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Getting the getter and setter of a property using RTTI

5 Comments

Introduction

One of the limitations of the TRttiProperty class is which not expose any direct relation between the property and the getter and setters. On this post we will check out how we can access to such info using the RTTI. So the aim is obtain a TRttiField and/or TRttiMethod instance to the getter and setter of the property. This will be done using a class helper like so.

  type
     TRttiPropertyHelper = class helper for TRttiProperty
  private
    function GetSetterField  : TRttiField;
    function GetGetterField  : TRttiField;
  public
    property SetterField : TRttiField read GetSetterField;
    property GetterField : TRttiField read GetGetterField;
    function SetterMethod (Instance : TObject) : TRttiMethod;
    function GetterMethod (Instance : TObject) : TRttiMethod;
  end;

Getting the address of the getter and setters.

The System.TypInfo.TPropInfo type contains two fields which holds the address of the getter and setters members

  PPropInfo = ^TPropInfo;
  TPropInfo = packed record
    PropType: PPTypeInfo;
    GetProc: Pointer;
    SetProc: Pointer;
    StoredProc: Pointer;
    Index: Integer;
                                                                                 
    Default: Integer;
    NameIndex: SmallInt;
    Name: TSymbolName;
    function NameFld: TTypeInfoFieldAccessor; inline;
    function Tail: PPropInfo; inline;
  end;

We need to examine such addresses to determine if represents a field or a method. So I will define a set of sample classes and dump the address of the getter and setter.

type
  TBar = class
  private
    FReadOnlyProp: string;
    FWriteOnlyProp: string;
    function GetReadBaseProp: string; virtual;
    procedure SetWriteBaseProp(const Value: string); virtual;
  public
    property ReadBaseProp: string read GetReadBaseProp;
    property WriteBaseProp: string write SetWriteBaseProp;
  end;

  TFoo = class(TBar)
  private
    function GetReadOnlyPropwGet: string;
    procedure SetWriteOnlyPropwSet(const Value: string);
    function GetReadBaseProp: string; override;
  public
    property ReadOnlyProp: string read FReadOnlyProp;
    property WriteOnlyProp: string Write FWriteOnlyProp;
    property ReadOnlyPropwGet: string read GetReadOnlyPropwGet;
    property WriteOnlyPropwSet: string write SetWriteOnlyPropwSet;
  end;

And now to obtain the addresses we will access the PropInfo member of each property in the class.

procedure DumpPropInfo(AClass: TObject);
var
  LContext: TRttiContext;
  LType: TRttiType;
  LProp: TRttiProperty;
  LPropInfo: PPropInfo;
begin
  //Get the typeinfo of the class
  LType := LContext.GetType(AClass.ClassInfo);

  for LProp in LType.GetProperties() do
    if LProp is TRttiInstanceProperty then
    begin
      //Get the pointer to the PPropInfo
      LPropInfo := TRttiInstanceProperty(LProp).PropInfo;
      //show the addresses of the getter and setter
      Writeln(Format('%-18s GetProc %p SetProc %p', [LProp.Name, LPropInfo.GetProc, LPropInfo.SetProc]));
    end;
end; 

If we run the above code passing an instance to the TFoo class we will obtain this output

ReadOnlyProp       GetProc FF000004 SetProc 00000000 //Field
WriteOnlyProp      GetProc 00000000 SetProc FF000008 //Field
ReadOnlyPropwGet   GetProc 004CCE70 SetProc 00000000 //Static method
WriteOnlyPropwSet  GetProc 00000000 SetProc 004CCE80 //Static method
ReadBaseProp       GetProc FE000000 SetProc 00000000 //virtual method
WriteBaseProp      GetProc 00000000 SetProc FE000004 //virtual method

Obtaining the TRttiField instance

A visual inspection of the returned values, indicates which the fields offsets are masked with the $FF000000 value. So with this info we can build a couple of helper functions to obtain an instance to the TRttiField .

First we need determine when a getter/setter represents a field.

