To answer your question:
String contatenation in C# (and .NET in general) is "safe", but doing it in a tight loop as you describe is likely to cause severe memory pressure and put strain on the garbage collector.
I would hazard a guess that the errors you speak of were related to resource exhaustion of some sort, but it would be helpful if you could provide more detail — for example, did you receive an exception? Did the application terminate abnormally?
Background:
.NET strings are immutable, so when you do a concatenation like this:
var stringList = new List<string> {"aaa", "bbb", "ccc", "ddd", //... };
string result = String.Empty;
foreach (var s in stringList)
{
result = result + s;
}
This is roughly equivalent to the following:
string result = "";
result = "aaa"
string temp1 = result + "bbb";
result = temp1;
string temp2 = temp1 + "ccc";
result = temp2;
string temp3 = temp2 + "ddd";
result = temp3;
// ...
result = tempN + x;
The purpose of this example is to emphasise that each time around the loop results in the allocation of a new temporary string.
Since the strings are immutable, the runtime has no alternative options but to allocate a new string each time you add another string to the end of your result.
Although the result string is constantly updated to point to the latest and greatest intermediate result, you are producing a lot of these un-named temporary string that become eligible for garbage collection almost immediately.
At the end of this concatenation you will have the following strings stored in memory (assuming, for simplicity, that the garbage collector has not yet run).
string a = "aaa";
string b = "bbb";
string c = "ccc";
// ...
string temp1 = "aaabbb";
string temp2 = "aaabbbccc";
string temp3 = "aaabbbcccddd";
string temp4 = "aaabbbcccdddeee";
string temp5 = "aaabbbcccdddeeefff";
string temp6 = "aaabbbcccdddeeefffggg";
// ...
Although all of these implicit temporary variables are eligible for garbage collection almost immediately, they still have to be allocated. When performing concatenation in a tight loop this is going to put a lot of strain on the garbage collector and, if nothing else, will make your code run very slowly. I have seen the performance impact of this first hand, and it becomes truly dramatic as your concatenated string becomes larger.
The recommended approach is to always use a StringBuilder if you are doing more than a few string concatenations. StringBuilder uses a mutable buffer to reduce the number of allocations that are necessary in building up your string.