Invoking Delphi dll from C#

Question

Good day!

I have strange problem. On Delphi side we have:

Function Func(str: String; res: double) : double; export; stdcall;
Begin
    Result := res;
End;

And on C# side:

[DllImport("Project1.dll")]
static extern double Func(string str, double res);

It's OK, if i will write like this:

Console.WriteLine(Func("this is my function", 0.1));

Result will be 0.1.

But if I will replace 0.1 with 0 (zero, and 0d, and 0.0 too), I will get SEHException (0x80004005).

Any ideas?

UPD.

Delphi 2007 (no way to change, too many to rebuild ^_^)

VS 2013 (.NET 4.5.1)

OS: Windows 8.1

Platform target x86 (in x64 it does not work at all).

Answer 1

Function Func(str: String; res: double): double; stdcall;

This function can only be called from Delphi because it uses the native Delphi string type. Indeed you can only call it from a Delphi version that has a binary compatible string type.

If you wish to interop with C# you will need to change your signature to use types that are valid for interop. For instance:

Function Func(str: PAnsiChar; res: double): double; stdcall;

On the C# side this is:

[DllImport(dllname, CharSet = CharSet.Ansi)]
static extern double Func(string str, double res);

As an alternative to using null-terminated character arrays, you could use the COM BSTR type if you preferred. I won't demonstrate that here. There are plenty of examples already available.

Answer 2

try

Console.WriteLine(Func("this is my function", Convert.ToDouble(0.0)));