Need to parse textbox values into an array of characters in Visual C #

I wish to assign the individual characters  in a textbox into an array of characters, buffer2 as follows:

I have the following textbox which has the value, "PLAIN" in it. I wish to assign each character in it to an array of characters, buffer2 so that buffer[0] = 'P', buffer[1]= 'L', buffer[2] = 'A' and so forth. Whatever I have tried, doesn't seem to work. Can someone show me a way to get it done? I either get index "out of bounds" error or  "cannot assign string to character" error.

Please help! Thanks.