textbox values getting one by one using arraylist

First of all, never use ArrayList, it is obsolete and inconvenient due to the need of element type cast, which is error-prone, too. Instead, use generic equivalent System.Collections.Generic.List. The type ArrayList was rendered obsolete with introduction of generics in v.2.0.

You need to split in a different way:

string[] lines = myTextBox.Text.Split(
    new string[] { System.Environment.NewLine });

Now, get line number of lines.Length and use it in a randomly generated index in this array using the class System.Random. Do not create an instance of the class Random in loop (a common mistake) — make it only once in the lifetime of your application. Use the method System.Random.Next(Int32, Int32) (with range 0 to lines.Length-1).

—SA

I am agree with
-SA

On the place of Arraylist use generic collection for example:

class Info
{
public string Name{get;set;}
}
 
List oListInfo = new List();
private string stringToArrayList(string MyValue)
{
string[] s = MyValue.Split(new string[] { Environment.NewLine });
foreach (string item in s)
oListInfo.Name = item ;
//Write other logic
}