ASP NET Core MVC model validation

In MVC as you are aware validation can be applied at the model level, similarly that can be done in ASP.NET Core.

ASP.NET Core Built-in Model Validation Attributes

The following are some of the common built-in validation attributes in ASP.NET Core

Attribute	Purpose
Required	Specifies the field is required
Range	Specifies the minimum and maximum value allowed
MinLength	Specifies the minimum length of a string
MaxLength	Specifies the maximum length of a string
Compare	Compares 2 properties of a model. For example, compare Email and ConfirmEmail properties
RegularExpression	Validates if the provided value matches the pattern specified by the regular expression

In controller action method to check if valiation is succesfull, we will use ModelState.IsValid Property.

Displaying Model Validation Errors

To display validation errors use asp-validation-for and asp-validation-summary tag helpers. 

To display the validation error associated with the Name property use asp-validation-for tag helper on a <span> element as shown below.

<div class="form-group row">

    <label asp-for="Name" class="col-form-label"></label>

    <div>

        <input asp-for="Name" class="form-control" placeholder="Name">

        <span asp-validation-for="Name"></span>

    </div>

</div>

To display a summary of all validation errors use asp-validation-summary tag helper on a <div> element as shown below.

<div asp-validation-summary="All">

</div>

The value for asp-validation-summary tag helper can be anyone from All, ModelOnly, None

Customizing Model Validation Error Message

If you want to change the default validation error message that can be done using the ErrorMessage property of the Required attribute as shown below.

public class Employee

{

    public int Id { get; set; }

    [Required(ErrorMessage = "The Email field is required")]

    public string Email { get; set; }

}