Related issues:

These are all closed, and as this is a pretty nasty and hard-to-track-down issue (you only find out it's a problem when running from full framework, and it's not at all obvious what causes it) I thought it better to raise the issue than stay quiet...

• Cannot change initonly field outside its .ctor regression #22485 ("Cannot change initonly field outside its .ctor regression")
• VS 15.5 Preview: VerificationException in otherwise valid function #22707 (VerificationException when using 15.5 preview)
• Consider switching to peverify-compat automatically when APTCA or similar attributes are used #22712 ("Consider switching to peverify-compat automatically when APTCA or similar attributes are used")

Version Used: Visual Studio 15.7.3; from the command line, csc reports version 2.8.3.62923 (7aafab5)

Steps to Reproduce:

1. Create a class library targeting just .NET Standard 2.0

Project:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <LangVersion>7.3</LangVersion>
    <!--<Features>peverify-compat</Features> -->
  </PropertyGroup>
</Project>

Code:

[assembly: System.Security.AllowPartiallyTrustedCallers]

namespace Library
{
    public readonly struct OuterStruct
    {
        private readonly ValueHolder implementation;

        public int Value => implementation.Value;

        public OuterStruct(int value) => implementation = new ValueHolder(value);
    }

    internal readonly struct ValueHolder
    {
        internal int Value { get; }

        internal ValueHolder(int value) => Value = value;
    }
}

2. Create a console app targeting the full framework, with a reference:

Project file:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net461</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <ProjectReference Include="..\Library\Library.csproj" />
  </ItemGroup>
</Project>

Code:

using Library;
using System;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            var outer = new OuterStruct(100);
            Console.WriteLine(outer.Value);
        }
    }
}

3. Run!

Expected Behavior:

Prints 100.

Actual Behavior:

Unhandled Exception: System.Security.VerificationException: Operation could destabilize the runtime.
   at Library.OuterStruct.get_Value()
   at ConsoleApp.Program.Main(String[] args)

Workarounds:

• Remove AllowPartiallyTrustedCallers
• Use the peverify-compat feature

What I would like to see:

• A warning when the compiler detects that it's emitting non-verifiable code that could easily break full frameworks calling it
• A good docs page that gives guidance to library developers as to which of these workarounds to use