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

You can:

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

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.)

I agree with the other answers in that it works generally and especially with the comment by E. Lippert that this is not documented behavior and therefore not future-proof code.

However, we also noticed another issue. If you're running your code in an environment with restricted permissions you might get an exception.

We've just had a case where our code worked fine on our machines, but we received a VerificationException when the code ran in a restricted environment. The culprit was a reflection call to the setter of a readonly field. It worked when we removed the readonly restriction of that field.