How do I properly pass a Delphi Record to a C# struct with Interop?

Question

In Delphi I have a structure like this:

  TCustomerInfo = Packed Record
    CustomerNo: Integer;
    FirstName: String[50];
    LastName: String[50];
  End;

With a dummy-proc like this:

procedure GetCustomer(CustomerNo: Integer; var CustomerInfo: TCustomerInfo);
begin
  CustomerInfo.CustomerNo := 19901;
  CustomerInfo.FirstName := 'JOHN';
  CustomerInfo.LastName := 'DOE';
end;

In C# I have this:

 [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi, Pack=1)]
 struct CUSTOMER_INFO
 {
  public Int32 CustomerNo;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst=50)]
  public string FirstName;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst=50)]
  public string LastName;
 }

With an imported Delphi function like this:

 [DllImport("Exceline.dll")]
 extern static void GetCustomer(Int32 CustomerNo, ref CUSTOMER_INFO CustomerInfo);

The idea is to make sure all memory allocation and storage is being handled by the C# application.

My problem is that nothing gets assigned to my C# struct upon return from GetCustomer :-/

Thomas Kjørnes · Accepted Answer · 2010-08-02 20:55:25Z

2

I finally came up with a solution which avoids all the Alloc/FreeHGlobal, but if this is truly bulletproff with regards to the garbage collector is another matter.

The solution is to first clear the TCustomer structure with FillChar then copy the data using the Move procedure.

The delphi record looks like this:

  TCustomer = packed record
    CustomerNo: Integer;
    FirstName: array [1..50] of Char;
    LastName: array [1..50] of Char;
  end;

Then i copy the string to the structure with a procedure:

procedure StrToBuf(Str: String; var buf);
begin
  Move(Pointer(str)^, buf, Length(str));
end;

Within a proc more or less like this:

procedure LoadCustomerFromQuery(var Query: TQuery; var Customer: TCustomer); stdcall;
begin

  FillChar(Customer, SizeOf(Customer), 0);

  StrToBuf(Query.FieldByName('FNAVN').AsString, Customer.FirstName);
  StrToBuf(Query.FieldByName('ENAVN').AsString, Customer.LastName);

  Customer.CustomerNo := Query.FieldByName('KUNDENR').AsInteger;

end;

Finally the C# struct looks something like this:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi, Pack=1)]
public struct TCustomer
{
    public Int32 CustomerNo;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 50)]
    public string FirstName;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 50)]
    public string LastName;
}

answered Aug 2, 2010 at 20:55

Thomas Kjørnes

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1 Comment

Tone Škoda Over a year ago

StrToBuf should have sizeof (char) in unicode Delphi.

Mark H · Accepted Answer · 2010-07-12 19:29:39Z

1

extern static void GetCustomer(Int32 CustomerNo, IntPtr CustomerInfo);
...
var info = default(CUSTOMER_INFO);
var ptr = Marshal.AllocHGlobal(Marshal.SizeOf(info));
Marshal.StructureToPtr(info, ptr, false);
GetCustomer(n, ptr);
Marshal.PtrToStructure(ptr, info);
Marshal.FreeHGlobal(ptr);

answered Jul 12, 2010 at 19:29

Mark H

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4 Comments

Thomas Kjørnes Over a year ago

Okay thanks, I'm getting closer with this, but I get an error on: Marshal.PtrToStructure(ptr, custInf) => The structure must not be a value class.

Thomas Kjørnes Over a year ago

Further it seems that C#'s ByValTStr isn't the right choice for Delphi's String[50]. I assume that Delphi use standard byte-len prefixed Pascal-strings, but I'm not allowed to use AnsiBStr in MarshalAs-attribute.

Mark H Over a year ago

Try info = (CUSTOMER_INFO)Marshal.PtrToStructure(ptr, typeof(CUSTOMER_INFO)); instead. As for the Delphi strings, they're an alias for AnsiString, which I think is just an array of chars, 1 byte per. I'm not sure if the length is prefixed - any chance of a dump on what you get?

Uli Gerhardt Over a year ago

A string[50] is essentially an array of 51 bytes where the first is the length byte. (If you want it zero-terminated you have to do this manually.)

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