What is the purpose of nameof

Question

Version 6 0 got a new feature of nameof  but I can t understand the purpose of it  as it just takes the variable name and changes it to a string on compilation   I thought it might have some purpose when using  lt T gt  but when I try to nameof T  it just prints me a T instead of the used type   Any idea on the purpose

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One of the usage of nameof keyword is for setting Binding in wpf programmatically   to set Binding you have to set Path with string  and with nameof keyword  it s possible to use Refactor option   For example  if you have IsEnable dependency property in your UserControl and you want to bind it to IsEnable of some CheckBox in your UserControl  you can use these two codes   CheckBox chk   new CheckBox    Binding bnd   new Binding   IsEnable     Source   this    chk SetBinding IsEnabledProperty  bnd     and  CheckBox chk   new CheckBox    Binding bnd   new Binding  nameof  IsEnable     Source   this    chk SetBinding IsEnabledProperty  bnd     It s obvious the first code can t refactor but the secend one

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I find that nameof increases the readability of very long and complex SQL statements in my applications  It makes the variables stand out of that sea of strings and eliminates your job of figuring out where the variables are used in your SQL statements   public bool IsFooAFoo string foo  string bar        var aVeryLongAndComplexQuery      SELECT yada  yada     -- long query in here     WHERE fooColumn     nameof foo       AND barColumn     nameof bar       -- long query here         SqlParameter   parameters             new SqlParameter nameof foo   SqlDBType VarChar  10   Value   foo            new SqlParameter nameof bar   SqlDBType VarChar  10   Value   bar

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Most common usage will be in input validation  such as    Currently void Foo string par       if  par    null  throw new ArgumentNullException  par         C  6 nameof void Foo string par       if  par    null  throw new ArgumentNullException nameof par        In first case  if you refactor the method changing par parameter s name  you ll probably forget to change that in the ArgumentNullException  With nameof you don t have to worry about that   See also  nameof  C  and Visual Basic Reference

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It has advantage when you use ASP Net MVC  When you use HTML helper to build some control in view it uses property names in name attribure of html input    Html TextBoxFor m   gt  m CanBeRenamed    It makes something like that    lt input type  text  name  CanBeRenamed    gt    So now  if you need to validate your property in Validate method you can do this   public IEnumerable lt ValidationResult gt  Validate ValidationContext validationContext      if  IsNotValid CanBeRenamed         yield return new ValidationResult          Property  nameof CanBeRenamed   is not valid         new         nameof CanBeRenamed               In case if you rename you property using refactoring tools  your validation will not be broken

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The most common use case I can think of is when working with the INotifyPropertyChanged interface   Basically everything related to WPF and bindings uses this interface   Take a look at this example   public class Model   INotifyPropertyChanged          From the INotifyPropertyChanged interface     public event PropertyChangedEventHandler PropertyChanged       private string foo      public String Foo               get   return this foo            set                       this foo   value                 Old code              PropertyChanged this  new PropertyChangedEventArgs  Foo                     New Code              PropertyChanged this  new PropertyChangedEventArgs nameof Foo                                    As you can see in the old way we have to pass a string to indicate which property has changed  With nameof we can use the name of the property directly  This might not seem like a big deal  But image what happens when somebody changes the name of the property Foo  When using a string the binding will stop working  but the compiler will not warn you  When using nameof you get a compiler error that there is no property argument with the name Foo    Note that some frameworks use some reflection magic to get the name of the property  but now we have nameof this is no longer neccesary

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Another use case of nameof is to check tab pages  instead of checking the index you can check the Name property of the tabpages as follow   if tabControl SelectedTab Name    nameof tabSettings            Do something     Less messy

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The ASP NET Core MVC project uses nameof in the AccountController cs and ManageController cs with the RedirectToAction method to reference an action in the controller   Example   return RedirectToAction nameof HomeController Index    Home      This translates to   return RedirectToAction  Index    Home      and takes takes the user to the  Index  action in the  Home  controller  i e   Home Index

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Your question already expresses the purpose  You must see this might be useful for logging or throwing exceptions  For example  public void DoStuff object input        if  input    null                throw new ArgumentNullException nameof input             This is good  If I change the name of the variable  the code will break instead of returning an exception with an incorrect message   Of course  the uses are not limited to this simple situation  You can use nameof whenever it would be useful to code the name of a variable or property  The uses are manifold when you consider various binding and reflection situations  It s an excellent way to bring what were run time errors to compile time

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The purpose of the nameof operator is to provide the source name of the artifacts   Usually the source name is the same name as the metadata name   public void M string p        if  p    null                throw new ArgumentNullException nameof p                     public int P       get               return p            set               p   value          NotifyPropertyChanged nameof P              But this may not always be the case   using i   System Int32      Console WriteLine nameof i       prints  i    Or   public static string Extension lt T gt  this T t        return nameof T   returns  T      One use I ve been giving to it is for naming resources    Display      ResourceType   typeof Resources       Name   nameof Resources Title Name       ShortName   nameof Resources Title ShortName       Description   nameof Resources Title Description       Prompt   nameof Resources Title Prompt      The fact is that  in this case  I didn t even need the generated properties to access the resources  but now I have a compile time check that the resources exist

