String manipulation (splitting, concatenation, etc.)

What I'm trying to do is take a 2D array of 1-char strings and merge it all into a single one, with each 'row' being a line worth (seperated by backslash-n).

The array is constructed as such: (called as 10, 40, '#')

 function Initialize (Width:int, Height:int, defChar:String) {
     var W: int = Width;
     var H: int = Height;
     var map = new String[W, H];
     for (var x = 0; x < W; x++) {
         for (var y = 0; y < H; y++) {
             map[x, y] = defChar;
             }
         }
     return map;
     }

And concatenated (well, unsuccessfully so) as this:

 function convertMapToGui (map:String[,]) {
 var mapGuiString: String;
 Debug.Log(map.Length);
 
 for (var x = 0; x < 10; x++) {
     for (var y = 0; y < 40; y++) {
         mapGuiString += (map[x,y].ToString);
         }
     mapGuiString += ('\n');
     }
 return mapGuiString;
 }

What I want is a 10x40 grid of #s. What I'm getting is a 10x40 grid of... "CompilerGenerated.__NewlineTest_convertMapToGui$callable0$30_43"

So what am I doing wrong here? And for that matter, are there any tutorials or useful pages you guys know off-hand for learning to work with strings and such in unity? Say, for an RPG-style dialogue system? I tried looking in the store for something to learn off of, but it's not showing me anything free.