c# GroupBy的動態拼接

public class Person
{
    public string FirstName{set;get;}
    public string LastName{set;get;}

    public Person(){}

    public Person(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
    }
}

List<Person> personList=new List<Person>();
personList.Add(new Person() { FirstName = "Mickey", LastName = "Mouse" });
personList.Add(new Person() { FirstName = "Mickey", LastName = "Mouse" });
personList.Add(new Person() { FirstName = "zhang", LastName = "san" });


string columnName="FirstName";
var dics=personList.GroupBy(x => GetPropertyValue(x, columnName)).ToDictionary(x=>x.Key,x=>x.Count());
foreach(var dic in dics)
{
    textBox1.AppendText(string.Format("{0},{1}\r\n",dic.Key,dic.Value));
}