About string ,and help me to solution one problom about zhe value and reference type! Thank you!

[C# Programming]

zhe subject is that
class Class1 {
private string str = "class1.str";
private int i = 0;
static void StringConvert(string str) {
str = "string being converted.";
}
static void StringConvert(Class1 c) {
c.str = "string being converted.";
}
static void Add(int i) {
i++;
}
static void AddWithRef(ref int i) {
i++;
}
static void Main() {
int i1 = 10;
int i2 = 20;
string str = "str";
Class1 c = new Class1();
Add(i1);
AddWithRef(ref i2);
Add(c.i);
StringConvert(str);
StringConvert(c);
Console.WriteLine(i1);
Console.WriteLine(i2);
Console.WriteLine(c.i);
Console.WriteLine(str);
Console.WriteLine(c.str);
Console.Read();
}
tell me why "Console.WriteLine(str);" and "Console.WriteLine(c.str);"
zhe two sentence have two different results!
I know that zhe "string" is reference type!
but here i am puzzled!