ComplexValueTypePropertiesSuck.cs

struct SuchMutableStruct
{
  public Matrix99x42 SoState;
  
  public bool IsInvalid { get; private set; }
  
  public bool Invalidate()
  {
    IsInvalid = true;
  }
}

public class StructUserClass
{
  public PropertyPlz { get; }
  
  public void DoDangerousOperation()
  {
    if (PropertyPlz.IsInvalid)
    {
      throw new Exception("Ooops I should not do this now");
    }
  }
}

public class Main
{
  public static void DoNaiveStuffFollowing2003sDoctrines()
  {
    StructUserClass c = new StructUserClass();
    c.PropertyPlz.Invalidate();
    c.DoDangerousOperation(); // Does this throw an exception? :P
  }
}

Let me guess, when you do invalidate you are doing it on a copy of the struct? So, no to the exception.

PS: I am finding this puzzles incredibly fun 😄