Responding to slack view_submission event with error object - .net-core

Question

I use ASP.NET Core 2.1 to build integration with Slack
Here is my modal in json format.
{
"type": "modal",
"callback_id": "send_sms",
"title": {
"type": "plain_text",
"text": "Send SMS",
"emoji": false
},
"submit": {
"type": "plain_text",
"text": "Submit"
},
"close": {
"type": "plain_text",
"text": "Cancel"
},
"blocks": [
{
"type": "divider"
},
{
"type": "input",
"block_id": "phone_number",
"label": {
"type": "plain_text",
"text": "Enter phone number",
"emoji": false
},
"element": {
"type": "plain_text_input",
"placeholder": {
"type": "plain_text",
"text": "Phone number",
"emoji": false
},
"action_id": "action_phone_number"
}
},
{
"type": "input",
"block_id": "message",
"label": {
"type": "plain_text",
"text": "Enter message",
"emoji": false
},
"element": {
"placeholder": {
"type": "plain_text",
"text": "Message",
"emoji": false
},
"action_id": "message",
"type": "plain_text_input",
"multiline": true
}
}
]
}
So when user submit form, my .net core application recieves an view_submission event and if phone number has invalid format I want to response to this event with error.
Slack docs says that I should response with such json object:
`{
"response_action": "errors",
"errors": {
"phone_number": "Invalid phone number format"
}
}`
During debugging I found out that my app actually DO load json from file and DO respond with a string that contains this json. But a still get this error on my modal
enter image description here
controller method returns Task> object but I'm not sure that it's correct
So my question is:
Does anybody know how I should response to this slack event using .net core?
As far as I understand even if I have validation error in my .net core app and I want to return error object to slack, I should respond with status 200 OK

Answer 1

Finnaly this question is resolved by returning
Content('json_string', "application/json");