Difference in lambda expressions between full .NET framework and .NET Core

Question

Is there a difference in the declaration of lambda expressions between the .NET Framework and .NET Core?

The following expressions compiles in .NET Core:

var lastShift = timeline.Appointments
                        .OfType<DriverShiftAppointment>()
                        .SelectMany(x => x.Activities.Where(x => !x.IsTheoretical))
                        .OrderByDescending(x => x.Start)
                        .FirstOrDefault();

But not in the .NET framework.

The problem in this expression is the following part

.SelectMany(x => x.Activities.Where(x => !x.IsTheoretical))

where x is declared twice (SelectMany and Where).

This is the error in the .NET Framework:

A local or parameter named 'x' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter

.NET framework

Reproducable example:

public class DemoClass
{
    public IList<int> Numbers { get; set; } = new List<int>();
}

class Program
{
    static void Main(string[] args)
    {
        var lst = new List<DemoClass>
        {
            new DemoClass
            {
                Numbers = new List<int>{ 1, 2, 3, 4, 5, 6, 7, 8 }
            },
            new DemoClass
            {
                Numbers = new List<int>{ 1, 2, 3, 4, 5, 6, 7, 8 }
            },
            new DemoClass
            {
                Numbers = new List<int>{ 1, 2, 3, 4, 5, 6, 7, 8 }
            }
        };

        var result = lst.SelectMany(x => x.Numbers.Where(x => x % 2 == 0))
                        .OrderByDescending(x => x);

        Console.WriteLine("Hello World!");
    }
}

Answer 1

It is telling you that problem is this line:

.SelectMany(x => x.Activities.Where(x => !x.IsTheoretical))

Using the x twice confuses it, it is ambiguous, try this:

.SelectMany(x => x.Activities.Where(y => !y.IsTheoretical))

But you are right, it compiles in core but not framework. It looks to be like this: https://github.com/dotnet/roslyn/issues/38377. Reading that link, it looks like this is a change in C# 8.0, and core is targeting 8.0 while framework is targeting 7.2.