Setting a "nullable" property on a .NET object

Question

I have some .NET interop code where I've managed to load objects and read properties, however I am having trouble with setting a property on an object. Here's the relevant parts of the Delphi code:

uses
  mscorlib_TLB, Winapi.ActiveX;

type
  // Irrelevant parts of the code omitted
  TDotNetObject = class(TObject)
  private
    FTarget: OleVariant;
    FType: _Type;
  public
    procedure SetProperty(const APropertyName: string; const AValue: OleVariant; const AIndex: Integer = -1);
  end;

function VariantToPSafeArray(const AValue: Variant): PSafeArray;
begin
  Result := PSafeArray(VarArrayAsPSafeArray(AValue));
end;

procedure TDotNetObject.SetProperty(const APropertyName: string; const AValue: OleVariant; const AIndex: Integer = -1);
var
  LPropertyInfo: _PropertyInfo;
  LIndex: PSafeArray;
begin
  if AIndex >= 0 then
    LIndex := VariantToPSafeArray(VarArrayOf([AIndex]))
  else
    LIndex := nil;
  LPropertyInfo := FType.GetProperty(APropertyName, BindingFlags_Instance or BindingFlags_Public or BindingFlags_NonPublic);
  if LPropertyInfo <> nil then
    LPropertyInfo.SetValue(FTarget, AValue, LIndex);
end;

procedure UpdateDefectStatus(const ADefectID, AStatus: Integer);
var
  LObject: TDotNetObject;
begin
  // ** Code to obtain the object omitted ***
  LObject.SetProperty('Status', AStatus);
end;

The mscorlib_TLB unit comes from JCL in Project JEDI, here:

https://github.com/project-jedi/jcl/blob/master/jcl/source/windows/mscorlib_TLB.pas

An error is thrown when LPropertyInfo.SetValue is called in TDotNetObject.SetProperty:

Project TestProject.exe raised exception class EOleException with message 'Object of type 'System.Int32' cannot be converted to type 'System.Nullable`1[MTData.Transport.Tracking.DefectReporting.DefectStatus]''.

The DefectStatus property on the C# object is declared as:

public DefectStatus? Status

(i.e. it's nullable)

The Status property type in C# is declared:

public enum DefectStatus
{
    /// <summary>
    /// Defect Reported.
    /// </summary>
    Reported,
    /// <summary>
    /// Defect assessed.
    /// </summary>
    Assessed,
    /// <summary>
    /// Defect on work order.
    /// </summary>
    OnWorkOrder,
    /// <summary>
    /// Defect closed.
    /// </summary>
    Closed
}

I found a solution for how to handle this situation using C# here:

https://stackoverflow.com/a/13270302/3164070

However I'm a bit lost as to how to do the same in Delphi. Any ideas?

EDIT

Given Olivier's answer, I have attempted to write some Delphi code to do the equivalent, which is as follows:

procedure InvokeToObject;
var
  LType, LDefectStatusType: _Type;
  LInvokeFlags: TOleEnum;
  LArgs: PSafeArray;
  LValue: Integer;
  LResult: OleVariant;
  LRes: HRESULT;
  LResHex: string;
begin
  LType := MTDataClr.GetCoreType('System.Enum');
  LDefectStatusType := MTDataClr.GetType('MTData.Transport.Tracking.DefectReporting.DefectStatus');
  LInvokeFlags := BindingFlags_InvokeMethod or BindingFlags_Static;
  LValue := 1;
  LArgs := VariantToPSafeArray(VarArrayOf([LDefectStatusType, LValue]));
  LRes := LType.InvokeMember_2('ToObject', LInvokeFlags, nil, Null, LArgs, nil, LResult);
  LResHex := IntToHex(LRes);
end;

The goal with this piece of code is just to invoke the ToObject method of the Enum type. Obtaining LType and LDefectStatusType succeeds, however the call to InvokeMember_2 does not, with a return code of: 0x80131512, which apparently is a Missing Member exception. Any ideas on what I'm doing wrong?

Answer 1

The issue is that an enum is not an int, which means SetValue() would need to perform a double conversion (Int32 to DefectStatus and DefectStatus to DefectStatus?), which it cannot (it can only perform one).

Here's a C# code that reproduces what you're trying to do:

using System;
using System.Reflection;

public enum DefectStatus
{
    Reported,
    Assessed,
    OnWorkOrder,
    Closed
}

public class Defect
{
    public DefectStatus? Status {get; set;}
}

public class Test
{
    public static void Main()
    {
        Defect def = new Defect();

        PropertyInfo pi = typeof(Defect).GetProperty("Status");

        // This throws an ArgumentException
//      pi.SetValue(def, 1, null);

        // Retrieve the Assessed enum via its numeric value
        object assessed = Enum.ToObject(typeof(DefectStatus), 1);

        // This works as expected
        pi.SetValue(def, assessed, null);

        Console.WriteLine(def.Status);
    }
}

So you need to retrieve the enum in Delphi. For that you will need to play with the API to access the Enum type and call ToObject on it.