gistfile1.cs

// Current (C# 5) syntax
var request = new CreateIndexRequest("new-index-name")
{
  IndexSettings = new IndexSettings
  {
    Settings = new Dictionary<string, object>
    {
      {"index.settings", "value"}
    },
    Mappings = new List<RootObjectMapping>
    {
      new RootObjectMapping
      {
        Name = "my_root_object",
        Properties = new Dictionary<PropertyNameMarker, IElasticType>
        {
          {"my_field", new StringMapping() { Analyzer = "default" } }
        }
      }
    }
  }
};

gistfile2.cs

// C# 6 dictionary initializer syntax
var request = new CreateIndexRequest("new-index-name")
{
  IndexSettings = new IndexSettings
  {
    Settings = {
      "index.settings" = "value"
    },
    Mappings = {
      new RootObjectMapping
      {
        Name = "my_root_object",
        Properties = {
          "my_field" = new StringMapping() { Analyzer = "default" } 
        }
      }
    }
  }
};

gistfile3.cs

// C# 6 member initializer syntax
var request = new CreateIndexRequest("new-index-name")
{
  IndexSettings = new IndexSettings
  {
    Settings = {
      $index_settings = "value" // Custom made-up serializer maps _ to .
    },
    Mappings = {
      new RootObjectMapping
      {
        Name = "my_root_object",
        Properties = {
          $my_field = new StringMapping() { Analyzer = "default" } 
        }
      }
    }
  }
};

gistfile4.json

{
  "settings": {
    "index.settings": "value"
  },
  "mappings": {
    "my_root_object": {
      "properties": {
        "my_field": { "analyzer": "default" }
      }
    }
  }
}