Getting "Cannot evaluate a security function" when using conditional breakpoints and in immediate window

Question

When developing .NET Core 2.0 applications, I'm having issues with conditional breakpoints, watch evaluations, and immediate window evaluations. I'm receiving the following error:

modifiers.GroupBy(c => c.Modifier.Group).ToList()
threw an exception of type 'System.ArgumentException'
Data: {System.Collections.ListDictionaryInternal}
HResult: -2147024809
HelpLink: null
InnerException: null
Message: "Cannot evaluate a security function."
ParamName: null
Source: null
StackTrace: null
TargetSite: null

What may cause this issue? Is there a workaround? This works just fine in a .NET Framework 4.x application.

---

Edit: Adding code sample

Using the following class:

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Group { get; set; }
}

In a normal console application, targeting .NET Core 2.0:

static void Main(string[] args)
{
    var people = new List<Person>
    {
        new Person { Age = 17, Name = "Person A", Group = "Group A" },
        new Person { Age = 20, Name = "Person B", Group = "Group A" },
        new Person { Age = 23, Name = "Person C", Group = "Group A" },
        new Person { Age = 17, Name = "Person D", Group = "Group B" },
        new Person { Age = 25, Name = "Person E", Group = "Group B" },
        new Person { Age = 40, Name = "Person F", Group = "Group B" },
    };
    // Make sure that System.Linq gets loaded
    people.Where(c => c.Name == "Person A").First();
}

Set a breakpoint after the initialization of the List<Person>. In the immediate window, trying to evaluate a GroupBy or a Where which is not a simple equality check, it will throw an ArgumentException with the message "Cannot evaluate a security function."

A couple of examples would be:

people.Where(c => c.Name.Contains("F")).ToList()

people.GroupBy(c => c.Group).ToList()

Note: The watch statements seems to be working in Visual Studio Code, but not in Visual Studio 2017 Preview 3.

Answer 1

That error seems to be due to the immediate window disallowing anything that may cause side-effects. Usually side-effects are allowed in the immediate window... but it does not like GroupBy on List<T> (while GroupBy on T[] is tolerated)

I reproduced using your example.

people.GroupBy(c => c.Group).ToList() throws
people.Where(c => c.Name.Contains("F")).ToList() does not.

It did not require a conditional breakpoint. My test was with a brand new .net core 2.0 project Debug Any CPU build. This was the stable release of VS Pro 2017 v15.3.1

To get around this issue, make a copy of your list into an array:

people.ToArray().GroupBy(c => c.Group).ToList()

I also recommend filing a bug report with MS.