Returning anonymous type in C

Question

I have a query that returns an anonymous type and the query is in a method  How do you write this   public  TheAnonymousType  TheMethod SomeParameter      using  MyDC TheDC   new MyDC             var TheQueryFromDB                                    select new   SomeVariable                                                 AnotherVariable                                      ToList           return  TheAnonymousType

User · Answer

Using C  7 0 we still can t return anonymous types but we have a support of tuple types and thus we can return a collection of tuple  System ValueTuple lt T1 T2 gt  in this case   Currently Tuple types are not supported in expression trees and you need to load data into memory   The shortest version of the code you want may look like this   public IEnumerable lt  int SomeVariable  object AnotherVariable  gt  TheMethod                  return  from data in TheDC Data         select new   data SomeInt  data SomeObject    ToList            Select data   gt   SomeVariable  data SomeInt  AnotherVariable  data SomeObject       Or using the fluent Linq syntax   return TheDC Data      Select data   gt  new  SomeVariable  data SomeInt  AnotherVariable  data SomeObject        ToList         Select data   gt   SomeVariable  data SomeInt  AnotherVariable  data SomeObject     Using C  7 1 we can omit properties names of tuple and they will be inferred from tuple initialization like it works with anonymous types   select  data SomeInt  data SomeObject     or Select data   gt   data SomeInt  data SomeObject

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Now with local functions especially  but you could always do it by passing a delegate that makes the anonymous type    So if your goal was to run different logic on the same sources  and be able to combine the results into a single list  Not sure what nuance this is missing to meet the stated goal  but as long as you return a T and pass a delegate to make T  you can return an anonymous type from a function      returning an anonymous type    look mom no casting void LookMyChildReturnsAnAnonICanConsume            if C  had first class functions you could do        var anonyFunc    name string id int    gt  new  Name name Id id       var items   new     new   Item1    hello   Item2   3          var itemsProjection  items Select x   gt  SomeLogic x Item1  x Item2   y  i    gt  new   Word   y  Count   i             same projection   same type     var otherSourceProjection   SomeOtherSource  y i    gt  new  Word y Count i        var q           from anony1 in itemsProjection         join anony2 in otherSourceProjection             on anony1 Word equals anony2 Word         select new  anony1 Word Source1Count anony1 Count Source2Count anony2 Count       var togetherForever   itemsProjection Concat otherSourceProjection  ToList       T SomeLogic lt T gt  string item1  int item2  Func lt string int T gt  f       return f item1 item2     IEnumerable lt T gt  SomeOtherSource lt T gt  Func lt string int T gt  f       var dbValues   new    Tuple Create  hello  1   Tuple Create  bye  2        foreach var x in dbValues          yield return f x Item1 x Item2

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With reflection   public object tst         var a   new           prop1    test1           prop2    test2              return a      public string tst2 object anonymousObject  string propName        return anonymousObject GetType   GetProperties            Where w   gt  w Name    propName           Select s   gt  s GetValue anonymousObject            FirstOrDefault   ToString        Sample   object a   tst    var val   tst2 a   prop2      Output   test2

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Another option could be using automapper  You will be converting to any type from your anonymous returned object as long public properties matches  The key points are  returning object  use linq and autommaper   or use similar idea returning serialized json  etc  or use reflection     using System Linq  using System Reflection  using AutoMapper  using Microsoft VisualStudio TestTools UnitTesting  using Newtonsoft Json   namespace UnitTestProject1        TestClass      public class UnitTest1                TestMethod          public void TestMethod1                         var data   GetData                 var firts   data First                 var info   firts GetType   GetProperties BindingFlags Instance   BindingFlags Public  First p   gt  p Name     Name                var value   info GetValue firts                Assert AreEqual value   One                         TestMethod          public void TestMethod2                         var data   GetData                 var config   new MapperConfiguration cfg   gt  cfg CreateMissingTypeMaps   true               var mapper   config CreateMapper                 var users   data Select mapper Map lt User gt   ToArray                 var firts   users First                 Assert AreEqual firts Name   One                         TestMethod          public void TestMethod3                         var data   GetJData                  var users   JsonConvert DeserializeObject lt User   gt  data                var firts   users First                 Assert AreEqual firts Name   One                        private object   GetData                          return new     new   Id   1  Name    One     new   Id   2  Name    Two                          private string GetJData                          return JsonConvert SerializeObject new     new   Id   1  Name    One     new   Id   2  Name    Two       Formatting None                      public class User                       public int Id   get  set                public string Name   get  set

User · Answer

You can only use dynamic keyword      dynamic obj   GetAnonymousType        Console WriteLine obj Name      Console WriteLine obj LastName      Console WriteLine obj Age         public static dynamic GetAnonymousType               return new   Name    John   LastName    Smith   Age 42          But with dynamic type keyword you will loose compile time safety  IDE IntelliSense etc

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You can return list of objects in this case   public List lt object gt  TheMethod SomeParameter      using  MyDC TheDC   new MyDC             var TheQueryFromDB                                    select new   SomeVariable                                                 AnotherVariable                                      ToList           return TheQueryFromDB

User · Answer

C  compiler is a two phase compiler  In the first phase it just checks namespaces  class hierarchies  Method signatures etc  Method bodies are compiled only during the second phase   Anonymous types are not determined until the method body is compiled    So the compiler has no way of determining the return type of the method during the first phase   That is the reason why anonymous types can not be used as return type   As others have suggested if you are using  net 4 0 or grater  you can use Dynamic   If I were you I would probably create a type and return that type from the method  That way it is easy for the future programmers who maintains your code and more readable

User · Answer

You can t   You can only return object  or container of objects  e g  IEnumerable lt object gt   IList lt object gt   etc

