How to configure AutoFoq with Foq's return strategy

This post explains how to configure AutoFixture.AutoFoq 3 so that null values are never returned when using Foq 1.5.1 and newer.

Foq’s behavior for mock objects that have not been explicitly setup, is to return null if the return type of a function is a reference type (e.g. a string):

type IInterface =
   abstract DoSomething : unit -> string

let sut = Mock<IInterface>().Create()
// No expectations have been setup.

let actual = sut.DoSomething()
// -> actual = null

AutoFixture is an opinionated library, and one of the opinions it holds is that nulls are invalid return values.

By the time AutoFixture.AutoFoq 3 was published Foq was still in version 1.0 with no specific hooks to override this behavior.

Foq now provides the necessary hooks to override this behavior and the rest of this post explains how to automate this when using AutoFixture.AutoFoq.


Ruben Bartelink originally discussed and proposed in Foq discussions about a returnStrategy argument for members that have not been explicitly setup:

let sut = Mock<IInterface>(returnStrategy = fun x -> "123" :> obj).Create()
// No expectations have been setup - fallback to returnStrategy function.

let actual = sut.DoSomething()
// -> actual = "123"

Configuring AutoFoq to use Foq’s returnStrategy argument

The existing AutoFoqCustomization has no specific hook to select Foq’s new returnStrategy argument. This can be addressed when necessary with the customization shown below:

module internal SynthesizerMethod =
    type private SynthesizerMethod<'T when 'T : not struct>
        (parameterInfos, builder) =
        interface IMethod with
            member this.Parameters = parameterInfos
            member this.Invoke parameters =
                    parameters |> Seq.toArray) :> obj

    let Create (targetType: Type, parameterInfos: ParameterInfo[], builder) =

module internal SynthesizerType =
    type Type with
        member this.GetPublicAndProtectedConstructors () =
                BindingFlags.Public |||
                BindingFlags.Instance |||

type internal SynthesizerMethodQuery (builder) =
    do if builder = null then raise <| ArgumentNullException("builder")
    interface IMethodQuery with
        member this.SelectMethods target =
            if target = null then raise <| ArgumentNullException("target")
            if target.IsInterface then
                seq { yield Create(target, Array.empty, builder) :?> IMethod }
                |> Seq.sortBy(fun x -> x.GetParameters().Length)
                |> ctor ->
                    Create(target, ctor.GetParameters(), builder) :?> IMethod)

    member this.SelectMethods targetType =
        (this :> IMethodQuery).SelectMethods targetType

type internal AutoFoqSynthesizeReturnValuesCustomization () =
    interface ICustomization with
        member this.Customize fixture =
            match fixture with
            | null -> raise (ArgumentNullException("fixture"))
            | _    -> fixture.Customizations.Add(

    member this.Customize fixture = (this :> ICustomization).Customize fixture

The only difference from the original AutoFoqCustomization is the usage of Foq’s returnStrategy argument in the SynthesizerMethodQuery class.

Typical usage

let CustomizationFillsReturnValues () =
    let fixture = Fixture().Customize(AutoFoqSynthesizeReturnValuesCustomization())

    let sut = fixture.Create<IInterface>()
    // No expectations have been setup - fallback to Foq's returnStrategy function.

    let actual = sut.DoSomething()
    // -> actual: "f5cdf6b1-a473-410f-95f3-f427f7abb0c7"

For any members that have not been explicitly setup Foq returns constrained non-deterministic values generated by AutoFixture's equivalence classes, for example random numbers.

Declarative usage

The above test can be also written declaratively using AutoData theories:

type TestConventionsAttribute() =
    inherit AutoDataAttribute(

[<Theory; TestConventions>]
let CustomizationFillsReturnValuesDecleratively (sut : IInterface) =
    let actual = sut.DoSomething()
    // -> actual: "f5cdf6b1-a473-410f-95f3-f427f7abb0c7"