如果 controller 方法中的参数构造在 .NET Core 3.1 中失败，如何更改 http 响应错误消息？

Question

I have a controller method which takes a SignUpDTO object as a parameter. Here is SignUpDTO class code, and the header of mentioned controller method:

[HttpPost("register")]
public async Task<ActionResult<UserDTO>> SignUpAsync([FromBody] SignUpDTO signUpData)

...

public class SignUpDTO
{
    [Required]
    [StringLength(100, MinimumLength = 3)]
    public string Username { get; set; }
    [Required]
    [RegularExpression(@"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,15}$")]
    public string Password { get; set; }
    [Required]
    [RegularExpression(@"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$")]
    public string MailAddress { get; set; }
}

Properties are decorated with some attributes which validates data. My problem is, if some invalid data comes with http request, the execution of the method does not even start. Here i include exemplary response sent by my api:

{
"errors": {
    "MailAddress": [
        "The field MailAddress must match the regular expression '^([\\w\\.\\-]+)@([\\w\\-]+)((\\.(\\w){2,3})+)$'."
    ]
},
"type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
"title": "One or more validation errors occurred.",
"status": 400,
"traceId": "|fcfb9908-4cd92abdc9dea235."
}

I would like to change JSON object sent with response, and send just simple message to be consistent with the other methods. The response should look like:

{
    "message": "Improper mail address!"
}

Is it possible to change default response if construction of the object fails? If not how i can solve my problem? Thank you

Answer 1

All of the attributes which extend ValidationAttribute allow you to provide a custom error message by setting the ErrorMessage property ( MSDN link ):

[RegularExpression(@"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$", ErrorMessage = "Improper mail address!")]

Answer 2

You can also change your return type to Task<IActionResult> this return type supports any possible object see documentation :

[HttpPost("register")]
public async Task<IActionResult> SignUpAsync([FromBody] SignUpDTO signUpData)
{
    if (signUpData == null || <any other validation>)
    {
        return BadRequest(<your response object with message only>);
    }

    /// your application code

    return Ok(<your UserDTO>);
}