bool IsValidFilename(string testName)
{
 Regex containsABadCharacter = new Regex("[" + Regex.Escape(System.IO.Path.InvalidPathChars) + "]");
 if (containsABadCharacter.IsMatch(testName) { return false; };

 return true;
}

var file = "myfile.txt";
var fileExists = System.IO.File.Exists(file);

var file = "mybadfile<\/.txt";
var invalidFileChars = System.IO.Path.GetInvalidFileNameChars();
var isInvalidFilename = invalidFileChars.Any(s => file.Contains(s));