Need Help with SortedList

[C# Programming]

Hi. I'm now using the SortedList to sort the strings from the text file. Is it possible to have a single key as the basis for comparing each string? If yes, I want to know how i will prevent the key from duplicating. Can someone help me?

As of now this is what I accomplished:


public static void SortByLastName(ref string[] arrayOfLastNames, ref string[] arrayOfFirstNames, ref string[] arrayOfPlateNum, ref string[] arrayOfType, ref string[] arrayOfYear)
{
Console.Clear();

int vehicleCountInt = 0;

Console.WriteLine("SHOW DATABASE: SORTING BY LAST NAME\n");

using (StreamReader countVehicle = new StreamReader("vehiclecount.txt"))
{
vehicleCountInt = Int32.Parse(countVehicle.ReadLine());

countVehicle.Close();
}

SortedList sortedLastName = new SortedList(vehicleCountInt);

using (StreamReader outputLastNames = new StreamReader("lastnames.txt"))
{

for (int i = 0; i < vehicleCountInt; i++)
{
arrayOfLastNames[i] = outputLastNames.ReadLine();

string keyVal = arrayOfLastNames[i];

sortedLastName.Add(keyVal, arrayOfLastNames[i]); //I'm having a run-time error here that throws the ArgumentException

if (!sortedLastName.ContainsKey(keyVal))
{
sortedLastName.Add(keyVal, arrayOfLastNames[i]);
}
else
{
sortedLastName.Remove(keyVal);
sortedLastName.Add(keyVal, arrayOfLastNames[i]);
}

Console.WriteLine("{0}", sortedLastName.Keys[i]);
}


outputLastNames.Close();
}

}