Description

Building a mock for a member that returns a unit type results in a System.InvalidProgramException: "Common Language Runtime detected an invalid program."

Repro steps

Compile and run the following program:

open Foq

type IMyInterface =
    abstract member foo: int -> unit

let mockMyInterface = Mock<IMyInterface>.With(fun mock ->
    <@ mock.foo(any()) --> () @>
)

[<EntryPoint>]
let main args = 
    mockMyInterface.foo(5)
    0 

Expected behavior

The mock should successfully call foo(5) without error.

Actual behavior

An exception of System.InvalidProgramException is thrown.

Known workarounds

It seems if I build the mock out via the Setup/Create approach this works correctly:

let mockMyInterface = 
    Mock<IMyInterface>()
        .Setup(fun x -> <@ x.foo() @>)
        .Returns(())
        .Create()

The documentation is sparse, so I'm unsure of the difference between the two (I opted for the .With() for brevity).

Related information

• Windows 10 x64
• For 1.7.2
• .NET 4.5/4.7 Framework causes this exception. Mono 5.x fails with a stacktrace and the message unknown type 0x01 in type_to_load_membase