Enough already. I'm obviously not incentivized/shamed into writing unit tests for things in UNTESTED dir.s. It's just making a mess of the file tree

FileManager/FileUtility.cs

@@ -0,0 +1,52 @@
+using System;
+using System.IO;
+
+namespace FileManager
+{
+    public static class FileUtility
+    {
+        public static string GetValidFilename(string dirFullPath, string filename, string extension, params string[] metadataSuffixes)
+        {
+            if (string.IsNullOrWhiteSpace(dirFullPath))
+                throw new ArgumentException($"{nameof(dirFullPath)} may not be null or whitespace", nameof(dirFullPath));
+
+            // file max length = 255. dir max len = 247
+
+            // sanitize
+            filename = GetAsciiTag(filename);
+            // manage length
+            if (filename.Length > 50)
+                filename = filename.Substring(0, 50) + "[...]";
+
+            // append id. it is 10 or 14 char in the common cases
+            if (metadataSuffixes != null && metadataSuffixes.Length > 0)
+                filename += " [" + string.Join("][", metadataSuffixes) + "]";
+
+            // this method may also be used for directory names, so no guarantee of extension
+            if (!string.IsNullOrWhiteSpace(extension))
+                extension = '.' + extension.Trim('.');
+
+            // ensure uniqueness
+            var fullfilename = Path.Combine(dirFullPath, filename + extension);
+            var i = 0;
+            while (File.Exists(fullfilename))
+                fullfilename = Path.Combine(dirFullPath, filename + $" ({++i})" + extension);
+
+            return fullfilename;
+        }
+
+        public static string GetAsciiTag(string property)
+        {
+            if (property == null)
+                return "";
+
+            // omit characters which are invalid. EXCEPTION: change colon to underscore
+            property = property.Replace(':', '_');
+
+            // GetInvalidFileNameChars contains everything in GetInvalidPathChars plus ':', '*', '?', '\\', '/'
+            foreach (var ch in Path.GetInvalidFileNameChars())
+                property = property.Replace(ch.ToString(), "");
+            return property;
+        }
+    }
+}