Question

I’m building a DLL in Delphi, and it needs to work similar to how the Windows API works. This DLL only has one exported function…

function DoSomething(var MyRecord: TMyRecord): Integer; stdcall;

…where TMyRecord = my record I will need to re-create in C#. If I’m not mistaken, this is exactly how the standard Windows API works. This record also contains a reference to another record type…

TMyOtherRecord = record
  SomeDC: HDC;
  SomeOtherInt: Integer;
end;

TMyRecord = record
  SomeInteger: Integer;
  SomeColor: TColor;
  SomeText: PChar;
  SomeTextSize: Integer;
  MyOtherRecord: TMyOtherRecord;
end;

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Question part 1:

I’d like to see if I can avoid using PChar, if at all possible. I don’t expect anything over 255 characters to be passed through. Is there another type I can use instead which won’t require me to use a size of string?

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Question part 2:

I need to double check that I am declaring this C# struct class correctly, because it needs to perfectly match the Record declared in Delphi…

public struct MyOtherRecord
{
  public IntPtr SomeDC;
  public int SomeOtherInt;
}

public struct MyRecord
{
  public int SomeInteger;
  public Color SomeColor;
  public string SomeText;
  public int SomeTextSize;
  public MyOtherRecord OtherReord = new MyOtherRecord();
}

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Question part 3:

Is it safe in this case to have a record inside of a record (or struct inside of a struct)? Pretty sure it is, but I need to make sure.

Answer 1

I’m going to assume that the information is flowing from C# to Delphi and not the other way, largely because that makes life a lot easier when writing the answer, and you didn’t state otherwise!

In that case the Delphi function declaration should be:

function DoSomething(const MyRecord: TMyRecord): Integer; stdcall;

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The first point is that you can’t expect System.Drawing.Color to be handled by the P/invoke marshaller. Declare the color as int and use ColorTranslator.ToWin32 and ColorTranslator.FromWin32 to handle the conversion.

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There’s nothing to be afraid of with PChar. You don’t need a field with the string length since the length is implicit in a PChar due to the null-terminator. Just declare the field as string in the C# struct, PChar in the Delphi record and let the P/invoke marshaller do its magic. Don’t try to write to the PChar content from Delphi. That will end in tears. If you want to pass a string back to the C# code then there are ways, but I won’t address them here.

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It’s perfectly fine to have inline structs. Nothing to worry about there. Don’t allocate them with new. Just treat them as value types (which they are) like int, double etc.

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In due course you will need to add StructLayout attributes and so on, declare your DLL function with DllImport and so on.

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To summarise, I would declare your structs like this:

Delphi

TMyOtherRecord = record
  SomeDC: HDC;
  SomeOtherInt: Integer;
end;

TMyRecord = record
  SomeInteger: Integer;
  SomeColor: TColor;
  SomeText: PChar;
  MyOtherRecord: TMyOtherRecord;
end;

function DoSomething(const MyRecord: TMyRecord): Integer; stdcall;

C#

[StructLayout(LayoutKind.Sequential)]
public struct MyOtherRecord
{
  public IntPtr SomeDC;
  public int SomeOtherInt;
}

[StructLayout(LayoutKind.Sequential)]
public struct MyRecord
{
  public int SomeInteger;
  public int SomeColor;
  public string SomeText;
  public MyOtherRecord MyOtherRecord;
}

[DllImport("mydll.dll")]
static extern int DoSomething([In] ref MyRecord MyRecord);

I’ve not marked the string with a MarshalAs since the default is to marshal it as a LPSTR which is the same as a Delphi 7 PChar.

I’ve only compiled this in my head so there may be a few wrinkles.