How do I cast an object to another object type with all the same properties except the new type has one more field

Question

I am trying to cast the type skill to type SkillViewModel.

This is where I try the cast:

    SkillViewModel skillViewModel = (SkillViewModel)skill;

This is my skill model that I am trying to convert:

    public class Skill
    {
         public int SkillId { get; set; }
         public string SkillName { get; set; }
         public string SkillDescription { get; set; }
         public int SkillLevelId { get; set; }
         public string SkillLevelDescription { get; set; }
         public int LevelId { get; set; }
         public int LevelNumber { get; set; }
    }

This is the SkillViewModel that I am trying to convert to. It inherits skill and has one added property RoleId:

    public class SkillViewModel : Skill
    {
         public int RoleId { get; set; }
    }

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SOLVED:

Rather than casting the skill object to type SkillViewModel, I just created a new instance and explicitly copied each in the skill to the SkillViewModel added the roleId at the end.

Like this:

    private SkillViewModel[] GetRoleSpecificSkills(Skill[] skills, int roleId)
    {
        var results = new List<SkillViewModel>();

        foreach (var skill in skills)
        {
            SkillViewModel skillViewModel = new SkillViewModel();
            skillViewModel.SkillId = skill.SkillId;
            skillViewModel.SkillName = skill.SkillName;
            skillViewModel.SkillDescription = skill.SkillDescription;
            skillViewModel.SkillLevelId = skill.SkillLevelId;
            skillViewModel.SkillLevelDescription = skill.SkillLevelDescription;
            skillViewModel.LevelId = skill.LevelId;
            skillViewModel.LevelNumber = skill.LevelNumber;
            skillViewModel.RoleId = roleId;

            results.Add(skillViewModel);
        }

        return results.ToArray();
    }

Answer 1

You can't cast parent to child this way. And this is very bad practice.

If you really need to downcast, try this:

var jsonParent = JsonConvert.SerializeObject(parent); 
Child c = JsonConvert.DeserializeObject<Child>(jsonParent);

Answer 2

Generally, you cannot cast from an incompatible type (such as a parent class) in C# nor any other statically typed programming language to that extent: a Skill object has a different representation in memory than a SkillViewModel object: Skill 's memory region is smaller by the size of the RoleId property, so if such casting is allowed by the compiler of a given language at all, then later if you try to access RoleId , it will result in a memory violation, because the offset of RoleId is beyond the boundary of the memory region allocated for the Skill object.

In case of converting from a parent class to a subclass, the best way is to create a constructor in SkillViewModel that takes a Skill object as a param, copies all its properties to its own instance variables and assigns some sane default value to the RoleId .
Note however that all these properties will now be referenced from 2 objects, so if any of them is mutable, then changing it in 1 object will also change it the second: this may have unexpected results sometimes, hence immutable objects are generally preferred whenever possible. Whenever you are making a new reference to a mutable object, you should consider making a deep clone of it instead: take into consideration performance and memory usage penalty on one hand and likeliness of someone making a bug on the other.

Answer 3

当您像这样投射时，您应该能够访问 RoleID 属性：

SkillViewModel skillViewModel = skill as SkillViewModel;