Dapper column to F# option property

Question

How do I get Dapper to convert data to an F# type with an option property? Simple example:

type SomeType = {
    Count : int option
}

connection.QueryAsync<SomeType>(...)

This throws:

System.InvalidOperationException
    A parameterless default constructor or one matching signature
    (System.Int32 count) is required for SomeType materialization

Using Nullable instead of option works:

type SomeType = {
    Count : Nullable<int>
}

But it's not as desirable for various reasons. For one thing, I have cases where I use string option (because F# doesn't allow null assignments normally, which is great), and Nullable<string> doesn't compile. Is there a way to configure/instruct Dapper to automatically deal with option types? I'd like to avoid using custom mappings every time.

In case it matters, using with Npgsql.

Answer 1

I don't think there's any support for adding handlers for open generics, so you'd have to add a type handler for each type you need.

You could define a handler like this:

type OptionHandler<'T>() =
    inherit SqlMapper.TypeHandler<option<'T>>()

    override __.SetValue(param, value) = 
        let valueOrNull = 
            match value with
            | Some x -> box x
            | None -> null

        param.Value <- valueOrNull    

    override __.Parse value =
        if isNull value || value = box DBNull.Value 
        then None
        else Some (value :?> 'T)

And register for the types you need like this:

SqlMapper.AddTypeHandler (OptionHandler<string>())
SqlMapper.AddTypeHandler (OptionHandler<int>())