How do I create dynamic properties in C

Question

I am looking for a way to create a class with a set of static properties   At run time  I want to be able to add other dynamic properties to this object from the database  I d also like to add sorting and filtering capabilities to these objects   How do I do this in C

User · Answer

I m not sure you really want to do what you say you want to do  but it s not for me to reason why   You cannot add properties to a class after it has been JITed   The closest you could get would be to dynamically create a subtype with Reflection Emit and copy the existing fields over  but you d have to update all references to the the object yourself   You also wouldn t be able to access those properties at compile time   Something like   public class Dynamic       public Dynamic Add lt T gt  string key  T value                AssemblyBuilder assemblyBuilder   AppDomain CurrentDomain DefineDynamicAssembly new AssemblyName  DynamicAssembly    AssemblyBuilderAccess Run           ModuleBuilder moduleBuilder   assemblyBuilder DefineDynamicModule  Dynamic dll            TypeBuilder typeBuilder   moduleBuilder DefineType Guid NewGuid   ToString             typeBuilder SetParent this GetType             PropertyBuilder propertyBuilder   typeBuilder DefineProperty key  PropertyAttributes None  typeof T   Type EmptyTypes            MethodBuilder getMethodBuilder   typeBuilder DefineMethod  get     key  MethodAttributes Public  CallingConventions HasThis  typeof T   Type EmptyTypes           ILGenerator getter   getMethodBuilder GetILGenerator            getter Emit OpCodes Ldarg 0           getter Emit OpCodes Ldstr  key           getter Emit OpCodes Callvirt  typeof Dynamic  GetMethod  Get   BindingFlags Instance   BindingFlags NonPublic  MakeGenericMethod typeof T             getter Emit OpCodes Ret           propertyBuilder SetGetMethod getMethodBuilder            Type type   typeBuilder CreateType             Dynamic child    Dynamic Activator CreateInstance type           child dictionary   this dictionary          dictionary Add key  value           return child             protected T Get lt T gt  string key                return  T dictionary key              private Dictionary lt string  object gt  dictionary   new Dictionary lt string object gt         I don t have VS installed on this machine so let me know if there are any massive bugs  well    other than the massive performance problems  but I didn t write the specification    Now you can use it   Dynamic d   new Dynamic    d   d Add  MyProperty   42   Console WriteLine d GetType   GetProperty  MyProperty   GetValue d  null      You could also use it like a normal property in a language that supports late binding  for example  VB NET

User · Answer

Use ExpandoObject like the ViewBag in MVC 3

User · Answer

You should look into DependencyObjects as used by WPF these follow a similar pattern whereby properties can be assigned at runtime  As mentioned above this ultimately points towards using a hash table   One other useful thing to have a look at is CSLA Net  The code is freely available and uses some of the principles patterns it appears you are after   Also if you are looking at sorting and filtering I m guessing you re going to be using some kind of grid  A useful interface to implement is ICustomTypeDescriptor  this lets you effectively override what happens when your object gets reflected on so you can point the reflector to your object s own internal hash table

User · Answer

If you need this for data-binding purposes  you can do this with a custom descriptor model    by implementing ICustomTypeDescriptor  TypeDescriptionProvider and or TypeCoverter  you can create your own PropertyDescriptor instances at runtime  This is what controls like DataGridView  PropertyGrid etc use to display properties   To bind to lists  you d need ITypedList and IList  for basic sorting  IBindingList  for filtering and advanced sorting  IBindingListView  for full  new row  support  DataGridView   ICancelAddNew  phew      It is a lot of work though  DataTable  although I hate it  is cheap way of doing the same thing  If you don t need data-binding  just use a hashtable  -p  Here s a simple example - but you can do a lot more

