Below is a little Dictionary extension method for mimicking upsert functionality on in-memory System.Collections.Generic. 
   1: public static class DictionaryExtensions
   2: {
   3:     public static TValue Upsert<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue value)             
   4:     {
   5:         if (dictionary.ContainsKey(key))
   6:         {
   7:             // Update value to existing key
   8:             dictionary[key] = value;
   9:         }
  10:         else
  11:         {
  12:             // Insert new key
  13:             dictionary.Add(key, value);
  14:         }
  15:         return value;
  16:     }
  17: }

Disclaimer: There are several instances where it is inappropriate to leverage upsert-like functionality for your in-memory hashtables. As with any particular code abstraction, the consumer will need to determine when it is fair to use the syntax shortcut.

As a short sidebar, if you use Code Snippet plugin for Live Writer, you’ll have to run as administrator as it operates on:

System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.mshtml, Version=7.0.3300.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'Microsoft.mshtml, Version=7.0.3300.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at WLWPluginBase.Win32.Win32IEHelper.GetSelectedText(IntPtr handle)
   at CodeSnippet.CodeSnippetPlugin.CreateContent(IWin32Window dialogOwner, String& content)
   at WindowsLive.Writer.PostEditor.ContentSources.ContentSourceManager.PerformInsertion(IContentSourceSite sourceSite, ContentSourceInfo contentSource)