In the System.TypInfo unit exists a private boolean function called IsField which indicates when a Pointer (GetProc, SetProc) represents a field.

function IsField(P: Pointer): Boolean; inline;
begin
  Result := (IntPtr(P) and PROPSLOT_MASK) = PROPSLOT_FIELD;
end;

Now using the above method and some additional code we can build the next function which returns a TRttiField instance for the getter of a property.

function GetPropGetterField(AProp : TRttiProperty) : TRttiField;
var
  LPropInfo : PPropInfo;
  LField: TRttiField;
  LOffset : Integer;
begin
  Result:=nil;
  //Is a readable property?
  if (AProp.IsReadable) and (AProp.ClassNameIs('TRttiInstancePropertyEx')) then
  begin
    //get the propinfo of the porperty
    LPropInfo:=TRttiInstanceProperty(AProp).PropInfo;
    //check if the GetProc represent a field
    if (LPropInfo<>nil) and (LPropInfo.GetProc<>nil) and IsField(LPropInfo.GetProc) then
    begin
      //get the offset of the field
      LOffset:= IntPtr(LPropInfo.GetProc) and PROPSLOT_MASK_F;
      //iterate over the fields of the class
      for LField in AProp.Parent.GetFields do
         //compare the offset the current field with the offset of the getter
         if LField.Offset=LOffset then
           Exit(LField);
    end;
  end;
end;

To obtain the setter field the code looks very similar but instead we inspect the SetProc member.

function GetPropSetterField(AProp : TRttiProperty) : TRttiField;
var
  LPropInfo : PPropInfo;
  LField: TRttiField;
  LOffset : Integer;
begin
  Result:=nil;
  //Is a writable property?
  if (AProp.IsWritable) and (AProp.ClassNameIs('TRttiInstancePropertyEx')) then
  begin
    //get the propinfo of the porperty
    LPropInfo:=TRttiInstanceProperty(AProp).PropInfo;
    //check if the GetProc represent a field
    if (LPropInfo<>nil) and (LPropInfo.SetProc<>nil) and IsField(LPropInfo.SetProc) then
    begin
      //get the offset of the field
      LOffset:= IntPtr(LPropInfo.SetProc) and PROPSLOT_MASK_F;
      //iterate over the fields of the class
      for LField in AProp.Parent.GetFields do
         //compare the offset the current field with the offset of the setter
         if LField.Offset=LOffset then
           Exit(LField);
    end;
  end;
end;

Obtaining the TRttiMethod instance

To obtain a TRttiMethod instance for the setter and getter, first we need to determine if the GetProc/SetProc represent a static o virtual method, then we need to obtain the real address of the method. Luckily exist the private function GetCodePointer in the System.TypInfo unit which do this task. Note that we need a instance to the object to resolve the code address.

function GetCodePointer(Instance: TObject; P: Pointer): Pointer; inline;
begin
  if (IntPtr(P) and PROPSLOT_MASK) = PROPSLOT_VIRTUAL then // Virtual Method
    Result := PPointer(PNativeUInt(Instance)^ + (UIntPtr(P) and $FFFF))^
  else // Static method
    Result := P;
end;

Now we can create a function to return a TRttiMethod for the getter of a property.

function GetPropGetterMethod(Instance: TObject; AProp : TRttiProperty) : TRttiMethod;
var
  LPropInfo : PPropInfo;
  LMethod: TRttiMethod;
  LCodeAddress : Pointer;
  LType : TRttiType;
  LocalContext: TRttiContext;
begin
  Result:=nil;
  if (AProp.IsReadable) and (AProp.ClassNameIs('TRttiInstancePropertyEx')) then
  begin
    //get the PPropInfo pointer
    LPropInfo:=TRttiInstanceProperty(AProp).PropInfo;
    if (LPropInfo<>nil) and (LPropInfo.GetProc<>nil) and not IsField(LPropInfo.GetProc) then
    begin
      //get the real address of the ,ethod
      LCodeAddress := GetCodePointer(Instance, LPropInfo^.GetProc);
      //get the Typeinfo for the current instance
      LType:= LocalContext.GetType(Instance.ClassType);
      //iterate over the methods of the instance
      for LMethod in LType.GetMethods do
      begin
         //compare the address of the currrent method against the address of the getter
         if LMethod.CodeAddress=LCodeAddress then
           Exit(LMethod);
      end;
    end;
  end;
end;

And for the setter is the same again but we inspect the SetProc instead.

