Return multiline response in asp.net core API

I'm just wondering the reason behind why you are expecting response in Multiple lines.

In the response body, you can use newline characters (\n) to separate each line. When the API is called, the client will receive a response with multiple lines of text.

Note that the actual response format and interpretation may depend on the client consuming the API. It's important to ensure that the client is capable of understanding and rendering multiple lines of text in the response.

Are you sure it is \n\r? If so then something unusual is happening. Newline characters are \r\n, not \n\r.

If it is \r\n insterad of \n\r then that is multiline. The \r\n characters indicate a newline, in both Windows and in email messages. There is a very good reason why those characters are used but that is off-topic for here.

 Environment.Newline() is \r\n in Windows.

Why do you say multiline? What is it that does not work?

To split the string into multiple strings, essentially getting multiple lines, use:

string[] lines = theText.Split(
    new string[] { Environment.NewLine },
    StringSplitOptions.None
);

You do not show the code. You show an exception that the code is getting. We can probably help better if we know what the code is that is getting that exception.

To return a response in multiline format in your ASP.Net Core API, you can use the following steps:

1. Update the formatting of the response message: Modify the response message to include line breaks using the appropriate line break characters. In this case, you can use \r\n for Windows-style line breaks.

2. Set the Response.ContentType property: Ensure that the content type of the response is set to "text/plain" to indicate that the response should be treated as plain text.

3. Modify the response formatting options: In your API configuration or startup code, configure the JSON serializer to use the desired formatting options.

using Microsoft.AspNetCore.Mvc;

// ...

[HttpGet]
public IActionResult YourAction()
{
    string errorMessage = "Field names are invalid.\r\nField names should be either from Case or Charge.";

    var result = new
    {
        error = new
        {
            data = 0,
            message = errorMessage
        }
    };

    return Content(errorMessage, "text/plain");
}

By setting the content type to "text/plain," the response will be treated as plain text, and line breaks will be respected when displaying the response.