definition statement if for property in c#

public class hamid
{
    private int[] arr = new int[10];
    public int Arr
    {
        get
        {
            return arr;
        }
        set
        {
            //i want have a 'if' statement for arr
            //for example if (values<0)
            //but i have error please help me 
        }
    }


}