# Data Annotation (Model Validation)
## 官方文件
主要：
https://learn.microsoft.com/zh-tw/aspnet/core/tutorials/first-mvc-app/validation?view=aspnetcore-8.0
包含其他：
https://learn.microsoft.com/zh-tw/aspnet/core/tutorials/first-mvc-app/validation?view=aspnetcore-8.0
## 原理
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Client-Side Validation: Data annotations automatically generate **client-side validation** logic, reducing server round-trips and improving user experience. For example, [Required], [EmailAddress], and [StringLength] will generate JavaScript validation code in Razor views.

## 範例
Here are some real-world examples of how you might use data annotations with ErrorMessage in an ASP.NET Core MVC application:

### Example 1: User Registration Form
In a user registration form, you might want to ensure that the username, email, and password fields are filled out correctly and **adhere to specific validation rules**.

public class RegisterViewModel
{
    [Required(ErrorMessage = "Username is required.")]
    [StringLength(20, ErrorMessage = "Username must be between 5 and 20 characters long.", MinimumLength = 5)]
    public string Username { get; set; }

    [Required(ErrorMessage = "Email is required.")]
    [EmailAddress(ErrorMessage = "Please enter a valid email address.")]
    public string Email { get; set; }

    [Required(ErrorMessage = "Password is required.")]
    [StringLength(100, ErrorMessage = "Password must be at least 8 characters long.", MinimumLength = 8)]
    [DataType(DataType.Password)]
    public string Password { get; set; }

    [Required(ErrorMessage = "Please confirm your password.")]
    [DataType(DataType.Password)]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
}

### Example 2: Product Entry Form
When adding a new product to an inventory system, you might want to validate fields like name, price, and stock quantity.

public class ProductViewModel
{
    [Required(ErrorMessage = "Product name is required.")]
    [StringLength(50, ErrorMessage = "Product name cannot exceed 50 characters.")]
    public string Name { get; set; }

    [Required(ErrorMessage = "Price is required.")]
    [Range(0.01, 10000.00, ErrorMessage = "Price must be between $0.01 and $10,000.00.")]
    public decimal Price { get; set; }

    [Required(ErrorMessage = "Stock quantity is required.")]
    [Range(0, int.MaxValue, ErrorMessage = "Stock quantity cannot be negative.")]
    public int Stock { get; set; }
}

### Example 3: Contact Form
A contact form might include fields like name, email, phone number, and message, all of which need to be validated.

public class ContactViewModel
{
    [Required(ErrorMessage = "Your name is required.")]
    [StringLength(100, ErrorMessage = "Name cannot exceed 100 characters.")]
    public string Name { get; set; }

    [Required(ErrorMessage = "Email is required.")]
    [EmailAddress(ErrorMessage = "Please enter a valid email address.")]
    public string Email { get; set; }

    [Phone(ErrorMessage = "Please enter a valid phone number.")]
    public string PhoneNumber { get; set; }

    [Required(ErrorMessage = "Message is required.")]
    [StringLength(500, ErrorMessage = "Message cannot exceed 500 characters.")]
    public string Message { get; set; }
}

### Example 4: Login Form
A simple login form might require validation on the email and password fields.

public class LoginViewModel
{
    [Required(ErrorMessage = "Email is required.")]
    [EmailAddress(ErrorMessage = "Please enter a valid email address.")]
    public string Email { get; set; }

    [Required(ErrorMessage = "Password is required.")]
    [DataType(DataType.Password)]
    public string Password { get; set; }
}

Example 5: Event Registration Form
An event registration form might require the user's name, email, and a selection of the event date.

public class EventRegistrationViewModel
{
    [Required(ErrorMessage = "Your name is required.")]
    [StringLength(100, ErrorMessage = "Name cannot exceed 100 characters.")]
    public string Name { get; set; }

    [Required(ErrorMessage = "Email is required.")]
    [EmailAddress(ErrorMessage = "Please enter a valid email address.")]
    public string Email { get; set; }

    [Required(ErrorMessage = "Please select an event date.")]
    [DataType(DataType.Date)]
    public DateTime EventDate { get; set; }
}

## 常用 Common Data Annotations
Here’s a list of some of the most common data annotations used in .NET Core:

1. [Required]
Ensures that the property is not left empty.
Example:

    [Required(ErrorMessage = "Name is required.")]
    public string Name { get; set; }

2. [StringLength]
- Sets a maximum and optionally a minimum length for a string property.
- Example:

    [StringLength(50, MinimumLength = 5, ErrorMessage = "Name must be between 5 and 50 characters.")]
    public string Name { get; set; }

3. [MaxLength]
Sets the maximum allowable length of a string or array.
Example:

    [MaxLength(100, ErrorMessage = "Description can't exceed 100 characters.")]
    public string Description { get; set; }

