Passing properties by reference in C

Question

I m trying to do do the following   GetString      inputString      ref Client WorkPhone   private void GetString string inValue  ref string outValue        if   string IsNullOrEmpty inValue                 outValue   inValue            This is giving me a compile error  I think its pretty clear what I m trying to achieve  Basically I want GetString to copy the contents of an input string to the WorkPhone property of Client   Is it possible to pass a property by reference

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The accepted answer is good if that function is in your code and you can modify it  But sometimes you have to use an object and a function from some external library and you can t change the property and function definition  Then you can just use a temporary variable   var phone   Client WorkPhone  GetString input  ref phone   Client WorkPhone   phone

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I wrote a wrapper using the ExpressionTree variant and c 7  if somebody is interested    public class Accessor lt T gt        private Action lt T gt  Setter      private Func lt T gt  Getter       public Accessor Expression lt Func lt T gt  gt  expr                var memberExpression    MemberExpression expr Body          var instanceExpression   memberExpression Expression          var parameter   Expression Parameter typeof T             if  memberExpression Member is PropertyInfo propertyInfo                        Setter   Expression Lambda lt Action lt T gt  gt  Expression Call instanceExpression  propertyInfo GetSetMethod    parameter   parameter  Compile                Getter   Expression Lambda lt Func lt T gt  gt  Expression Call instanceExpression  propertyInfo GetGetMethod     Compile                      else if  memberExpression Member is FieldInfo fieldInfo                        Setter   Expression Lambda lt Action lt T gt  gt  Expression Assign memberExpression  parameter   parameter  Compile                Getter   Expression Lambda lt Func lt T gt  gt  Expression Field instanceExpression fieldInfo   Compile                          public void Set T value    gt  Setter value        public T Get     gt  Getter        And use it like   var accessor   new Accessor lt string gt       gt  myClient WorkPhone   accessor Set  12345    Assert Equal accessor Get     12345

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Just a little expansion to Nathan s Linq Expression solution  Use multi generic param so that the property doesn t limited to string   void GetString lt TClass  TProperty gt  string input  TClass outObj  Expression lt Func lt TClass  TProperty gt  gt  outExpr        if   string IsNullOrEmpty input                 var expr    MemberExpression  outExpr Body          var prop    PropertyInfo  expr Member          if   prop GetValue outObj  Equals input                         prop SetValue outObj  input  null

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If you want to get and set the property both  you can use this in C 7   GetString      inputString            gt  client WorkPhone  x   gt  client WorkPhone   x    void GetString string inValue   Func lt string gt  get  Action lt string gt  set  outValue        if   string IsNullOrEmpty outValue                 outValue set inValue

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Properties cannot be passed by reference   Make it a field then  and use the property to reference it publicly   public class MyClass       public class MyStuff               string foo   get  set               private ObservableCollection lt MyStuff gt   collection       public ObservableCollection lt MyStuff gt  Items   get   return  collection           public MyClass                  collection   new ObservableCollection lt MyStuff gt             this LoadMyCollectionByRef lt MyStuff gt  ref  collection              public void LoadMyCollectionByRef lt T gt  ref ObservableCollection lt T gt  objects collection                   Load refered collection

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without duplicating the property  void Main         var client   new Client        NullSafeSet  test   s   gt  client Name   s       Debug Assert person Name     test         NullSafeSet     s   gt  client Name   s       Debug Assert person Name     test         NullSafeSet null  s   gt  client Name   s       Debug Assert person Name     test       void NullSafeSet string value  Action lt string gt  setter        if   string IsNullOrEmpty value                 setter value

