Null you can't but what you can insert is string.Empty. That has the same general principle as null, but it is techincally a value so it should work. Without more info and knowing what you are trying to accomplish though I can't say for sure.
I'm not sure why you would want to enter a null value like this. In this example with the addition of the null you could initialize the array by setting its size and then enter it's values.
string AutoComp = new string;
AutoComp = "name1";
AutoComp = "name2";
AutoComp = "name3";
// Doing it this way will allow to you work through your code and enter a value for AutoComp where ever you please. Keeping scope in mind of course. :)/>
I would use null value for string variable when I would want to save a record in my database and at the same time wanted some field in a record to remain null value in my database. Otherwise, I don't see any reason to use null value for a string variable.
One more thing that might be important: Usually if you wanted to insert a record with some of the fields being null, you would use DBNull.Value. If using NHibernate, you would use null instead.