Retrieving Property name from lambda expression

Question

Is there a better way to get the Property name when passed in via a lambda expression  Here is what i currently have   eg    GetSortingInfo lt User gt  u   gt  u UserId     It worked by casting it as a memberexpression only when the property was  a string  because not all properties are strings i had to use object but then it would return a unaryexpression for those   public static RouteValueDictionary GetInfo lt T gt  this HtmlHelper html       Expression lt Func lt T  object gt  gt  action  where T   class       var expression   GetMemberInfo action       string name   expression Member Name       return GetInfo html  name      private static MemberExpression GetMemberInfo Expression method        LambdaExpression lambda   method as LambdaExpression      if  lambda    null          throw new ArgumentNullException  method         MemberExpression memberExpr   null       if  lambda Body NodeType    ExpressionType Convert                memberExpr                  UnaryExpression lambda Body  Operand as MemberExpression            else if  lambda Body NodeType    ExpressionType MemberAccess                memberExpr   lambda Body as MemberExpression             if  memberExpr    null          throw new ArgumentException  method         return memberExpr

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There s an edge case when it comes to Array Length  While  Length  is exposed as a property  you can t use it in any of the previously proposed solutions   using Contract   System Diagnostics Contracts Contract  using Exprs   System Linq Expressions   static string PropertyNameFromMemberExpr Exprs MemberExpression expr        return expr Member Name     static string PropertyNameFromUnaryExpr Exprs UnaryExpression expr        if  expr NodeType    Exprs ExpressionType ArrayLength          return  Length        var mem expr   expr Operand as Exprs MemberExpression       return PropertyNameFromMemberExpr mem expr      static string PropertyNameFromLambdaExpr Exprs LambdaExpression expr             if  expr Body is Exprs MemberExpression    return PropertyNameFromMemberExpr expr Body as Exprs MemberExpression       else if  expr Body is Exprs UnaryExpression     return PropertyNameFromUnaryExpr expr Body as Exprs UnaryExpression        throw new NotSupportedException       public static string PropertyNameFromExpr lt TProp gt  Exprs Expression lt Func lt TProp gt  gt  expr        Contract Requires lt ArgumentNullException gt  expr    null       Contract Requires lt ArgumentException gt  expr Body is Exprs MemberExpression    expr Body is Exprs UnaryExpression        return PropertyNameFromLambdaExpr expr      public static string PropertyNameFromExpr lt T  TProp gt  Exprs Expression lt Func lt T  TProp gt  gt  expr        Contract Requires lt ArgumentNullException gt  expr    null       Contract Requires lt ArgumentException gt  expr Body is Exprs MemberExpression    expr Body is Exprs UnaryExpression        return PropertyNameFromLambdaExpr expr       Now example usage   int   someArray   new int 1   Console WriteLine PropertyNameFromExpr       gt  someArray Length       If PropertyNameFromUnaryExpr didn t check for ArrayLength   someArray  would be printed to the console  compiler seems to generate direct access to the backing Length field  as an optimization  even in Debug  thus the special case

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I leave this function if you want to get multiples fields        lt summary gt          Get properties separated by    Ex  to invoke  d   gt  new   d FirstName  d LastName              lt  summary gt           lt typeparam name  T  gt  lt  typeparam gt           lt param name  exp  gt  lt  param gt           lt returns gt  lt  returns gt      public static string GetFields lt T gt  Expression lt Func lt T  object gt  gt  exp                MemberExpression body   exp Body as MemberExpression          var fields   new List lt string gt             if  body    null                        NewExpression ubody   exp Body as NewExpression              if  ubody    null                  foreach  var arg in ubody Arguments                                        fields Add  arg as MemberExpression  Member Name                                        return string Join      fields

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public string GetName lt TSource  TField gt  Expression lt Func lt TSource  TField gt  gt  Field        return  Field Body as MemberExpression      UnaryExpression Field Body  Operand as MemberExpression  Member Name      This handles member and unary expressions  The difference being that you will get a UnaryExpression if your expression represents a value type whereas you will get a MemberExpression if your expression represents a reference type  Everything can be cast to an object  but value types must be boxed  This is why the UnaryExpression exists  Reference   For the sakes of readability   Jowen   here s an expanded equivalent   public string GetName lt TSource  TField gt  Expression lt Func lt TSource  TField gt  gt  Field        if  object Equals Field  null                 throw new NullReferenceException  Field is required               MemberExpression expr   null       if  Field Body is MemberExpression                expr    MemberExpression Field Body            else if  Field Body is UnaryExpression                expr    MemberExpression   UnaryExpression Field Body  Operand            else               const string Format    Expression   0   not supported            string message   string Format Format  Field            throw new ArgumentException message   Field               return expr Member Name

