call extern function GetComputerName in C#

using C#.net 2010.
I'm trying to call the extern function GetComputerName and its not working
declared in my class:
[DllImport("kernel32")] static extern int GetComputerName(string lpBuffer, ref int nSize);
and calling it here:
//find computer name int Length = 30; string computerName = GetComputerName(computerName, ref Length);
does anyone know what I'm doing wrong?
its giving me compile error "cannot implicitly convert  int to string" but not sure how to fix that
thanks
MC
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