Enjoy!

Console.WriteLine(String.Join(",", new List<uint> { 1, 2, 3, 4, 5 }));

First Parameter: ","
Second Parameter: new List<uint> { 1, 2, 3, 4, 5 })

String.Join will take a list as a the second parameter and join all of the elements using the string passed as the first parameter into one single string.

Yup..

List<Customer__c> customers = [Select Id, LegacyCustomerNo__c FROM Customer__c WHERE state__c = 'PA'];
String[] tmp1 = New String[]{};
String[] tmp2 = New String[]{};
String idString, customernostring;

for(Customers__c c : customers__c){
   tmp1.add(c.id);
   tmp2.add(c.legacycustomerno__c);
}

idString = string.join(tmp1,',');
customernostring = string.join(tmp2,',');

In .NET 4 you don't need the ToArray() call - string.Join is overloaded to accept IEnumerable<T> or just IEnumerable<string>.

There are potentially more efficient ways of doing it before .NET 4, but do you really need them? Is this actually a bottleneck in your code?

You could iterate over the list, work out the final size, allocate a StringBuilder of exactly the right size, then do the join yourself. That would avoid the extra array being built for little reason - but it wouldn't save much time and it would be a lot more code.

This is very easy , Isn't it ?

string sCategories = string.Join(",", categories.Select(x => x.Name));

Depends on your result and description, I guess your code is something like:

b = """58738121385139B
9135A4251EF0
2B631B53C383"""
print(', '.join(b))
result:
5, 8, 7, 3, 8, 1, 2, 1, 3, 8, 5, 1, 3, 9, B, 
, 9, 1, 3, 5, A, 4, 2, 5, 1, E, F, 0, 
, 2, B, 6, 3, 1, B, 5, 3, C, 3, 8, 3

You should use split to achieve it

print(', '.join(b.split("\n")))
result:
58738121385139B, 9135A4251EF0, 2B631B53C383