Auto generation of code

public string GetLatestOrderId()
{
    string ReceivedId = string.Empty;
    string displayString = string.Empty;
    String query = "SELECT MAX(OrderReceivedNo) FROM [Order_Received]";
    String data = DataManager.RunExecuteScalar(ConnectionString.Constr, query);
    ReceivedId = data;
    if (string.IsNullOrEmpty(ReceivedId))
    {
        ReceivedId = "OR0000";//It is the start index of alpha numeric value
    }
    int len = ReceivedId.Length;
    string splitNo = ReceivedId.Substring(2, len - 2);//This line will ignore the string values and read only the numeric values
    int num = Convert.ToInt32(splitNo);
    num++;// Increment the numeric value
    displayString = ReceivedId.Substring(0, 2) + num.ToString("0000");//Concatenate the string value and the numeric value after the increment
    return displayString;
}

public static IEnumerable<string> Numbers()
{
    return Enumerable.Range(0xA0000, 0xFFFF9 - 0xA0000 + 1)
        .Select(x => x.ToString("X"));
}
You could also have an id generator class:

public class IdGenerator
{
    private const int Min = 0xA0000;
    private const int Max = 0xFFFF9;
    private int _value = Min - 1;

    public string NextId()
    {
        if (_value < Max)
        {
            _value++;
        }
        else
        {
            _value = Min;
        }
        return _value.ToString("X");
    }
}