I have written the code but still comes error

I have written this code but still can't find the function. Please help me rectify the problem.

The code:-

public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
Test t = new Test();
IntPtr fooPtr = Test.plus(5, 7);
int LENGTH = 0;
var foo = new int[LENGTH];
Marshal.Copy(fooPtr, foo, 0, LENGTH);
MessageBox.Show(LENGTH.ToString());
}
}
internal sealed class Test
{
[DllImport("F://test_Multiple//MFCApplication2//Debug//MFCApplication2.exe", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
public static extern IntPtr plus(int a, int b);
}