CanwriteReadOnlyReflection.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace CanwriteReadOnlyReflection
{
    [TestClass]
    public class UnitTest1
    {
        public class TargetType
        {
            private readonly object _foo;

            public object Foo { get { return _foo; } }
        }
        [TestMethod]
        public void TestMethod1()
        {
            // instance
            var target = new TargetType();
            
            // reflected type, can be obtained through several other means
            var reflection = target.GetType();
            // reflected field
            var field = reflection.GetField("_foo", System.Reflection.BindingFlags.GetField | System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
            field.SetValue(target, "bar");

            // assert that private readonly instance member has been modified by fetched it via read-only public property
            Assert.AreEqual("bar", target.Foo);
        }
    }
}