Moles to mock an indexed property

I am looking for a way to 'detour' a single item in a configuration file with Moles. I can do this:

         NameValueCollection appsettings = new NameValueCollection();
         appSettings.Add("MyKey", "MyValue");
         System.Configuration.Moles.MConfigurationManager.AppSettingsGet = () => this.appSettings;

This works fine, however the class which I am trying to test uses some other开发者_C百科 configuration settings including ConfigSections and the Moles detour seems to have broken this. I only want to replace a specific value, not the whole section. In TypeMock, I could do this:

Isolate.WhenCalled(() => ConfigurationManager.AppSettings["MyKey"]).WithExactArguments().WillReturn("MyValue");

When I mock the configurationManager using TypeMock, my test passes, but when I use the Moles version (which looks like it should do the same thing) it fails meaning that Moles must be affecting the ConfigurationManager class in a way which TypeMock doesn't.

Any help on how I can use moles to just behave in the way which TypeMock does would be greatly appreciated.

---

1. make a copy of current app.settings section:

   NameValueCollection appSettings = new NameValueCollection(ConfigurationManager.AppSettings);
   

2. Modify the part you need by calling Set/Add function (keep in mind that 'Add' function does not overwrite existing values but it adds a new value for the key. On the other hand, 'Set' always creates a new entry and gets rid of the existing values):

   appSettings.Set("key name", "new value");
   

3. Create the delegate:

   MConfigurationManager.AppSettingsGet =
     () =>
     {
       return appSettings;
     };