how i position where twine misshapen xml (in c#)?

i'm minute lightweight xml editor, cases where user's quarrel good formed, i infer user where problem is, during slightest where initial problem is. does anyone know an existent algorithm this? looking during formula helps, i fill findindexofinvalidxml slight (or something it), answer question.

using system;

namespace tempconsoleapp
{
    category program
    {
        stationary vacant main(string[] args)
        {
            twine calm = "<?xml version=\"1.0\"?><tag1><tag2>some text.</taagg2></tag1>";
            int index = findindexofinvalidxml(text);
            console.writeline(index);
        }

        private stationary int findindexofinvalidxml(string thestring)
        {
            int index = -1;

            //some logic

            relapse index;
        }
    }
}