How do I use a string in TRttiMethod.Invoke as parameter properly?

Question

I'm trying to generalize the content validation of visual components with the Text-property using RTTI but when I try to pass a string value into TRttiMethod.Invoke, I get the Message "Invalid Typecast". (Actually "Ungültige Typumwandlung" but I guess, that was a fitting translation.)

The code below is stripped of all security measures, assertions and so on, assuming all passed objects are just perfect.

procedure ValidateTextFieldAndSetFocus(const Field: TObject; const Validator: TObject; const errorStates: array of TStringValidationResult; const sErrorMessage: string);
var
  context  : TRttiContext;
  objField : TRttiType;
  objValid : TRttiType;
  prop     : TRttiProperty;
  execute  : TRttiMethod;
  I        : Integer;
  validResult : TStringValidationResult;
  value    : TValue;
begin
  context  := TRttiContext.Create;
  objField := context.GetType(Field.ClassInfo);
  objValid := context.GetType(Validator.ClassInfo);
  prop     := objField.GetProperty('Text');
  value    := prop.GetValue(Field);
  execute  := objValid.GetMethod('Execute');
  for I := 0 to High(errorStates) do
    if execute.Invoke(Validator,[value]).TryAsType<TStringValidationResult>(validResult) then
      if validResult = errorStates[I] then
      begin
        SetFocusIfCan(Field);
        raise Exception.Create(sErrorMessage);
      end;
end;

The Validator's Execute only has one string-Parameter. I've seen examples where strings were passed directly into the array of TValue, but then I get the same typecast error.

edit:

The actual error appears in execute.Invoke(Validator,[value]).

Example

TNoSemicolonNullValidator = class
  class function Execute(const aStr: string): TStringValidationResult;
end;

procedure TestValidation;
var
  Validator : TNoSemicolonNullValidator;
begin
  Validator := TNoSemicolonNullValidator.Create;
  try
    ValidateTextFieldAndSetFocus(Edit1,Validator,[svInvalid],'Edit1 is invalid!');
  finally
    Validator.Free;
  end;
end;

Solarism

Please provide a minimal reproducible example so that we don't have to guess the parts that are missing – Eudoxia

As it stands, this question should be closed as being off topic. Because you have not provided a reproduction. Once you remedy that (see the link in the comment above), we will be able to answer. – Eudoxia

Actually the code provides enough information to spot the mistake. – Mckay

@StefanGlienke So you don't believe in submitting complete reproductions? Personally I like to encourage best practices and teach people how to isolate problems and submit minimal reproductions. I'm quite sure you feel the same way when people submit issues to Spring4D. I feel very strongly that we should be educating people of the importance of complete code to reproduce problems. I'm disappointed that you don't seem to share that view. – Eudoxia

Answer 1

You are calling a class function here but you are passing a TObject as first parameter (which is the hidden Self argument of non static methods). On a class method the Self parameter must not be an instance but the class of it. So the correct call would be:

execute.Invoke(validator.ClassType, [value]);

Here is a minimal example to prove that:

program Project1;

{$APPTYPE CONSOLE}

uses
  Rtti,
  SysUtils;

type
  TValidator = class
    class function Execute(const s: string): Boolean;
  end;

class function TValidator.Execute(const s: string): Boolean;
begin
  Writeln(s);
end;

var
  ctx: TRttiContext;
  v: TValidator;
begin
  v := TValidator.Create;
  try
    ctx.GetType(TValidator).GetMethod('Execute').Invoke(v, ['test']);
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  try
    ctx.GetType(TValidator).GetMethod('Execute').Invoke(v.ClassType, ['test']);
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;
end.

Mckay