A lazier StringBuilder...

StringBuilderEx

    sealed class StringBuilderEx
    {
        readonly List<Func<string>> parts;
 
        Func<string, string> replace = null;
 
        public int Capacity
        {
            get
            {
                return parts.Capacity;
            }
        }
 
        public StringBuilderEx(int n)
        {
            parts = new List<Func<string>> (n);
        }
 
        public StringBuilderEx()
        {
            parts = new List<Func<string>> ();
        }
 
        public void Append(string s)
        {
            parts.Add (() => s);
        }
        
        public void AppendLine(string s)
        {
            parts.Add (() => s + Environment.NewLine);
        }
 
        public void AppendFormat(string format, params object[] objs)
        {
            parts.Add (() => string.Format (format, objs));
        }
 
        public void AppendFormat(Func<string> format, params object[] objs)
        {
            parts.Add (() => string.Format (format(), objs));
        }
 
        public void AppendFormat(Func<string> format, params Func<object>[] objs)
        {
            Func<string> thunk = () =>
                {
                    var evaluated_objs = new object[objs.Length];
 
                    for (var i = 0; i < objs.Length; i++)
                        evaluated_objs[i] = objs[i] ();
 
                    return string.Format (format (), evaluated_objs);
                };
 
            parts.Add (thunk);
        }
 
        public void Append(Func<string> f)
        {
            parts.Add (f);
        }
 
        public void AppendLine(Func<string> f)
        {
            parts.Add (() => f () + Environment.NewLine);
        }
 
        public void InsertAt(int n, Func<string> f)
        {
            parts.Insert (n, f);
        }
 
        public void Clear()
        {
            parts.Clear ();
        }
 
        public StringBuilderEx Replace(char oldChar, char newChar)
        {
            replace = s => s.Replace (oldChar, newChar);
 
            return this;
        }
 
        public StringBuilderEx Replace(string oldString, string newString)
        {
            replace = s => s.Replace (oldString, newString);
 
            return this;
        }
 
        public void RemoveAt(int n)
        {
            parts.RemoveAt (n);
        }
 
        public override string ToString()
        {
            var temp_array = new string[parts.Count];
 
            if (replace != null)
                for (var i = 0; i < parts.Count; i++)
                    temp_array[i] = replace (parts[i] ());
            else
                for (var i = 0; i < parts.Count; i++)
                    temp_array[i] = parts[i] ();
            
            
            return string.Join ("", temp_array);
        }
    }