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As others have already pointed out  the nameof operator does insert the name that the element was given in the sourcecode    I would like to add that this is a really good idea in terms of refactoring since it makes this string refactoring safe  Previously  I used a static method which utilized reflection for the same purpose  but that has a runtime performance impact  The nameof operator has no runtime performance impact  it does its work at compile time  If you take a look at the MSIL code you will find the string embedded  See the following method and its disassembled code   static void Main string   args        Console WriteLine nameof args        Console WriteLine  regular text          striped nops from the listing IL 0001 ldstr args IL 0006 call System Void System Console  WriteLine System String  IL 000C ldstr regular text IL 0011 call System Void System Console  WriteLine System String  IL 0017 ret   However  that can be a drawback if you plan to obfuscate your software  After obfuscation the embedded string may no longer match the name of the element  Mechanisms that rely on this text will break  Examples for that  including but not limited to are  Reflection  NotifyPropertyChanged      Determining the name during runtime costs some performance  but is safe for obfuscation  If obfuscation is neither required nor planned  I would recommend using the nameof operator

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Another use-case where nameof feature of C  6 0 becomes handy - Consider a library like Dapper which makes DB retrievals much easier  Albeit this is a great library  you need to hardcode property field names within query  What this means is that if you decide to rename your property field  there are high chances that you will forget to update query to use new field names  With string interpolation and nameof features  code becomes much easier to maintain and typesafe   From the example given in link  without nameof  var dog   connection Query lt Dog gt   select Age    Age  Id    Id   new   Age    int  null  Id   guid       with nameof  var dog   connection Query lt Dog gt    select  nameof Dog Age      Age   nameof Dog Id      Id   new   Age    int  null  Id   guid

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The MSDN article lists MVC routing  the example that really clicked the concept for me  among several others  The  formatted  description paragraph reads    When reporting errors in code  hooking up model-view-controller  MVC  links  firing property changed events  etc    you often want to capture the string name of a method  Using nameof helps keep your code valid when renaming definitions  Before you had to use string literals to refer to definitions  which is brittle when renaming code elements because tools do not know to check these string literals   The accepted   top rated answers already give several excellent concrete examples

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Previously we were using something like that      Some form  SetFieldReadOnly       gt  Entity UserName           Base form  private void SetFieldReadOnly Expression lt Func lt object gt  gt  property        var propName   GetPropNameFromExpr property       SetFieldsReadOnly propName      private void SetFieldReadOnly string propertyName                Reason - compile time safety  No one can silently rename property and break code logic  Now we can use nameof

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It s really useful for ArgumentException and its derivatives   public string DoSomething string input         if input    null                 throw new ArgumentNullException nameof input                    Now if someone refactors the name of the input parameter the exception will be kept up to date too   It is also useful in some places where previously reflection had to be used to get the names of properties or parameters   In your example nameof T  gets the name of the type parameter - this can be useful too   throw new ArgumentException nameof T     Type  typeof T   does not support this method         Another use of nameof is for enums - usually if you want the string name of an enum you use  ToString     enum MyEnum       FooBar   7        Console WriteLine MyEnum FooBar ToString       gt   FooBar    This is actually relatively slow as  Net holds the enum value  i e  7  and finds the name at run time   Instead use nameof   Console WriteLine nameof MyEnum FooBar     gt   FooBar    Now  Net replaces the enum name with a string at compile time     Yet another use is for things like INotifyPropertyChanged and logging - in both cases you want the name of the member that you re calling to be passed to another method      Property with notify of change public int Foo       get   return this foo        set               this foo   value          PropertyChanged this  new PropertyChangedEventArgs nameof this Foo              Or        Write a log  audit or trace for the method called void DoSomething     params            Log nameof DoSomething    Message

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What about cases where you want to reuse the name of a property  for example when throwing exception based on a property name  or handling a PropertyChanged event  There are numerous cases where you would want to have the name of the property   Take this example   switch  e PropertyName        case nameof SomeProperty         break            opposed to     case  SomeOtherProperty         break        In the first case  renaming SomeProperty will change the name of the property too  or it will break compilation  The last case doesn t   This is a very useful way to keep your code compiling and bug free  sort-of     A very nice article from Eric Lippert why infoof didn t make it  while nameof did

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Let s say you need to print the name of a variable in your code  If you write  int myVar   10  print  quot myVar quot     quot  value is  quot    myVar toString      and then if someone refactors the code and uses another name for myVar  he she would have to look for the string value in your code and change it accordingly  Instead  if you write  print nameof myVar     quot  value is  quot    myVar toString      It would help to refactor automatically

[c#] What is the purpose of nameof?

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