Can I change a private readonly field in C# using reflection?

Question

I am wondering, since a lot of things can be done using reflection, can I change a private readonly field after the constructor completed its execution?
(note: just curiosity)

public class Foo
{
 private readonly int bar;

 public Foo(int num)
 {
  bar = num;
 }

 public int GetBar()
 {
  return bar;
 }
}

Foo foo = new Foo(123);
Console.WriteLine(foo.GetBar()); // display 123
// reflection code here...
Console.WriteLine(foo.GetBar()); // display 456

Answer 1

You can:

typeof(Foo)
   .GetField("bar",BindingFlags.Instance|BindingFlags.NonPublic)
   .SetValue(foo,567);

Answer 2

The obvious thing is to try it:

using System;
using System.Reflection;

public class Test
{
    private readonly string foo = "Foo";

    public static void Main()
    {
        Test test = new Test();
        FieldInfo field = typeof(Test).GetField
            ("foo", BindingFlags.Instance | BindingFlags.NonPublic);
        field.SetValue(test, "Hello");
        Console.WriteLine(test.foo);
    }        
}

This works fine. (Java has different rules, interestingly - you have to explicitly set the Field to be accessible, and it will only work for instance fields anyway.)

Answer 3

The answer is yes, but more importantly:

Why would you want to? Intentionally breaking encapsulation seems like a horrifically bad idea to me.

Using reflection to change a readonly or constant field is like combining the Law of Unintended Consequences with Murphy's Law.

Answer 4

You asked why you would want to break the encapsulation like that.

I use an entity helper class to hydrate entities. This uses reflection to get all the properties of a new empty entity, and matches the property/field name to the column in the resultset, and set's it using propertyinfo.setvalue().

I don't want anyone else to be able to change the value, but I don't want to take all the effort to custom code hydration methods for every entity either.

My many of my stored procs return resultsets that don't correspond directly to tables or views, so the code gen ORM's do nothing for me.