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This is a general implementation to get the string name of fields properties indexers methods extension methods delegates of struct class interface delegate array  I have tested with combinations of static instance and non-generic generic variants     involves recursion public static string GetMemberName this LambdaExpression memberSelector        Func lt Expression  string gt  nameSelector   null     recursive func     nameSelector   e   gt    or move the entire thing to a separate recursive method               switch  e NodeType                        case ExpressionType Parameter                  return   ParameterExpression e  Name              case ExpressionType MemberAccess                  return   MemberExpression e  Member Name              case ExpressionType Call                  return   MethodCallExpression e  Method Name              case ExpressionType Convert              case ExpressionType ConvertChecked                  return nameSelector   UnaryExpression e  Operand               case ExpressionType Invoke                  return nameSelector   InvocationExpression e  Expression               case ExpressionType ArrayLength                  return  Length               default                  throw new Exception  not a proper member selector                          return nameSelector memberSelector Body       This thing can be written in a simple while loop too     iteration based public static string GetMemberName this LambdaExpression memberSelector        var currentExpression   memberSelector Body       while  true                switch  currentExpression NodeType                        case ExpressionType Parameter                  return   ParameterExpression currentExpression  Name              case ExpressionType MemberAccess                  return   MemberExpression currentExpression  Member Name              case ExpressionType Call                  return   MethodCallExpression currentExpression  Method Name              case ExpressionType Convert              case ExpressionType ConvertChecked                  currentExpression     UnaryExpression currentExpression  Operand                  break              case ExpressionType Invoke                  currentExpression     InvocationExpression currentExpression  Expression                  break              case ExpressionType ArrayLength                  return  Length               default                  throw new Exception  not a proper member selector                        I like the recursive approach  though the second one might be easier to read  One can call it like    someExpr   x   gt  x Property ExtensionMethod   0     or someExpr   x   gt  Static Method   Field    or someExpr   x   gt  VoidMethod      or someExpr        gt  localVariable    or someExpr   x   gt  x    or someExpr   x   gt   Type x    or someExpr        gt  Array 0  Delegate null     etc  string name   someExpr GetMemberName      to print the last member   Note    In case of chained expressions like A B C   C  is returned   This doesn t work with consts  array indexers or enums  impossible to cover all cases

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Starting with  NET 4 0 you can use ExpressionVisitor to find properties   class ExprVisitor   ExpressionVisitor       public bool IsFound   get  private set        public string MemberName   get  private set        public Type MemberType   get  private set        protected override Expression VisitMember MemberExpression node            if   IsFound  amp  amp  node Member MemberType    MemberTypes Property                IsFound   true              MemberName   node Member Name              MemberType   node Type                    return base VisitMember node             Here is how you use this visitor   var visitor   new ExprVisitor    visitor Visit expr   if  visitor IsFound        Console WriteLine  First property in the expression tree  Name  0   Type  1    visitor MemberName  visitor MemberType FullName     else       Console WriteLine  No properties found

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I ve updated  Cameron s answer to include some safety checks against Convert typed lambda expressions   PropertyInfo GetPropertyName lt TSource  TProperty gt   Expression lt Func lt TSource  TProperty gt  gt  propertyLambda      var body   propertyLambda Body    if    body is MemberExpression member       amp  amp    body is UnaryExpression unary        amp  amp   member   unary Operand as MemberExpression     null       throw new ArgumentException   Expression   propertyLambda              does not refer to a property        if    member Member is PropertyInfo propInfo       throw new ArgumentException   Expression   propertyLambda              refers to a field  not a property        var type   typeof TSource     if   propInfo DeclaringType GetTypeInfo   IsAssignableFrom type GetTypeInfo         throw new ArgumentException   Expresion   propertyLambda               refers to a property that is not from type   type          return propInfo

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This is another answer   public static string GetPropertyName lt TModel  TProperty gt  this HtmlHelper lt TModel gt  htmlHelper                                                                        Expression lt Func lt TModel  TProperty gt  gt  expression                var metaData   ModelMetadata FromLambdaExpression expression  htmlHelper ViewData            return metaData PropertyName

