WoofWare.Whippet.Plugin.InterfaceMock 0.1.2

WoofWare.Whippet.Plugin.InterfaceMock

This is a Whippet plugin for defining mocks for interfaces.

It is a copy of the corresponding Myriad HttpClient plugin in WoofWare.Myriad, taken from commit d59ebdfccb87a06579fb99008a15f58ea8be394e.

Usage

Define a file like Client.fs:

type IPublicType =
    abstract Mem1 : string * int -> string list
    abstract Mem2 : string -> int

In your fsproj:

<Project>
    <ItemGroup>
        <Compile Include="Client.fs" />
        <Compile Include="GeneratedClient.fs">
            <WhippetFile>Client.fs</WhippetFile>
            <WhippetParamIPublicType>InterfaceMock</WhippetParamIPublicType>
        </Compile>
    </ItemGroup>

    <ItemGroup>
        
        <PackageReference Include="WoofWare.Whippet.Plugin.InterfaceMock" WhippetPlugin="true" Version="" />
        <PackageReference Include="WoofWare.Whippet" Version="" PrivateAssets="all" />
    </ItemGroup>
</Project>

(This plugin follows a standard convention taken by WoofWare.Whippet.Plugin plugins, where you use Whippet parameters with the same name as each input type, whose contents are a !-delimited list of the generators which you wish to apply to that input type.)

The generator produces a type like this:

/// Mock record type for an interface
type internal PublicTypeMock =
    {
        Mem1 : string * int -> string list
        Mem2 : string -> int
    }

    static member Empty : PublicTypeMock =
        {
            Mem1 = (fun x -> raise (System.NotImplementedException "Unimplemented mock function"))
            Mem2 = (fun x -> raise (System.NotImplementedException "Unimplemented mock function"))
        }

    interface IPublicType with
        member this.Mem1 (arg0, arg1) = this.Mem1 (arg0, arg1)
        member this.Mem2 (arg0) = this.Mem2 (arg0)

What's the point?

Reflective mocking libraries like Foq in my experience are a rich source of flaky tests. The Grug-brained developer would prefer to do this without reflection, and this reduces the rate of strange one-in-ten-thousand "failed to generate IL" errors. But since F# does not let you partially update an interface definition, we instead stamp out a record, thereby allowing the programmer to use F#'s record-update syntax.

Features

You may supply an isInternal : bool argument:

<Compile Include="GeneratedClient.fs">
    <WhippetFile>Client.fs</WhippetFile>
    <WhippetParamIPublicType>InterfaceMock(false)</WhippetParamIPublicType>
</Compile>

By default, we make the resulting record type at most internal (never public), since this is intended only to be used in tests; but you can instead make it public by setting the false boolean.

Instead of configuring the client with <WhippetParamMyType>InterfaceMock</WhippetParamMyType>, you may choose to add an attribute called InterfaceMock (with an optional "isInternal" argument) to any type you wish to use as an input. You may use WoofWare.Whippet.Plugin.InterfaceMock.Attributes to provide this attribute, or you may define it yourself.