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It is actually possible to return an anonymous type from a method in a particular use-case  Let s have a look   With C  7 it is possible to return anonymous types from a method  although it comes with a slight constraint  We are going to use a new language feature called local function together with an indirection trick  another layer of indirection can solve any programming challenge  right     Here s a use-case I recently identified  I want to log all configuration values after I have loaded them from AppSettings  Why  Because there s some logic around missing values that revert to default values  some parsing and so on  An easy way to log the values after applying the logic is to put them all in a class and serialize it to a logfile  using log4net   I also want to encapsulate the complex logic of dealing with settings and separate that from whatever I need to do with them  All without creating a named class that exists just for a single use   Let s see how to solve this using a local function that creates an anonymous type   public static HttpClient CreateHttpClient            I deal with configuration values in this slightly convoluted way         The benefit is encapsulation of logic and we do not need to        create a class  as we can use an anonymous class         The result resembles an expression statement that        returns a value  similar to expressions in F       var config   Invoke      gt                   slightly complex logic with default value            in case of missing configuration value             this is what I want to encapsulate          int  acquireTokenTimeoutSeconds   null          if  int TryParse ConfigurationManager AppSettings  AcquireTokenTimeoutSeconds    out int i                         acquireTokenTimeoutSeconds   i                        more complex logic surrounding configuration values                 construct the aggregate configuration class as an anonymous type          var c   new                       AcquireTokenTimeoutSeconds                   acquireTokenTimeoutSeconds    DefaultAcquireTokenTimeoutSeconds                     more properties                        log the whole object for monitoring purposes             this is also a reason I want encapsulation          Log InfoFormat  Config  0    c           return c                  use this configuration in any way necessary           the rest of the method concerns only the factory         i e  creating the HttpClient with whatever configuration        in my case this      return new HttpClient               local function that enables the above expression     T Invoke lt T gt  Func lt T gt  func    gt  func Invoke        I have succeeded in constructing an anonymous class and also encapsulated the logic of dealing with complex settings management  all inside CreateHttpClient and within its own  expression   This might not be exactly what the OP wanted but it is a lightweight approach with anonymous types that is currently possible in modern C

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In C  7 we can use tuples to accomplish this   public List lt  int SomeVariable  string AnotherVariable  gt  TheMethod SomeParameter      using  MyDC TheDC   new MyDC             var TheQueryFromDB                                select new   SomeVariable                                             AnotherVariable                                  ToList           return TheQueryFromDB                  Select s   gt                         SomeVariable   s SomeVariable                        AnotherVariable   s AnotherVariable                     ToList            You might need to install System ValueTuple nuget package though

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You cannot return anonymous types  Can you create a model that can be returned  Otherwise  you must use an object   Here is an article written by Jon Skeet on the subject  Code from the article   using System   static class GrottyHacks       internal static T Cast lt T gt  object target  T example                return  T  target           class CheesecakeFactory       static object CreateCheesecake                 return new   Fruit  Strawberry   Topping  Chocolate                static void Main                 object weaklyTyped   CreateCheesecake            var stronglyTyped   GrottyHacks Cast weaklyTyped              new   Fruit     Topping                 Console WriteLine  Cheesecake   0    1                 stronglyTyped Fruit  stronglyTyped Topping                         Or  here is another similar article  Or  as others are commenting  you could use dynamic

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You can use the Tuple class as a substitute for an anonymous types when returning is necessary   Note  Tuple can have up to 8 parameters   return Tuple Create variable1  variable2     Or  for the example from the original post   public List lt Tuple lt SomeType  AnotherType gt  gt  TheMethod SomeParameter      using  MyDC TheDC   new MyDC             var TheQueryFromDB                                    select Tuple Create                                        ToList           return TheQueryFromDB ToList              http   msdn microsoft com en-us library system tuple v vs 110  aspx

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Three options   Option1   public class TheRepresentativeType       public     SomeVariable  get set       public     AnotherVariable  get set      public IEnumerable lt TheRepresentativeType gt  TheMethod SomeParameter       using  MyDC TheDC   new MyDC              var TheQueryFromDB                                    select new TheRepresentativeType  SomeVariable                                                 AnotherVariable                                      ToList          return TheQueryFromDB            Option 2   public IEnumerable TheMethod SomeParameter       using  MyDC TheDC   new MyDC              var TheQueryFromDB                                    select new TheRepresentativeType  SomeVariable                                                 AnotherVariable                                      ToList         return TheQueryFromDB            you can iterate it as object  Option 3   public IEnumerable lt dynamic gt  TheMethod SomeParameter       using  MyDC TheDC   new MyDC              var TheQueryFromDB                                    select new TheRepresentativeType  SomeVariable                                                 AnotherVariable                                      ToList          return TheQueryFromDB    You may need to call  Cast lt dynamic gt     but I m not sure           and you will be able to iterate it as a dynamic object and access their properties directly

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public List lt SomeClass gt  TheMethod SomeParameter      using  MyDC TheDC   new MyDC             var TheQueryFromDB                                    select new SomeClass  SomeVariable                                                 AnotherVariable                                      ToList           return TheQueryFromDB ToList             public class SomeClass     public string SomeVariable get set     public string AnotherVariable get set       Creating your own class and querying for it is the best solution I know As much as I know you can not use anonymous type return values in another method  because it won t just be recognized However  they can be used in the same method  I used to return them as IQueryable or IEnumerable  though it still does not let you see what is inside of the anonymous type variable   I run into something like this before while I was trying to refactor some code  you can check it here   Refactoring and creating separate methods

User · Answer

You can return dynamic which will give you a runtime checked version of the anonymous type but only in  NET 4

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