StringExtensions.cs

 internal static class StringExtensions
    {
        public static string ReplaceCaseInsensitive(this string source, object replacements)
        {
            replacements
                .GetType()
                .GetProperties()
                .Select(p => new {p.Name, Value = p.GetValue(replacements) as string ?? ""})
                .ToList()
                .ForEach(p => { source = ReplaceCaseInsensitive(source, string.Format("{{{0}}}", p.Name), p.Value); });
            
            return source;
        }

        private static string ReplaceCaseInsensitive(string originalString, string oldValue, string newValue)
        {
            var startIndex = 0;
            while (true)
            {
                startIndex = originalString.IndexOf(oldValue, startIndex, StringComparison.InvariantCultureIgnoreCase);
                if (startIndex == -1)
                    break;

                originalString = originalString.Substring(0, startIndex) + newValue + originalString.Substring(startIndex + oldValue.Length);
                startIndex += newValue.Length;
            }
            return originalString;
        }
    }
    
    [TestFixture]
    public class StringExtensionTests
    {
        [Test]
        public void Can_perform_basic_replace()
        {
            ////Act
            var result = "{test}".ReplaceCaseInsensitive(new {Test = "dimmer"});

            ////Assert
            Assert.That(result, Is.EqualTo("dimmer"));
        }

        [Test]
        public void Can_perform_double_replace()
        {
            ////Act
            var result = "{test}{test2}".ReplaceCaseInsensitive(new { Test = "dimmer", Test2 = "dimmer2" });

            ////Assert
            Assert.That(result, Is.EqualTo("dimmerdimmer2"));
        }

        [Test]
        public void Null_value_is_ok()
        {
            ////Act
            var result = "{test}{test2}".ReplaceCaseInsensitive(new { Test = "dimmer", Test2 = (string)null });

            ////Assert
            Assert.That(result, Is.EqualTo("dimmer"));
        }

        [Test]
        public void Null_source_is_ok()
        {
            ////Act
            var result = "".ReplaceCaseInsensitive(new { Test = "dimmer", Test2 = (string)null });

            ////Assert
            Assert.That(result, Is.EqualTo(""));
        }

        [Test]
        public void Case_is_ignored()
        {
            ////Act
            var result = "{tesT}".ReplaceCaseInsensitive(new { Test = "dimmer" });

            ////Assert
            Assert.That(result, Is.EqualTo("dimmer"));
        }

        [Test]
        public void Non_existent_is_ignored()
        {
            ////Act
            var result = "nothing".ReplaceCaseInsensitive(new { Test = "dimmer" });

            ////Assert
            Assert.That(result, Is.EqualTo("nothing"));
        }
    }