study this example:
put this code in a method; put a breakpoint on the indicated line; single step through the code (use F11); observe the values being created.
string example = @"name=Card&value=%2a%2a%2a%2a%2a%2a%2a%2a%2a%2a%2a%2a0015";
string[] splitter = new string[] {"name=", "&value="};
StringBuilder sb = new StringBuilder();
string[] splitline = example.Split(splitter, StringSplitOptions.RemoveEmptyEntries);
sb.AppendLine($"{splitline[0]} {splitline[1]}");
string result = sb.ToString();
use a StringBuilder to avoid allocating new strings. study the String.Split method: [
^], [
^] and the StringBuilder class: [
^]