C# - How to copy/create an object as a decendent
.net-3.5, c#, inheritance
Solution
The instance you've created is a `MyClassA`. That is its runtime type, not `MyClassB`. You cannot cast a `MyClassA` instance to a `MyClassB` at runtime because `MyClassB` is a more specific type than `MyClassA`.
You need to create a brand-new instance of `MyClassB`. One way to clean this up is to create a constructor that takes a `MyClassA`, e.g.
public class MyClassB : MyClassA
{
public MyClassB(MyClassA a, string z)
{
this.PropertyA = a.PropertyA;
this.PropertyB = a.PropertyB;
// etc.
this.PropertyZ = z;
}
public string PropertyZ { get; set; }
}
Problem
This is easier to explain with an example. Given these two classes: ``` public class MyClassA { public String Property_A { get; set; } public String Property_B { get; set; } public String Property_C { get; set; } public String Property_D { get; set; } ... public String Property_Y { get; set; } } public class MyClassB: MyClassA { public String Property_Z { get; set; } } ``` Suppose I have fully created instance of MyClassA (with all properties from A - Y filled in). Then I need to make an instance of MyClassB which is exactly the same as my instance of MyClassA but with Property_Z filled in (with a custom value of course). How can I do this? Doing this does not work (throws and Invalid Cast Exception): ``` MyClassB myInstanceB = (myClassB) myInstanceA; myInstance.Property_Z = myCustomValue; ``` I have not needed to do anything like this since my C++ days and I am stumped. Any ideas? UPDATE: I found a solution to my problem in how I create the instances. It is below. I did not mark it as the answer because it did not exactly fit my question.