How to make such a form to remove values from ArrayList with StringBuilder?

The problem is that StringBuilder is not the same as ArrayList. In ArrayList, the string values are maintained as separate strings, accessible by index. You can loop through them.

StringBuilder is a big string. You are adding parts to the string but once they are added, they are no longer considered discreet strings, they are part of the larger string.

So if you build a string like this:

StringBuilder sb = new StringBuilder();
sb.Append("How");
sb.Append(" now,");
sb.Append(" brown");
sb.Append(" cow?");

...you no longer have four strings, you have one string:

"How now, brown cow?"

Even if you added with AppendLine that just means there is a line break inserted into the text.

Theoretically, you could maintain the locations of the lines added in a dictionary:

StringBuilder sb = new StringBuilder();
Dictionary<string,int> dictSB = new Dictionary<string,int>();

dictSB.Add("brown", sb.Length);
sb.Append("brown")


If you wanted to remove "brown" you could do it like this.

int brownLocation = dictSb["brown];
sb.Remove(brownLocation, "brown".Length);

Then you'd have to iterate through the dictionary entries and decrement all those that have larger values than the (former) location of "brown" by the length of brown.

But why would you want to do all that? If you need the strings to stay discrete, keep them in a collection of some sort. What are you trying to accomplish?