4. [MinLength]
- Sets the minimum allowable length of a string or array.
- Example:

    [MinLength(10, ErrorMessage = "Description must be at least 10 characters long.")]
    public string Description { get; set; }

5. [Range]
- Validates that a number or date falls within a specified range.
- Example:

    [Range(18, 65, ErrorMessage = "Age must be between 18 and 65.")]
    public int Age { get; set; }

6. [RegularExpression]
- Validates that the value of a property matches a specified regular expression pattern.
- Example:

    [RegularExpression(@"^[a-zA-Z0-9]*$", ErrorMessage = "Only alphanumeric characters are allowed.")]
    public string Username { get; set; }

7. [EmailAddress]
- Validates that the value is a valid email address format.
- Example:

    [EmailAddress(ErrorMessage = "Invalid email address.")]
    public string Email { get; set; }

8. [Phone]
- Validates that the value is a valid phone number format.
- Example:

    [Phone(ErrorMessage = "Invalid phone number.")]
    public string PhoneNumber { get; set; }

9. [Compare]
- Compares two properties and validates that their values are equal.
- Often used for confirming passwords.
- Example:

    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
10. [CreditCard]

- Validates that the value is a valid credit card number format.
- Example:

[CreditCard(ErrorMessage = "Invalid credit card number.")]
public string CreditCardNumber { get; set; }

11. [Url]
- Validates that the value is a valid URL format.
- Example:

[Url(ErrorMessage = "Invalid URL.")]
public string WebsiteUrl { get; set; }

12. [DataType]
- Specifies the type of data, such as `Date`, `Time`, `Currency`, `Password`, etc.
- Helps with rendering the correct HTML input types and formatting.
- Example:

[DataType(DataType.Date)]
public DateTime BirthDate { get; set; }

13. [Display]
- Specifies how the data field is displayed.
- Commonly used to set display names, prompts, or formatting strings.
- Example:

[Display(Name = "Full Name")]
public string Name { get; set; }

14. [DisplayFormat]
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- Specifies formatting options for the data field.
- Often used for formatting dates, numbers, etc.
- Example:

[DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
public DateTime DateOfBirth { get; set; }

### Usage Example
Here’s a complete example of a model using several of these data annotations:

public class UserViewModel
{
[Required(ErrorMessage = "Name is required.")]
[StringLength(50, ErrorMessage = "Name must be less than 50 characters.")]
public string Name { get; set; }

[EmailAddress(ErrorMessage = "Invalid email address.")]
public string Email { get; set; }

[Phone(ErrorMessage = "Invalid phone number.")]
public string PhoneNumber { get; set; }

[Range(18, 65, ErrorMessage = "Age must be between 18 and 65.")]
public int Age { get; set; }

[Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
public string ConfirmPassword { get; set; }

[DataType(DataType.Password)]
public string Password { get; set; }
}

## 用法 Usage
To display the validation error messages to the user in an ASP.NET Core MVC application, **you can use the ModelState and ValidationSummary or individual ValidationMessageFor helpers** in your view.
-> 重點：ModelState + ValidationSummary(整體)/ValidationMessageFor(個體)

### 1. Model Definition
You've already defined the model with validation attributes:

public class UserViewModel
{
[Required(ErrorMessage = "Username is required.")]
[StringLength(20, ErrorMessage = "Username must be between 5 and 20 characters long.", MinimumLength = 5)]
public string Username { get; set; }
}

### 2. Controller Action
In your controller, when you post the form data, you should check if the model state is valid:

[HttpPost]
public IActionResult Register(UserViewModel model)
{
if (ModelState.IsValid)
{
// Proceed with registration logic
return RedirectToAction("Success");
}

// If we got this far, something failed; redisplay the form with validation errors
return View(model);
}

### 3. View (Razor Page)
In your view, you can use ValidationSummary and ValidationMessageFor to display error messages.

@model UserViewModel

<form asp-action="Register" method="post">
<div class="form-group">
<label asp-for="Username"></label>
<input asp-for="Username" class="form-control" />
<span asp-validation-for="Username" class="text-danger"></span>
</div>

<button type="submit" class="btn btn-primary">Register</button>
</form>

<div>
@Html.ValidationSummary(false, "", new { @class = "text-danger" })
</div>

### Explanation:
**ValidationMessageFor**: The asp-validation-for tag helper is used to display the validation error message for a specific property, like Username.

**ValidationSummary**: This displays a summary of all validation errors in the form. If you set the first parameter to false, it only shows model-level errors and not property-specific ones. If set to true, it shows all errors.

### Result:
If the user submits the form without filling in the username or with a username that doesn't meet the requirements, the corresponding error message ("Username is required." or "Username must be between 5 and 20 characters long.") will be displayed next to the input field.