using System;
using System.Runtime.InteropServices;

class Example
{
    // Use DllImport to import the Win32 MessageBox function.
    [DllImport("user32.dll", CharSet = CharSet.Unicode)]
    public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);

    static void Main()
    {
        // Call the MessageBox function using platform invoke.
        MessageBox(new IntPtr(0), "Hello World!", "Hello Dialog", 0);
    }
}

http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.aspx

[1] Add 'using System.Runtime.InteropServices;' 
[2] Add DllImport and function declaration
[3] call function.

This example involves passing strings and  passing IntPtr.

C# calling a DLL API, in this case, MessageBox in user32.dll

C#