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I have done the INotifyPropertyChanged implementation similar to the method below  Here the properties are stored in a dictionary in the base class shown below  It is of course not always desirable to use inheritance  but for view models I think it is acceptable and gives very clean property references in the view model classes    public class PhotoDetailsViewModel       PropertyChangedNotifierBase lt PhotoDetailsViewModel gt        public bool IsLoading               get   return GetValue x   gt  x IsLoading             set   SetPropertyValue x   gt  x IsLoading  value                public string PendingOperation               get   return GetValue x   gt  x PendingOperation             set   SetPropertyValue x   gt  x PendingOperation  value                public PhotoViewModel Photo               get   return GetValue x   gt  x Photo             set   SetPropertyValue x   gt  x Photo  value               The somewhat more complex base class is shown below  It handles the translation from lambda expression to property name  Note that the properties are really pseudo properties since only the names are used  But it will appear transparent to the view model and references to the properties on the view model   public class PropertyChangedNotifierBase lt T gt    INotifyPropertyChanged       readonly Dictionary lt string  object gt   properties   new Dictionary lt string  object gt          protected U GetValue lt U gt  Expression lt Func lt T  U gt  gt  property                var propertyName   GetPropertyName property            return GetValue lt U gt  propertyName              private U GetValue lt U gt  string propertyName                object value           if    properties TryGetValue propertyName  out value                         return default U                      return  U value             protected void SetPropertyValue lt U gt  Expression lt Func lt T  U gt  gt  property  U value                var propertyName   GetPropertyName property            var oldValue   GetValue lt U gt  propertyName            if  Object ReferenceEquals oldValue  value                         return                     properties propertyName    value           RaisePropertyChangedEvent propertyName              protected void RaisePropertyChangedEvent lt U gt  Expression lt Func lt T  U gt  gt  property                var name   GetPropertyName property           RaisePropertyChangedEvent name              protected void RaisePropertyChangedEvent string propertyName                if  PropertyChanged    null                        PropertyChanged this  new PropertyChangedEventArgs propertyName                         private static string GetPropertyName lt U gt  Expression lt Func lt T  U gt  gt  property                if  property    null                        throw new NullReferenceException  property                       var lambda   property as LambdaExpression           var memberAssignment    MemberExpression  lambda Body          return memberAssignment Member Name             public event PropertyChangedEventHandler PropertyChanged

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I m using an extension method for pre C  6 projects and the nameof   for those targeting C  6   public static class MiscExtentions       public static string NameOf lt TModel  TProperty gt  this object  object  Expression lt Func lt TModel  TProperty gt  gt  propertyExpression                var expression   propertyExpression Body as MemberExpression          if  expression    null                        throw new ArgumentException  Expression is not a property                        return expression Member Name            And i call it like   public class MyClass        public int Property1   get  set        public string Property2   get  set        public int   Property3   get  set        public Subclass Property4   get  set        public Subclass   Property5   get  set       public class Subclass       public int PropertyA   get  set        public string PropertyB   get  set          result is Property1 this NameOf  MyClass o    gt  o Property1      result is Property2 this NameOf  MyClass o    gt  o Property2      result is Property3 this NameOf  MyClass o    gt  o Property3      result is Property4 this NameOf  MyClass o    gt  o Property4      result is PropertyB this NameOf  MyClass o    gt  o Property4 PropertyB      result is Property5 this NameOf  MyClass o    gt  o Property5     It works fine with both fields and properties

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This might be optimal  public static string GetPropertyName lt TResult gt  Expression lt Func lt TResult gt  gt  expr        var memberAccess   expr Body as MemberExpression      var propertyInfo   memberAccess  Member as PropertyInfo      var propertyName   propertyInfo  Name       return propertyName

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Here is another way to get the PropertyInfo based off this answer  It eliminates the need for an object instance        lt summary gt      Get metadata of property referenced by expression  Type constrained       lt  summary gt  public static PropertyInfo GetPropertyInfo lt TSource  TProperty gt  Expression lt Func lt TSource  TProperty gt  gt  propertyLambda        return GetPropertyInfo  LambdaExpression  propertyLambda           lt summary gt      Get metadata of property referenced by expression       lt  summary gt  public static PropertyInfo GetPropertyInfo LambdaExpression propertyLambda           https   stackoverflow com questions 671968 retrieving-property-name-from-lambda-expression     MemberExpression member   propertyLambda Body as MemberExpression      if  member    null          throw new ArgumentException string Format               Expression   0   refers to a method  not a property                propertyLambda ToString           PropertyInfo propInfo   member Member as PropertyInfo      if  propInfo    null          throw new ArgumentException string Format               Expression   0   refers to a field  not a property                propertyLambda ToString           if propertyLambda Parameters Count      0          throw new ArgumentException String Format               Expression   0   does not have any parameters  A property expression needs to have at least 1 parameter                propertyLambda ToString           var type   propertyLambda Parameters 0  Type      if  type    propInfo ReflectedType  amp  amp           type IsSubclassOf propInfo ReflectedType           throw new ArgumentException String Format               Expression   0   refers to a property that is not from type  1                 propertyLambda ToString                type        return propInfo      It can be called like so   var propertyInfo   GetPropertyInfo  User u    gt  u UserID

