void Main()
{
    List<Demo1> demo1 = new List<Demo1> { new Demo1 { PropertyToSet = "Start" } };
    List<Demo2> demo2 = new List<Demo2> { new Demo2 { PropertyToSet = "Start" } };

    demo1.ToList().ForEach(c => c.PropertyToSet = "End");
    demo2.ToList().ForEach(c => c.PropertyToSet = "End");

    Console.WriteLine(demo1.First().PropertyToSet); //End
    Console.WriteLine(demo2.First().PropertyToSet); //Start
}

public class  Demo1 {public string PropertyToSet;}
public struct Demo2 {public string PropertyToSet;}