Missing DLLImport even though there is a "using InteropServices"

Question

I have the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using MapsApp.DB;

namespace MapsApp
{
    public partial class _Default : System.Web.UI.Page
    {
        [DLLImport("GeoUrbanApp.exe")]
        public static extern double CalcFigure(double east, double north, double size);
...

I am trying to call the CalcFigure function from the .exe. I've added it in the references, and trying to import it. All I get is:

The type or namespace name 'DLLImport' could not be found (are you missing a using directive or an assembly reference?)

The type or namespace name 'DLLImportAttribute' could not be found (are you missing a using directive or an assembly reference?)

The solution most people find online is the "using System.Runtime.InteropServices;" but I have it.

Answer 1

Try adding

using System.Runtime.InteropServices;

to your class, that's the namespace the DllImportAttribute resides in.

Answer 2

It's DllImport not DLLImport

:)