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now in C  6 you can simply use nameof like this nameof User UserId    which has many benefits  among them is that this is done at compile time  not runtime   https   msdn microsoft com en-us magazine dn802602 aspx

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static void Main string   args        var prop   GetPropertyInfo lt MyDto gt      gt    MyProperty        MyDto dto   new MyDto        dto MyProperty   666       var value   prop GetValue dto          value    666    class MyDto       public int MyProperty   get  set       public static PropertyInfo GetPropertyInfo lt TSource gt  Expression lt Func lt TSource  object gt  gt  propertyLambda        Type type   typeof TSource        var member   propertyLambda Body as MemberExpression      if  member    null                var unary   propertyLambda Body as UnaryExpression          if  unary    null                        member   unary Operand as MemberExpression                      if  member    null                throw new ArgumentException string Format  Expression   0   refers to a method  not a property                propertyLambda ToString                 var propInfo   member Member as PropertyInfo      if  propInfo    null                throw new ArgumentException string Format  Expression   0   refers to a field  not a property                propertyLambda ToString                 if  type    propInfo ReflectedType  amp  amp   type IsSubclassOf propInfo ReflectedType                 throw new ArgumentException string Format  Expression   0   refers to a property that is not from type  1                 propertyLambda ToString    type               return propInfo

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With C  7 pattern matching   public static string GetMemberName lt T gt  this Expression lt T gt  expression        switch  expression Body                case MemberExpression m              return m Member Name          case UnaryExpression u when u Operand is MemberExpression m              return m Member Name          default              throw new NotImplementedException expression GetType   ToString               Example   public static RouteValueDictionary GetInfo lt T gt  this HtmlHelper html       Expression lt Func lt T  object gt  gt  action  where T   class       var name   action GetMemberName        return GetInfo html  name        Update  C  8 pattern matching   public static string GetMemberName lt T gt  this Expression lt T gt  expression    gt      expression Body switch               MemberExpression m   gt              m Member Name          UnaryExpression u when u Operand is MemberExpression m   gt              m Member Name              gt              throw new    NotImplementedException expression GetType   ToString

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I ve found that some of the suggested answers which drill down into the MemberExpression UnaryExpression don t capture nested subproperties   ex  o   gt  o Thing1 Thing2 returns Thing1 rather than Thing1 Thing2   This distinction is important if you re trying to work with EntityFramework DbSet Include        I ve found that just parsing the Expression ToString   seems to work fine  and comparatively quickly   I compared it against the UnaryExpression version  and even getting ToString off of the Member UnaryExpression to see if that was faster  but the difference was negligible   Please correct me if this is a terrible idea   The Extension Method       lt summary gt      Given an expression  extract the listed property name  similar to reflection but with familiar LINQ lambdas   Technique  via https   stackoverflow com a 16647343 1037948      lt  summary gt       lt remarks gt Cheats and uses the tostring output -- Should consult performance differences lt  remarks gt       lt typeparam name  TModel  gt the model type to extract property names lt  typeparam gt       lt typeparam name  TValue  gt the value type of the expected property lt  typeparam gt       lt param name  propertySelector  gt expression that just selects a model property to be turned into a string lt  param gt       lt param name  delimiter  gt Expression toString delimiter to split from lambda param lt  param gt       lt param name  endTrim  gt Sometimes the Expression toString contains a method call  something like  Convert x    so we need to strip the closing part from the end lt  param gt       lt returns gt indicated property name lt  returns gt  public static string GetPropertyName lt TModel  TValue gt  this Expression lt Func lt TModel  TValue gt  gt  propertySelector  char delimiter        char endTrim               var asString   propertySelector ToString       gives you   o   gt  o Whatever      var firstDelim   asString IndexOf delimiter      make sure there is a beginning property indicator  the     in  o Whatever  -- this may not be necessary       return firstDelim  lt  0           asString           asString Substring firstDelim 1  TrimEnd endTrim      --   fn  GetPropertyNameExtended    Checking for the delimiter might even be overkill   Demo  LinqPad   Demonstration   Comparison code -- https   gist github com zaus 6992590

