Form validation of empty strings

Question

How can I make the form first name input field not accept empty strings with space characters " "

    <form asp-action="SaveRegistration" autocomplete="off">
        <div asp-validation-summary="ModelOnly" class="text-danger"></div>
        <div class="form-group">
            <label asp-for="FirstName" class="control-label"></label>
            <input asp-for="FirstName" class="form-control" />
            <span asp-validation-for="FirstName" class="text-danger"></span>
        </div>

Model:

public class ContactInfo
{
    [Required(ErrorMessage = "This field is required")]
    [StringLength(50)]
    [DisplayName("First name")]
    public string FirstName { get; set; }
}

With [Required] attribute the user can still submit with a string with just space characters " "

I know it's a simple question, but I am novice in ASP.NET MVC

you know, you can use [StringLength] and.. what deters you from using @Html.TextBoxFor()? — Bagus Tesa
– Bagus Tesa, Commented Jul 27, 2018 at 7:08
@BagusTesa I am already using StringLength. How is Html.TextBoxFor better than <input asp-for="FirstName" ? — Don Box
– Don Box, Commented Jul 27, 2018 at 7:30
@Bola How does MinLength helps? I want to prevent user from entering string of just space characters, I don't want to restrict to a minimum length — Don Box
– Don Box, Commented Jul 27, 2018 at 7:34
[Required(AllowEmptyStrings = false)] - is this work? Sounds like you can write custom RequiredAttribute with IsNullOrWhiteSpace check. — Tetsuya Yamamoto
– Tetsuya Yamamoto, Commented Jul 27, 2018 at 7:34

Jeremy Thompson · Accepted Answer · 2023-04-14 05:47:42Z

Usage:

[NotNullOrWhiteSpaceValidator]
public string FirstName { get; set; }

How to make your own Attributes:

using System;
using System.ComponentModel.DataAnnotations;

public class NotNullOrWhiteSpaceValidatorAttribute : ValidationAttribute
{
    public NotNullOrWhiteSpaceValidatorAttribute() : base("Invalid Field") { }
    public NotNullOrWhiteSpaceValidatorAttribute(string Message) : base(Message) { }

    public override bool IsValid(object value)
    {
        if (value == null) return false;

        if (string.IsNullOrWhiteSpace(value.ToString())) return false;

        return true;        
    }

    protected override ValidationResult IsValid(Object value, ValidationContext validationContext)
    {
        if (IsValid(value)) return ValidationResult.Success;
        return new ValidationResult("Value cannot be empty or white space.");
    }
}

Here is implementation code for a bunch more validators: digits, email, etc: https://github.com/srkirkland/DataAnnotationsExtensions/tree/master/DataAnnotationsExtensions

Thanks! This is server side validation, correct? The form needs to be posted in order for the validation to kick in
Correct, many people add client-side JavaScript validation, you should always have server-side validation as shown encase JavaScript is disabled on the clients' browser, eg Chrome or even Nintendo Wii. Typically Validation methods will be called via if (Model.IsValid) in the Controller. It's also possible to call these validations earlier in the pipeline...

SBFrancies · Accepted Answer · 2018-07-27 07:41:40Z

2

You could always use a regular expression.

For example:

[RegularExpression(@".*\S+.*$", ErrorMessage = "Field Cannot be Blank Or Whitespace"))]
public string FirstName { get; set; }

answered Jul 27, 2018 at 7:41

SBFrancies

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15 Comments

Don Box Over a year ago

Nice. Albeit regular expressions are nasty.

Bagus Tesa Over a year ago

as in, it works on C# side but sometimes wont on js side... look behind.

Don Box Over a year ago

@BagusTesa "sometims wont" ? What does that even mean?

Don Box Over a year ago

@SBFrancies I will mark your comment as answer, however I was looking for something better than RegExp. This can be avoided using ViewState.IsValid or having a custom client validation

SBFrancies Over a year ago

Thanks - I agree, not a fan of regular expressions personally.

|

shakram02 · Accepted Answer · 2020-10-02 09:07:12Z

2

The default behaviour is to reject empty strings. You can change this behaviour by setting AllowEmptyStrings to true docs

[Required(ErrorMessage = "This field is required")]

answered Oct 2, 2020 at 9:07

shakram02

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Comments

Abdul Hadee · Accepted Answer · 2020-04-03 05:26:49Z

Note:This is for .net MVC 4+

There is a Implementation of AllowEmptyStrings within [Required(ErrorMessage = " is required.")].

And this method returns true if an empty string is allowed; otherwise, false. The default value for this method is set to false. So no need to worry ! Test and verify the same.It should not allow single or multiple empty string.If not explicitly make it false and see if its work.

FYR` {

public class RequiredAttribute : ValidationAttribute
    {
        //
        // Summary:
        //     Initializes a new instance of the 
           System.ComponentModel.DataAnnotations.RequiredAttribute
        //     class.
        public RequiredAttribute();

        //
        // Summary:
        //     Gets or sets a value that indicates whether an empty string is 
       // allowed.
        //
        // Returns:
        //     true if an empty string is allowed; otherwise, false. The default 
        // value is false.
        public bool AllowEmptyStrings { get; set; }

        //
        // Summary:
        //     Checks that the value of the required data field is not empty.
        //
        // Parameters:
        //   value:
        //     The data field value to validate.
        //
        // Returns:
        //     true if validation is successful; otherwise, false.
        //
        // Exceptions:
        //   T:System.ComponentModel.DataAnnotations.ValidationException:
        //     The data field value was null.
        public override bool IsValid(object value);
    }

}`

Saurabh · Accepted Answer · 2018-07-30 05:29:19Z

-1

First

You can try stringlength or minlength data annotation

Second

you can use parsley.js

Its an amazing library for client side form validation .

I will recommend you to use both client side and server side validation .

answered Jul 30, 2018 at 5:29

Saurabh

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1 Comment

Jeremy Thompson Over a year ago

String Length or MinLength doesn't prevent whitespace.being valid

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