How do I P/Invoke to a function that might not be there?

Question

Have the following in native code, need to write in managed code:

HINSTANCE hUser = LoadLibrary("user32.dll"); /* Can't fail -- it's already loaded */
BOOL (*dpi)() = (BOOL (*)())GetProcAddress(hUser, "SetProcessDPIAware");
if(dpi) dpi();

The function SetProcessDPIAware does not exist on the lowest end platform we support, so we would run into loader problems simply declaring the function and trying to call it.

However, I have to make a runtime decision whether or not to call SetProcessDPIAware based on conditions other than operating system version so I cannot use a manifest.

The solution is to have `GetProcAddress` as a P/Invoke function as well, and call that from managed code. — Dai, Oct 29 '14 at 21:46
Duplicate mark is incorrect as the duplicate question does not have an answer I can use. — Joshua, Oct 29 '14 at 22:16
How do you decide at runtime then? The whole block is inside an if statement. — Joshua, Oct 29 '14 at 23:00

score 1 · Accepted Answer · answered Oct 29 '14 at 21:48

You could P/Invoke the LoadLibrary and GetProcAddress in a similar way:

using System;
using System.Runtime.InteropServices;

static class Program
{
    [DllImport("kernel32", SetLastError = true)]
    static extern IntPtr LoadLibrary(string lpFileName);

    [DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
    static extern IntPtr GetProcAddress(IntPtr hModule, string procName);

    delegate bool MyDelegate();

    static void Main()
    {
        var hUser = LoadLibrary("user32.dll");
        var res = GetProcAddress(hUser, "SetProcessDPIAware");
        if (res != IntPtr.Zero)
        {
            // The function we are looking for exists =>
            // we can now get the corresponding delegate and invoke it
            var del = (MyDelegate)Marshal.GetDelegateForFunctionPointer(res, typeof(MyDelegate));

            bool x = del();
            Console.WriteLine(x);
        }
    }
}

How do I P/Invoke to a function that might not be there?

1 Answers1