List<string> MainStringList = new List<string>();

public void RunTest()
{
    MainStringList.Add("Original string");

    Console.WriteLine("Before SetList: {0}", MainStringList[0]);
    SetList(MainStringList);

    Console.WriteLine("After SetList: {0}", MainStringList[0]);

}

public void SetList(List<string> listReference)
{
    listReference = new List<string>();
    listReference.Add("Changed string");
}

// Output:
// Before SetList: Original string
// After SetList: Original string

// -------------------------------

List<string> MainStringList = new List<string>();

public void RunTest()
{
    MainStringList.Add("Original string");

    Console.WriteLine("Before SetList: {0}", MainStringList[0]);
    SetList(ref MainStringList);

    Console.WriteLine("After SetList: {0}", MainStringList[0]);

}

public void SetList(ref List<string> listReference)
{
    listReference = new List<string>();
    listReference.Add("Changed string");
}

// Output:
// Before SetList: Original string
// After SetList: Changed string