Question

If an object is readonly or const, is it possible to cast that object to make it writable?
Something similar to C++ const_cast.

Answer 1

It’s not possible in C#, just like it’s not possible in C++. In C++, if the object is really const, you cannot const_cast the constness away and write to it without invoking undefined behaviour:

struct foo { const int x; };

foo a;
int& b = const_cast<int&>(a.x);
b = 17; // invokes undefined behaviour

A readonly field in C# only means that the field itself cannot be reassigned. It’s akin to T *const or T& in C++. You can change the referenced object at will through its members.

class Foo { public int x; }
class Bar { public readonly Foo y = new Foo(); }

Bar a = new Bar();
a.y.x = 3; // valid
a.y = new Foo(); // invalid

Well, I’m not telling the whole truth. You can cheat and change readonly fields through reflection¹:

typeof(string).GetField("Empty").SetValue(null, "bar");
// this effectively makes string.Empty equal to "bar", with disastrous consequences
// It requires full trust though.
// Obviously, it's evil.

If it is a const field however, not even this trick will work.

const fields are hardcoded in assemblies that use them, instead of keeping references to the original assembly:

// Assembly A.dll
public class Foo { public static const int X = 42; }

// Assembly B.dll
int y = Foo.X;
// this is the equivalent to:
int y = 42;

This means that if you recompile A.dll and change the value of Foo.X to 23, B.dll will still use 42 until it is recompiled.

All that said, if you want to have a field that you want to change, just don’t make it readonly. If you want it to be mutable by the class, but immutable from the outside, make it private and add a read-only property (note: this is not the same as a readonly field):

class Foo
{
    private int bar;
    public int Bar
    {
        get { return bar; }
    }
}

---

¹_{This is not really guaranteed, but it works on the Microsoft implementations. If you’re wondering why this hack works at all, you can read Eric Lippert’s explanation. Be sure to also read the answer about readonly on value types. And it goes without saying, don’t do this at home.}