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I found another way you can do it was to have the source and property strongly typed and explicitly infer the input for the lambda  Not sure if that is correct terminology but here is the result  public static RouteValueDictionary GetInfo lt T P gt  this HtmlHelper html  Expression lt Func lt T  P gt  gt  action  where T   class       var expression    MemberExpression action Body      string name   expression Member Name       return GetInfo html  name      And then call it like so  GetInfo  User u    gt  u UserId    and voila it works

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I recently did a very similar thing to make a type safe OnPropertyChanged method   Here s a method that ll return the PropertyInfo object for the expression  It throws an exception if the expression is not a property   public PropertyInfo GetPropertyInfo lt TSource  TProperty gt       TSource source      Expression lt Func lt TSource  TProperty gt  gt  propertyLambda        Type type   typeof TSource        MemberExpression member   propertyLambda Body as MemberExpression      if  member    null          throw new ArgumentException string Format               Expression   0   refers to a method  not a property                propertyLambda ToString           PropertyInfo propInfo   member Member as PropertyInfo      if  propInfo    null          throw new ArgumentException string Format               Expression   0   refers to a field  not a property                propertyLambda ToString           if  type    propInfo ReflectedType  amp  amp           type IsSubclassOf propInfo ReflectedType           throw new ArgumentException string Format               Expression   0   refers to a property that is not from type  1                 propertyLambda ToString                type         return propInfo      The source parameter is used so the compiler can do type inference on the method call  You can do the following  var propertyInfo   GetPropertyInfo someUserObject  u   gt  u UserID

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Well  there s no need to call  Name ToString    but broadly that is about it  yes  The only consideration you might need is whether x Foo Bar should return  Foo    Bar   or an exception - i e  do you need to iterate at all    re comment  for more on flexible sorting  see here

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I created an extension method on ObjectStateEntry to be able to flag properties  of Entity Framework POCO classes  as modified in a type safe manner  since the default method only accepts a string  Here s my way of getting the name from the property   public static void SetModifiedProperty lt T gt  this System Data Objects ObjectStateEntry state  Expression lt Func lt T gt  gt  action        var body    MemberExpression action Body      string propertyName   body Member Name       state SetModifiedProperty propertyName

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Here s an update to method proposed by Cameron  The first parameter is not required   public PropertyInfo GetPropertyInfo lt TSource  TProperty gt       Expression lt Func lt TSource  TProperty gt  gt  propertyLambda        Type type   typeof TSource        MemberExpression member   propertyLambda Body as MemberExpression      if  member    null          throw new ArgumentException string Format               Expression   0   refers to a method  not a property                propertyLambda ToString           PropertyInfo propInfo   member Member as PropertyInfo      if  propInfo    null          throw new ArgumentException string Format               Expression   0   refers to a field  not a property                propertyLambda ToString           if  type    propInfo ReflectedType  amp  amp           type IsSubclassOf propInfo ReflectedType           throw new ArgumentException string Format               Expresion   0   refers to a property that is not from type  1                 propertyLambda ToString                type         return propInfo      You can do the following   var propertyInfo   GetPropertyInfo lt SomeType gt  u   gt  u UserID   var propertyInfo   GetPropertyInfo  SomeType u    gt  u UserID     Extension methods   public static PropertyInfo GetPropertyInfo lt TSource  TProperty gt  this TSource source      Expression lt Func lt TSource  TProperty gt  gt  propertyLambda  where TSource   class       return GetPropertyInfo propertyLambda      public static string NameOfProperty lt TSource  TProperty gt  this TSource source      Expression lt Func lt TSource  TProperty gt  gt  propertyLambda  where TSource   class       PropertyInfo prodInfo   GetPropertyInfo propertyLambda       return prodInfo Name      You can   SomeType someInstance   null  string propName   someInstance NameOfProperty i   gt  i Length   PropertyInfo propInfo   someInstance GetPropertyInfo i   gt  i Length

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I was playing around with the same thing and worked this up  It s not fully tested but seems to handle the issue with value types  the unaryexpression issue you ran into   public static string GetName Expression lt Func lt object gt  gt  exp        MemberExpression body   exp Body as MemberExpression       if  body    null           UnaryExpression ubody    UnaryExpression exp Body         body   ubody Operand as MemberExpression             return body Member Name

[c#] Retrieving Property name from lambda expression

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