XML Serialization - Required attribute - asp.net-core

Question

I have a class that should represent a controller's action parameter and I'd like its properties to be "required" (meaning, you get a status code 400 or something in case it's passed as null). I managed to get it done using System.ComponentModel.DataAnnotations, but the ErrorMessage that I pass to the constructor of the Required attribute is never shown.
[XmlRoot(ElementName = "root")]
public class Request
{
[XmlElement(ElementName = "prop")]
[Required(ErrorMessage = "The property is required.")]
public string Property { get; set; }
[XmlElement(ElementName = "another")]
[Required(ErrorMessage = "The property is required.")]
public string Another { get; set; }
}
Action:
[HttpPost]
public IActionResult Post([FromBody] Request value)
{
return Ok(value); //ignore this, it's just for testing purposes...
}
However, if I don't pass the Property value, I get a 400 that doesn't contain the ErrorMessage I passed earlier. Am I missing something here?
<ValidationProblemDetails xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Title>One or more validation errors occurred.</Title>
<Status>400</Status>
</ValidationProblemDetails>
My Startup has Xml formatters added to it:
services.AddMvc(options =>
{
options.RespectBrowserAcceptHeader = true;
options.InputFormatters.Insert(0, new XmlSerializerInputFormatter(options));
options.OutputFormatters.Insert(0, new XmlSerializerOutputFormatter());
}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
The body of the request looks like this, and it doesn't have "Property":
<root>
<another>Test</another>
<!-- Property "Property" is missing here -->
</root>

Answer 1

Kudos to Code Rethinked for the huge help - Customizing automatic HTTP 400 error response in ASP.NET Core Web APIs.
An approach that I managed to figure out eventually includes the use of services.Configure in my Startup.ConfigureServices method.
services.Configure<ApiBehaviorOptions>(options =>
{
options.InvalidModelStateResponseFactory = context =>
{
return new OkObjectResult(new CustomResponse(someStatusCode, context))
{
ContentTypes = { "application/xml" }
};
};
});
So, I made a class named CustomResponse that holds the status code I want to retrieve and all the validation errors (including the ones where my Required property was not passed to the API).
[XmlRoot(ElementName = "rcemsTrxSubReqAck")]
public class CustomResponse
{
[XmlElement(ElementName = "Status")]
public string Status { get; set; }
[XmlArray(ElementName = "Errors"), XmlArrayItem(ElementName = "Error")]
public string[] Errors { get; set; }
public CustomResponse(int status, ActionContext context)
{
Status = status;
Errors = ConstructErrorMessages(context);
}
private string[] ConstructErrorMessages(ActionContext context)
{
if (context == null)
{
return null;
}
string[] arr = new string[context.ModelState.ErrorCount];
int i = 0;
foreach (var keyModelStatePair in context.ModelState)
{
var key = keyModelStatePair.Key;
var errors = keyModelStatePair.Value.Errors;
if (errors != null && errors.Count > 0)
{
if (errors.Count == 1)
{
var errorMessage = GetErrorMessage(errors[0]);
arr[i] = $"{key}: {errorMessage}";
}
else
{
var errorMessages = new string[errors.Count];
for (var j = 0; j < errors.Count; j++)
{
errorMessages[j] = GetErrorMessage(errors[j]);
}
arr[i] = $"{key}: {errorMessages.ToString()}";
}
i++;
}
}
return arr;
}
private string GetErrorMessage(ModelError error)
{
return string.IsNullOrEmpty(error.ErrorMessage) ? "The input was not valid." : error.ErrorMessage;
}
}