Most of .NET developers are familiar with collection initializers syntax. Long story short, this language feature allows you to replace this ugly syntax<\/p>\n
\r\nvar numbers = new List();\r\nnumbers.Add(1);\r\nnumbers.Add(2);\r\nnumbers.Add(3);\r\nnumbers.Add(4);\r\n<\/pre>\nwith much shorter and cleaner one<\/p>\n
\r\nvar numbers = new List { 1, 2, 3, 4 };\r\n<\/pre>\nThe more interesting thing is the fact that you can leverage this syntax not only for build-in collections but also for your custom classes. In order to do that your class must implement IEnumerable<\/i> interface and expose public Add<\/i> method. Let’s assume that we have some real case scenario and we have class which looks like that<\/p>\n\r\npublic class StatusMessages : IEnumerable\r\n{\r\n private List messages = new List();\r\n\r\n public IEnumerator GetEnumerator()\r\n {\r\n return this.messages.GetEnumerator();\r\n }\r\n\r\n IEnumerator IEnumerable.GetEnumerator()\r\n {\r\n return this.messages.GetEnumerator();\r\n }\r\n}\r\n\r\npublic class StatusMessage\r\n{\r\n public string Text { get; set; } = string.Empty;\r\n public string SourceSystem { get; set; } = string.Empty;\r\n public int StatusCode { get; set; }\r\n\r\n public StatusMessage()\r\n {\r\n }\r\n\r\n public StatusMessage(string text)\r\n {\r\n this.Text = text;\r\n }\r\n\r\n public override string ToString()\r\n {\r\n return $\"Text: {this.Text} StatusCode: {this.StatusCode} SourceSystem: {this.SourceSystem}\";\r\n }\r\n}\r\n<\/pre>\nThis is a simple wrapper over the List. At this moment we can’t use collection initializer when constructing objects of this classs. However by adding method Add<\/i> <\/p>\n\r\npublic class StatusMessages : IEnumerable\r\n{\r\n \/\/ rest of the code omitted for brevity\r\n public void Add(StatusMessage message)\r\n {\r\n this.messages.Add(message);\r\n }\r\n \/\/ rest of the code omitted for brevity\r\n}\r\n<\/pre>\nwe can create a new instance of that class just like we would do with List<\/i><\/p>\n\r\nvar messagesAddInitializer = new StatusMessages\r\n {\r\n new StatusMessage(\"y\"),\r\n new StatusMessage(\"x\"),\r\n };\r\n<\/pre>\nWhat is more, it is possible to have multiple overloads of Add<\/i> method in our class, so for instance if we add<\/p>\n\r\npublic class StatusMessages : IEnumerable\r\n{\r\n \/\/ rest of the code omitted for brevity\r\n public void Add(string text)\r\n {\r\n this.Add(new StatusMessage(text));\r\n }\r\n \/\/ rest of the code omitted for brevity\r\n}\r\n<\/pre>\nwe are able to use this syntax<\/p>\n
\r\nvar messagesAddStringInitializer = new StatusMessages\r\n {\r\n \"y\",\r\n \"x\"\r\n };\r\n<\/pre>\nIn addition it is even possible to use a mix of Add<\/i> overloads<\/p>\n\r\nvar messagesAddAndTextInitializers = new StatusMessages\r\n {\r\n new StatusMessage(\"y\"),\r\n \"x\"\r\n };\r\n<\/pre>\nAdd<\/i> method can also have multiple arguments<\/p>\n\r\npublic class StatusMessages : IEnumerable\r\n{\r\n \/\/ rest of the code omitted for brevity\r\n public void Add(int statusCode, string text)\r\n {\r\n this.Add(new StatusMessage(text) { StatusCode = statusCode });\r\n }\r\n \r\n public void Add(int statusCode, string text, string sourceSystem)\r\n {\r\n this.Add(new StatusMessage(text) { StatusCode = statusCode,SourceSystem = sourceSystem });\r\n }\r\n \/\/ rest of the code omitted for brevity\r\n}\r\n<\/pre>\nin that case we are able to use this syntax<\/p>\n
\r\nvar messagesMultipleArgumentsInitializers = new StatusMessages\r\n {\r\n { 1, \"x\" },\r\n {1,\"x\",\"source\" }\r\n };\r\n<\/pre>\nThe last thing I would like to point out is that Add<\/i> function doesn’t have to be an instance method, we can easily implement it as an extension method and still leverage the collection initializer syntax. <\/p>\n\r\nnamespace AbusingCollectionInitializers\r\n{\r\n public static class StatusMessagesExtensions\r\n {\r\n public static void Add(this StatusMessages statusMessages, int statusCode)\r\n {\r\n statusMessages.Add(new StatusMessage() { StatusCode = statusCode });\r\n }\r\n }\r\n}\r\n<\/pre>\nNow with proper namespace import we can write this code<\/p>\n
\r\nusing AbusingCollectionInitializers;\r\n\/\/ rest of the code omitted for brevity\r\nvar messagesExtensionInitializers = new StatusMessages { 1,2,3 };\r\n<\/pre>\n