Question

I have a model class, which I am using to create the form

Public Class Users
    Public Property Id As Integer
    Public Property UserName As String
    Public Property UserNin As Int16
    Public Property UserType As String
    Public Property Password As String

    Property UserTypes As IEnumerable(Of SelectListItem) = {
            New SelectListItem With {.Value = "admin", .Text = "Admin"},
            New SelectListItem With {.Value = "doctor", .Text = "Doctor"},
            New SelectListItem With {.Value = "reception", .Text = "Receptionist"}
    }
End Class

My View class, using the above ViewModel to generate form utilising HTML Helpers

<% Using (Html.BeginForm("Create", "User", FormMethod.Post))%>
    <table>
        <tr>
            <td>
                User Name
            </td>
        </tr>
        <tr>
            <td>
                <%= Html.TextBoxFor(Function(x) x.UserName)%>
            </td>
        </tr>
        <tr>
            <td>
                User NIN
            </td>
        </tr>
        <tr>
            <td>
                <%= Html.TextBoxFor(Function(x) x.UserNin)%>
            </td>
        </tr>
        <tr>
            <td>
                Password
            </td>
        </tr>
        <tr>
            <td>
                <%= Html.PasswordFor(Function(x) x.Password)%>
            </td>
        </tr>
        <tr>
            <td>
                <%= Html.DropDownListFor(Function(x) x.UserType, Model.UserTypes)%>
            </td>
        </tr>
        <tr>
            <td>
                <input type="submit" value="Add New User" />
            </td>
        </tr>
    </table>
<% End Using%>

Now what is the best way to add validation to this case?

---

Update:

Here is what I have tried in another case, but still I dont see any validation, the action is called and processes.

Here is my model class

Public Class LoginUser
    <Required()>
    Public Property UserName As String

    <Required()>
    <StringLength(8)>
    Public Property Password As String

End Class

This is the partial View

<% Using (Html.BeginForm("Login", "User", FormMethod.Post))%>
<% Html.EnableClientValidation()%>
<table ID="loginTable" runat="server">
    <tr>
        <td>
            <label for="username">UserName</label>
        </td>
    </tr>
    <tr>
        <td>
            <%= Html.TextBoxFor(Function(x) x.UserName)%>
            <%= Html.ValidationMessageFor(Function(x) x.UserName) %>
        </td>
    </tr>
    <tr>
        <td>
            <label for="password">Password</label>
        </td>
    </tr>
    <tr>
        <td>
            <%= Html.TextBoxFor(Function(x) x.Password)%>
            <%= Html.ValidationMessageFor(Function(x) x.Password)%>
        </td>
    </tr>
    <tr>
        <td>
            <input type="submit" value="Login" />
        </td>
    </tr>
</table>
<% End Using%>

Login Action

<HttpPost()>
Function Login() As ActionResult
    If ModelState.IsValid Then
        Dim sql As String
        Dim username As String = Request.Form("username").ToString
        Dim password As String = Request.Form("password").ToString
        Dim dbHelper As New DBHelper(False)

        sql = "SELECT * FROM " & _tblName & " WHERE username = '" & username & "' and password = '" & password & "'"
        Try
            Dim dr As DataRow = dbHelper.ExecuteAndGetRow(sql)

            If Convert.ToInt16(dr.Item(0).ToString) > 0 Then
                Dim nin As String = dr.Item(4)
                Session("loggedin") = 1
                Session("logged") = dr.Item(0)
                Session("logged_nin") = dr.Item(4)
                ViewData("message") = "Login Successful"
                ViewData("show_loginForm") = False
            Else
                ViewData("message") = "Login failed"
                ViewData("show_loginForm") = True
            End If
        Catch ex As Exception
            ViewData("message") = "Login failed"
            ViewData("show_loginForm") = True
        End Try


        Return View()
    Else
        Return View()
    End If
End Function

Answer 1

You could use data annotations. For example if the UserName field was required you decorate it with the <Required> attribute:

<Required>
Public Property UserName As String

and inside the view you put a corresponding placeholder for the error message:

<%= Html.TextBoxFor(Function(x) x.UserName) %>
<%= Html.ValidationMessageFor(Function(x) x.UserName) %>

or if you wanted to centralize all error messages in a single place you could use the ValidationSummary helper:

<%= Html.ValidationSummary() %>

And inside the POST controller action you could verify if the model is valid using ModelState.IsValid boolean property:

<HttpPost()>
Function Create(model As Users) As ActionResult
    If ModelState.IsValid Then
        ' validation succeeded => do some processing and redirect
        Return RedirectToAction("Create")
    End If

    ' validation failed => redisplay the same view so that the user
    ' can fix the errors
    Return View(model)
End Function

And here’s an article on MSDN which explains this as well.