1. Add Data Annotation Attributes in Your Model

ASP.NET Core automatically validates these before entering your controller action.

Example Model

public class RegisterRequest
{
    [Required(ErrorMessage = "Username is required")]
    [StringLength(20, MinimumLength = 3)]
    public string Username { get; set; }

    [Required]
    [EmailAddress(ErrorMessage = "Invalid Email format")]
    public string Email { get; set; }

    [Required]
    [MinLength(6, ErrorMessage = "Password must be at least 6 characters")]
    public string Password { get; set; }

    [Range(18, 60, ErrorMessage = "Age must be between 18 and 60")]
    public int Age { get; set; }
}

2. Validate in Controller Using ModelState

ASP.NET Core automatically populates ModelState.

[HttpPost("register")]
public IActionResult Register([FromBody] RegisterRequest model)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState); // returns all validation errors
    }

    return Ok("Registration successful");
}

3. Enable Automatic Validation (Built-in in ASP.NET Core 3+)

From .NET Core 3 and above, automatic model validation is enabled by default.

Meaning:

1. You don't need to manually check ModelState.IsValid

2. The framework returns 400 BadRequest with validation errors automatically.

But if you prefer manual checking (recommended for custom responses), use the previous method.

4. Optional: Customize Validation Response

If you want cleaner error messages:

if (!ModelState.IsValid)
{
    var errors = ModelState.Values
        .SelectMany(v => v.Errors)
        .Select(e => e.ErrorMessage)
        .ToList();

    return BadRequest(new { Success = false, Errors = errors });
}

Bonus: Custom Validation Attribute Example

public class NoSpaceAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value == null) return true;
        return !value.ToString().Contains(" ");
    }
}

Use it like:

[NoSpace(ErrorMessage = "Username cannot contain spaces")]
public string Username { get; set; }

Summary