Invalid string format in model validation message in .NET

I recently encountered a strange case or runtime error in System.ComponentModel.DataAnnotations namespace.

I have a model below and, with it, a few model validation attributes.

public class MyModel
{
    [Required]
    [MinLength(1)]
    [MaxLength(64)]
    [RegularExpression("^[0-9\\p{L}\\-_,.!?\\[\\]{}()<> ]*$", ErrorMessage = "is invalid. It should only contain letters, numbers, spaces, unicode and special characters ( ) [ ] { } < > - _ , . ? !")]
    [NoWhitespaceOnStartAndEnd(ErrorMessage = "is invalid. It should not start or end with space character(s).")]
    public string Name { get; set; }
}

Nothing weird on the first look. The code also compiles fine. But when I try to validate the property Name with some strange inputs, I get the exception below:

System.FormatException: Input string was not in a correct format.
   at System.Text.ValueStringBuilder.ThrowFormatInvalidString()
   at System.Text.ValueStringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ReadOnlySpan`1 args)
   ...

This does not look like an error of the validator but something internally. After some digging, I discovered that ErrorMessage was the culprit. In .NET internals, I found this:

public override string FormatErrorMessage(string name)
{
    SetupRegex();

    return string.Format(CultureInfo.CurrentCulture, ErrorMessageString, name, Pattern);
}

That makes sense, the ErrorMessage string is formatted using string.Format(). I isolated the case and ran the code below a few times until I tried curly braces. I got the same error:

var str = string.Format("Miha {}");
> System.FormatException: Input string was not in a correct format.
  + System.Text.StringBuilder.FormatError()
  + System.Text.StringBuilder.AppendFormatHelper(System.IFormatProvider, string, System.ParamsArray)
  + string.FormatHelper(System.IFormatProvider, string, System.ParamsArray)
  + string.Format(string, object[])

Now let’s escape them:

> var str = string.Format("Miha {{}}");
> Console.WriteLine(str);
Result: Miha {}

Now I only need to escape curly braces { } with double curly braces {{ }} in my model validation error message to:

[RegularExpression("^[0-9\\p{L}\\-_,.!?\\[\\]{}()<> ]*$", ErrorMessage = "is invalid. It should only contain letters, numbers, spaces, unicode and special characters ( ) [ ] {{ }} < > - _ , . ? !")]

And that’s it! :D