If absoluteDir and relativeFile exist for the sole purpose of being used to create absoluteFile, use should probably stick with plain strings for them and leaving only absoluteFile as a FileInfo.
var absoluteDir = @"c:\dir";
var relativeFile = @"subdir\file";
var absoluteFile = new FileInfo(Path.Combine(absoluteDir, relativeFile));
If otherwise you really need them to be typed, then you should use Path.Combine applied to the OriginalPath of each of them, such as in:
var absoluteDir = new DirectoryInfo(@"c:\dir");
var relativeFile = new FileInfo(@"subdir\file");
var absoluteFile = new FileInfo(Path.Combine(absoluteDir.OriginalPath), relativeFile.OriginalPath));