User · Answer

Create a Hashtable called  Properties  and add your properties to it

User · Answer

I have done exactly this with an ICustomTypeDescriptor interface and a Dictionary   Implementing ICustomTypeDescriptor for dynamic properties   I have recently had a requirement to bind a grid view to a record object that could have any number of properties that can be added and removed at runtime  This was to allow a user to add a new column to a result set to enter an additional set of data   This can be achieved by having each data  row  as a dictionary with the key being the property name and the value being a string or a class that can store the value of the property for the specified row  Of course having a List of Dictionary objects will not be able to be bound to a grid  This is where the ICustomTypeDescriptor comes in   By creating a wrapper class for the Dictionary and making it adhere to the ICustomTypeDescriptor interface the behaviour for returning properties for an object can be overridden   Take a look at the implementation of the data  row  class below        lt summary gt      Class to manage test result row data functions      lt  summary gt  public class TestResultRowWrapper   Dictionary lt string  TestResultValue gt   ICustomTypeDescriptor         - METHODS -----------------------------------------------------------------------------------------------------------------       region Methods           lt summary gt          Gets the Attributes for the object          lt  summary gt      AttributeCollection ICustomTypeDescriptor GetAttributes                 return new AttributeCollection null                   lt summary gt          Gets the Class name          lt  summary gt      string ICustomTypeDescriptor GetClassName                 return null                  lt summary gt          Gets the component Name          lt  summary gt      string ICustomTypeDescriptor GetComponentName                 return null                  lt summary gt          Gets the Type Converter          lt  summary gt      TypeConverter ICustomTypeDescriptor GetConverter                 return null                  lt summary gt          Gets the Default Event          lt  summary gt           lt returns gt  lt  returns gt      EventDescriptor ICustomTypeDescriptor GetDefaultEvent                 return null                  lt summary gt          Gets the Default Property          lt  summary gt      PropertyDescriptor ICustomTypeDescriptor GetDefaultProperty                 return null                  lt summary gt          Gets the Editor          lt  summary gt      object ICustomTypeDescriptor GetEditor Type editorBaseType                return null                  lt summary gt          Gets the Events          lt  summary gt      EventDescriptorCollection ICustomTypeDescriptor GetEvents Attribute   attributes                return new EventDescriptorCollection null                   lt summary gt          Gets the events          lt  summary gt      EventDescriptorCollection ICustomTypeDescriptor GetEvents                 return new EventDescriptorCollection null                   lt summary gt          Gets the properties          lt  summary gt      PropertyDescriptorCollection ICustomTypeDescriptor GetProperties Attribute   attributes                List lt propertydescriptor gt  properties   new List lt propertydescriptor gt                Add property descriptors for each entry in the dictionary         foreach  string key in this Keys                        properties Add new TestResultPropertyDescriptor key                         Get properties also belonging to this class also         PropertyDescriptorCollection pdc   TypeDescriptor GetProperties this GetType    attributes            foreach  PropertyDescriptor oPropertyDescriptor in pdc                        properties Add oPropertyDescriptor                      return new PropertyDescriptorCollection properties ToArray                     lt summary gt          gets the Properties          lt  summary gt      PropertyDescriptorCollection ICustomTypeDescriptor GetProperties                 return   ICustomTypeDescriptor this  GetProperties null                   lt summary gt          Gets the property owner          lt  summary gt      object ICustomTypeDescriptor GetPropertyOwner PropertyDescriptor pd                return this              endregion Methods        ---------------------------------------------------------------------------------------------------------------------------     Note  In the GetProperties method I Could Cache the PropertyDescriptors once read for performance but as I m adding and removing columns at runtime I always want them rebuilt  You will also notice in the GetProperties method that the Property Descriptors added for the dictionary entries are of type TestResultPropertyDescriptor  This is a custom Property Descriptor class that manages how properties are set and retrieved  Take a look at the implementation below        lt summary gt      Property Descriptor for Test Result Row Wrapper      lt  summary gt  public class TestResultPropertyDescriptor   PropertyDescriptor         - PROPERTIES --------------------------------------------------------------------------------------------------------------       region Properties           lt summary gt          Component Type          lt  summary gt      public override Type ComponentType               get   return typeof Dictionary lt string  TestResultValue gt                      lt summary gt          Gets whether its read only          lt  summary gt      public override bool IsReadOnly               get   return false                    lt summary gt          Gets the Property Type          lt  summary gt      public override Type PropertyType               get   return typeof string                 endregion Properties        - CONSTRUCTOR -------------------------------------------------------------------------------------------------------------       region Constructor           lt summary gt          Constructor          lt  summary gt      public TestResultPropertyDescriptor string key            base key  null                     endregion Constructor        - METHODS -----------------------------------------------------------------------------------------------------------------       region Methods           lt summary gt          Can Reset Value          lt  summary gt      public override bool CanResetValue object component                return true                  lt summary gt          Gets the Value          lt  summary gt      public override object GetValue object component                  return   Dictionary lt string  TestResultValue gt  component  base Name  Value                  lt summary gt          Resets the Value          lt  summary gt      public override void ResetValue object component                  Dictionary lt string  TestResultValue gt  component  base Name  Value   string Empty                  lt summary gt          Sets the value          lt  summary gt      public override void SetValue object component  object value                  Dictionary lt string  TestResultValue gt  component  base Name  Value   value ToString                    lt summary gt          Gets whether the value should be serialized          lt  summary gt      public override bool ShouldSerializeValue object component                return false              endregion Methods        ---------------------------------------------------------------------------------------------------------------------------     The main properties to look at on this class are GetValue and SetValue  Here you can see the component being casted as a dictionary and the value of the key inside it being Set or retrieved  Its important that the dictionary in this class is the same type in the Row wrapper class otherwise the cast will fail  When the descriptor is created the key  property name  is passed in and is used to query the dictionary to get the correct value   Taken from my blog at   ICustomTypeDescriptor Implementation for dynamic properties

