the contained value to be a value type, so it can be used for storing values of any types. When obtained from DataFrame<R, C> or Series<K, T>, the Value will never be Double.NaN or null (but this is not, in general, checked when constructing the value). The type is only used in C#-friendly API. F# operations generally use expose standard F# option<T> type instead. However, there the OptionalValue module contains helper functions for using this type from F# as well as Missing and Present active patterns.