What is the difference between const and readonly in C#?

Question

What is the difference between const and readonly in C#?

When would you use one over the other?

Answer 1

Yet another gotcha: readonly values can be changed by "devious" code via reflection.

var fi = this.GetType()
             .BaseType
             .GetField("_someField", 
                       BindingFlags.Instance | BindingFlags.NonPublic);
fi.SetValue(this, 1);

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