User · Answer

I m not sure what your reasons are  and even if you could pull it off somehow with Reflection Emit  I  not sure that you can   it doesn t sound like a good idea  What is probably a better idea is to have some kind of Dictionary and you can wrap access to the dictionary through methods in your class  That way you can store the data from the database in this dictionary  and then retrieve them using those methods

User · Answer

If it is for binding  then you can reference indexers from XAML  Text   Binding  FullName      Here it is referencing the class indexer with the key  FullName

User · Answer

Couldn t you just have your class expose a Dictionary object  Instead of  attaching more properties to the object   you could simply insert your data  with some identifier  into the dictionary at run time

User · Answer

Why not use an indexer with the property name as a string value passed to the indexer

User · Answer

You might use a dictionary  say  Dictionary lt string object gt  properties    I think in most cases where something similar is done  it s done like this  In any case  you would not gain anything from creating a  real  property with set and get accessors  since it would be created only at run-time and you would not be using it in your code     Here is an example  showing a possible implementation of filtering and sorting  no error checking    using System  using System Collections Generic  using System Linq   namespace ConsoleApplication1        class ObjectWithProperties           Dictionary lt string  object gt  properties   new Dictionary lt string object gt              public object this string name                get                    if  properties ContainsKey name                        return properties name                                     return null                            set                   properties name    value                                      class Comparer lt T gt    IComparer lt ObjectWithProperties gt  where T   IComparable            string m attributeName           public Comparer string attributeName               m attributeName   attributeName                     public int Compare ObjectWithProperties x  ObjectWithProperties y                return   T x m attributeName   CompareTo  T y m attributeName                          class Program            static void Main string   args                    create some objects and fill a list             var obj1   new ObjectWithProperties                obj1  test     100              var obj2   new ObjectWithProperties                obj2  test     200              var obj3   new ObjectWithProperties                obj3  test     150              var objects   new List lt ObjectWithProperties gt  new ObjectWithProperties    obj1  obj2  obj3                     filtering              Console WriteLine  Filtering                 var filtered   from obj in objects                          where  int obj  test    gt   150                          select obj              foreach  var obj in filtered                   Console WriteLine obj  test                                   sorting              Console WriteLine  Sorting                 Comparer lt int gt  c   new Comparer lt int gt   test                objects Sort c               foreach  var obj in objects                    Console WriteLine obj  test

User · Answer

As a replacement for some of orsogufo s code  because I recently went with a dictionary for this same problem myself  here is my    operator   public string this string key        get   return properties ContainsKey key    properties key    null         set               if  properties ContainsKey key                         properties key    value                    else                       properties Add key  value                       With this implementation  the setter will add new key-value pairs when you use     if they do not already exist in the dictionary   Also  for me properties is an IDictionary and in constructors I initialize it to new SortedDictionary lt string  string gt

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