Something like below; this specific example uses C# 3, but you can do
similar with C# 2 - you just lose the ability to use
left.AddRange(right) - you have to do SomeClass.AddRange(left,right)
[perhaps renaming the method].

Marc

using System;
using System.Collections.Generic;
using System.Xml.Linq;
static class Program
{
   static void Main()
   {
       // dictionary
       var left = new Dictionary<int, string>();
       left.Add(1, "Fred");
       left.Add(2, "Barney");

       var right = new Dictionary<int, string>();
       right.Add(3, "Wilma");
       right.Add(4, "Betty");

       left.AddRange(right);
   }
}

public static class DictionaryExt
{
   public static void AddRange<TKey, TValue>(
       this IDictionary<TKey, TValue> destination,
       IDictionary<TKey, TValue> values)
   {
       if (destination == null) throw new
ArgumentNullException("destination");
       if (values == null) throw new ArgumentNullException("values");

       foreach (var pair in values)
       {
           destination.Add(pair);
       }
   }
   public static void Merge<TKey, TValue>(
       this IDictionary<TKey, TValue> destination,
       IDictionary<TKey, TValue> values)
   {
       if (destination == null) throw new
ArgumentNullException("destination");
       if (values == null) throw new ArgumentNullException("values");

       foreach (var pair in values)
       {
           destination[pair.Key] = pair.Value;
       }
   }
}