Question

On page load the default value of the dropdownlist tell the user to select posible values.
Which is either Male or Female.If user did not select either of these values:Male or Female, the Genereate PatientNumber button should be disabled.
Otherwise the patient gender is generated base on the values select.
Currently if the dropdown is at default value i still can generate patientNumber.
Some one help me the cause of the error.i prefer the correct code.

protected void patient_num_Click(object sender, EventArgs e)
    {
        String gender = drl_gender.Text.ToString();
        string patientNumber = " ";
        RegistrationNumber Register_patient = new RegistrationNumber();
        patient_num.Enabled = false;
        if (gender=="Select Gender")
        {
            patient_num.Enabled = false;


        }
        else if (gender=="Male")
        {
            patient_num.Enabled = true;
            patientNumber = Register_patient.GeneratePatientNumber(Gender.Male).ToString();
            patientNumber = patientNumber.Replace("/", "-");
            txtpatientNum.Text = patientNumber;


        }
        else
        {
            patient_num.Enabled = true;
            patientNumber = Register_patient.GeneratePatientNumber(Gender.Female).ToString();
            patientNumber = patientNumber.Replace("/", "-");
            txtpatientNum.Text = patientNumber;

        }


    }

Answer 1

The problem in your code is that you are using the Text property of the DropDownList to determine what the user has selected(use SelectedValue instead). The Text property returns the text of the currently selected item but "" if no item is selected:

MSDN:

The Text property gets and sets the same value that the SelectedValuee
property does. The SelectedValue property is commonly used to
determine the value of the selected item in the ListControl control.
If no item is selected, an empty string (“”) is returned.

Now have a look at your code(remember String.Empty when nothing is selected):

if (gender=="Select Gender")
{
    patient_num.Enabled = false;
}
....
else
{
    patient_num.Enabled = true;
    // here we are!
    patientNumber = Register_patient.GeneratePatientNumber(Gender.Female).ToString();
    patientNumber = patientNumber.Replace("/", "-");
    txtpatientNum.Text = patientNumber;
}

---

The solution:

Use a RequiredFieldValidator instead to ensure that the user has selected a gender. You can use the InitialValue property to tell it the value for your "-- please select --" item.

   <asp:DropDownList id="DdlGender" runat="server" >
        <asp:ListItem Text="-- please select --" Value="-1"></asp:ListItem>
        <asp:ListItem Text="female" Value="1"></asp:ListItem>
        <asp:ListItem Text="male" Value="2"></asp:ListItem>
    </asp:DropDownList>

  <asp:RequiredFieldValidator id="RequiredGender" 
       InitialValue="-1" 
       ControlToValidate="DdlGender"
       ErrorMessage="Please select gender!"
       runat="server"/>