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Another trick not yet mentioned is to have the class which implements a property  e g  Foo of type Bar  also define a delegate delegate void ActByRef lt T1 T2 gt  ref T1 p1  ref T2 p2   and implement a method ActOnFoo lt TX1 gt  ref Bar it  ActByRef lt Bar TX1 gt  proc  ref TX1 extraParam1   and possibly versions for two and three  extra parameters  as well  which will pass its internal representation of Foo to the supplied procedure as a ref parameter   This has a couple of big advantages over other methods of working with the property   The property is updated  in place   if the property is of a type that s compatible with  Interlocked  methods  or if it is a struct with exposed fields of such types  the  Interlocked  methods may be used to perform atomic updates to the property  If the property is an exposed-field structure  the fields of the structure may be modified without having to make any redundant copies of it  If the  ActByRef  method passes one or more  ref  parameters through from its caller to the supplied delegate  it may be possible to use a singleton or static delegate  thus avoiding the need to create closures or delegates at run-time  The property knows when it is being  worked with    While it is always necessary to use caution executing external code while holding a lock  if one can trust callers not to do too do anything in their callback that might require another lock  it may be practical to have the method guard the property access with a lock  such that updates which aren t compatible with  CompareExchange  could still be performed quasi-atomically    Passing things be ref is an excellent pattern  too bad it s not used more

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This is not possible  You could say  Client WorkPhone   GetString inputString  Client WorkPhone     where WorkPhone is a writeable string property and the definition of GetString is changed to  private string GetString string input  string current         if   string IsNullOrEmpty input             return input            return current      This will have the same semantics that you seem to be trying for   This isn t possible because a property is really a pair of methods in disguise  Each property makes available getters and setters that are accessible via field-like syntax  When you attempt to call GetString as you ve proposed  what you re passing in is a value and not a variable  The value that you are passing in is that returned from the getter get WorkPhone

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What you could try to do is create an object to hold the property value  That way you could pass the object and still have access to the property inside

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Properties cannot be passed by reference  Here are a few ways you can work around this limitation   1  Return Value  string GetString string input  string output        if   string IsNullOrEmpty input                 return input            return output     void Main         var person   new Person        person Name   GetString  test   person Name       Debug Assert person Name     test        2  Delegate  void GetString string input  Action lt string gt  setOutput        if   string IsNullOrEmpty input                 setOutput input            void Main         var person   new Person        GetString  test   value   gt  person Name   value       Debug Assert person Name     test        3  LINQ Expression  void GetString lt T gt  string input  T target  Expression lt Func lt T  string gt  gt  outExpr        if   string IsNullOrEmpty input                 var expr    MemberExpression  outExpr Body          var prop    PropertyInfo  expr Member          prop SetValue target  input  null            void Main         var person   new Person        GetString  test   person  x   gt  x Name       Debug Assert person Name     test        4  Reflection  void GetString string input  object target  string propertyName        if   string IsNullOrEmpty input                 var prop   target GetType   GetProperty propertyName           prop SetValue target  input            void Main         var person   new Person        GetString  test   person  nameof Person Name        Debug Assert person Name     test

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You can t ref a property  but if your functions need both get and set access you can pass around an instance of a class with a property defined   public class Property lt T gt        public delegate T Get        public delegate void Set T value       private Get get      private Set set      public T Value           get               return get                      set               set value                       public Property Get get  Set set            this get   get          this set   set            Example   class Client       private string workPhone     this could still be a public property if desired     public readonly Property lt string gt  WorkPhone     this could be created outside Client if using a regular public property     public int AreaCode   get  set        public Client             WorkPhone   new Property lt string gt               delegate      return workPhone                 delegate  string value    workPhone   value              class Usage       public void PrependAreaCode Property lt string gt  phone  int areaCode            phone Value   areaCode ToString      -    phone Value            public void PrepareClientInfo Client client            PrependAreaCode client WorkPhone  client AreaCode

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This is covered in section 7 4 1 of the C  language spec   Only a variable-reference can be passed as a ref or out parameter in an argument list   A property does not qualify as a variable reference and hence cannot be used

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To vote on this issue  here is one active suggestion of how this could be added to the language  I m not saying this is the best way to do this  at all   feel free to put out your own suggestion  But allowing properties to be passed by ref like Visual Basic already can do would hugely help simplify some code  and quite often   https   github com dotnet csharplang issues 1235

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