function GetPropSetterMethod(Instance: TObject; AProp : TRttiProperty) : TRttiMethod;
var
  LPropInfo : PPropInfo;
  LMethod: TRttiMethod;
  LCodeAddress : Pointer;
  LType : TRttiType;
  LocalContext: TRttiContext;
begin
  Result:=nil;
  if (AProp.IsWritable) and (AProp.ClassNameIs('TRttiInstancePropertyEx')) then
  begin
    //get the PPropInfo pointer
    LPropInfo:=TRttiInstanceProperty(AProp).PropInfo;
    if (LPropInfo<>nil) and (LPropInfo.SetProc<>nil) and not IsField(LPropInfo.SetProc) then
    begin
      LCodeAddress := GetCodePointer(Instance, LPropInfo^.SetProc);
      //get the Typeinfo for the current instance
      LType:= LocalContext.GetType(Instance.ClassType);
      //iterate over the methods
      for LMethod in LType.GetMethods do
      begin
         //compare the address of the currrent method against the address of the setter
         if LMethod.CodeAddress=LCodeAddress then
           Exit(LMethod);
      end;
    end;
  end;
end;

TRttiPropertyHelper

Finally we can implement the methods of our helper for the TRttiProperty class.

function TRttiPropertyHelper.GetGetterField: TRttiField;
begin
 Result:= GetPropGetterField(Self);
end;

function TRttiPropertyHelper.GetSetterField: TRttiField;
begin
 Result:= GetPropSetterField(Self);
end;

function TRttiPropertyHelper.GetterMethod(Instance: TObject): TRttiMethod;
begin
 Result:= GetPropGetterMethod(Instance, Self);
end;

function TRttiPropertyHelper.SetterMethod(Instance: TObject): TRttiMethod;
begin
 Result:= GetPropSetterMethod(Instance, Self);
end;

Now we can use the helper like this

procedure DumpPropInfoExt(AClass: TObject);
var
  LContext: TRttiContext;
  LType: TRttiType;
  LProp: TRttiProperty;
  LPropInfo: PPropInfo;

  LField: TRttiField;
  LMethod: TRttiMethod;
begin
  LType := LContext.GetType(AClass.ClassInfo);
  for LProp in LType.GetProperties() do
    if LProp is TRttiInstanceProperty then
    begin
      LPropInfo := TRttiInstanceProperty(LProp).PropInfo;
      Writeln(Format('%-18s GetProc %p SetProc %p',
        [LProp.Name, LPropInfo.GetProc, LPropInfo.SetProc]));

      if LProp.IsReadable then
      begin
        LField := LProp.GetterField;
        if LField <> nil then
          Writeln(Format('  Getter Field Name %s', [LField.Name]))
        else
        begin
          LMethod := LProp.GetterMethod(AClass);
          if LMethod <> nil then
            Writeln(Format('  Getter Method Name %s', [LMethod.Name]))
        end;
      end;

      if LProp.IsWritable then
      begin
        LField := LProp.SetterField;
        if LField <> nil then
          Writeln(Format('  Setter Field Name %s', [LField.Name]))
        else
        begin
          LMethod := LProp.SetterMethod(AClass);
          if LMethod <> nil then
            Writeln(Format('  Setter Method Name %s', [LMethod.Name]))
        end;
      end;

    end;
end;

Limitations

Exist some limitations to use the above code.

  1. Delphi Array Properties are not supported for the GetProperties method. Instead you must use the GetIndexedProperties method to get a list of TRttiIndexedProperty and from there you can access to the ReadMethod and WriteMethod properties.
  2. The getters and setters methods of the property must emit RTTI info this implies which depending of the visibility of these methods you will need instruct to the compiler generate RTTI info for the methods, adding a sentence like this to your class {$RTTI EXPLICIT METHODS([vcPrivate])}

 

You can download the sample code from Github.

Rodrigo.

Author: Rodrigo

Just another Delphi guy.

5 thoughts on “Getting the getter and setter of a property using RTTI

  1. I have do something similar and I would like to add a comment:

    In your limitations, the first point (Array Properties) is only valid for XE (and maybe other early versions of eRTTI). They are supported in later versions. This was one of the gotchas I came across in back porting to XE. The class is “TRttiIndexedProperty”

    This is more a question than a comment. In GetPropSetterField/GetPropGetterField, you use/do “for LField in AProp.Parent.GetFields do”. Can’t the similar technique be used in GetPropSetterMethod/GetPropGetterMethod : for LMethod in AProp.Parent.GetMethods do” ?

    • Nicholas, About the Array properties. I will check later. Now about your second question The AProp.Parent returns the class where the property is defined, So for example for the ReadBaseProp property the AProp.Parent is ‘TBar’, but the implementation for the GetReadBaseProp method is located in TFoo, So the Address will be not found if I use AProp.Parent, Because that the Instance is used to get the methods.

  2. I’m using this examples in code, i just found out that is isn;t working right if you define a few private fields which you don’t use as properties. Then the offsets don’t match.

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