C #, struct members are references, what happens

You know. struct (struct) is a value type, and Class (class) is a reference type, and now we consider a problem, what happens if the struct has members of a reference type?
Now we have a structure oneline, to introduce its characteristics, it can be regarded as a line of string, but at the same time it is composed of several parts, I quote: "ABCD", in some applications, I may need to know which elements of this string is composed of, is a/b/c/ D or ABC/D, and considering that this line can be added, so I used a ArrayList to put these elements, later called, I know, oh, is abc/d, rather than a/b/c/d, so as to make the corresponding treatment;
Now, let's look at my test code (with the debugger picture):
Oneline line_1=new oneline ("test");
Oneline line_2=line_1;
Oneline line_3=line_1.copy ();
Line_1.add ("New_line");
Return
Line_2 and line_3 should be the same if the reference type in the structure is to copy a copy when the struct is assigned to another structure.
I took it for granted when I was writing a program. The result is a very crazy exception, see figure, for example, when you use line_2 in the following code, if you think it has only "test" this element, the error is inevitable.
Finding out the problem, I wrote a copy method to return the copy of the structure, and the problem never appeared, as we can see from the graph that Line_3 is running as we expected.

Finally, briefly describe what this anomaly has taught me today.
The assignment of a struct should be a shallow copy equivalent to a class (a member is a value that duplicates a value, a member is a reference to copy a reference), not a full copy.
Appledotnet@hotmail.com 2004/07/08 attached oneline structure code see later



public struct Oneline
{
int Length;
String line;
ArrayList AL;
Public Oneline (string line)//initialization
{
This. Line=line;
This. Length=line.length;
This. Al=new ArrayList ();
This. AL. ADD (line);
}
public bool Add (string newline)//Add one line
{
foreach (Object obj in AL)
if (((string) obj) ==newline) return false;
This. AL. ADD (newline);
This. Line=this. Line+newline;
This. Length=this. Line.length;
return true;
}
Override public string ToString ()
{
return line;
}
Public oneline Copy ()//Returns a copy of the replica
{
Oneline ol=new Oneline (this. Al[0]. ToString ());
for (int i=1;i< AL. count;i++)
Ol. ADD (Al[i]. ToString ());
return OL;
}
}