我如何从c#中的字符串中获取位？

string str = "Hello, World!";
int startIndex = 0;
int length = 5;

string result = str.Substring(startIndex, length);
Console.WriteLine(result); // 输出 "Hello"

string str = "Hello, World!";
int index = 4;

char result = str[index];
Console.WriteLine(result); // 输出 "o"