So you want to load a file and display it on your window, but the file's encoding isn't always exactly what D expects? Here's a way to load 8-bit text into a DFL control, where the text is either ASCII, UTF-8, UTF-8 with BOM, or ANSI.

import dfl.all, dfl.internal.utf;
import std.file, std.utf, std.stream;
char[] from8Bit(void[] data)
{
 ubyte[] utf8bom = ByteOrderMarks[BOM.UTF8];
 if(data.length >= utf8bom.length
 && cast(ubyte[])data[0 .. utf8bom.length] == utf8bom)
 {
 // It has UTF-8 BOM, so must be valid UTF-8.
 return cast(char[])data[utf8bom.length .. data.length];
 }
 char[] str;
 str = cast(char[])data;
 try
 {
 // Check if valid UTF-8 or ASCII.
 std.utf.validate(str);
 }
 catch
 {
 // Fall back to ANSI.
 str = dfl.internal.utf.fromAnsi(str.ptr, str.length);
 }
 return str;
}

Load the file into your program like so:

 textBox1.text = from8Bit(std.file.read("foo.txt"));

and you shouldn't get any invalid UTF-8 errors unless it is in fact UTF-8 which is invalid.

Note that when it falls back to ANSI, it has the problems of ANSI where it might be using the wrong codepage. In this case, I don't think Notepad would do any better either.