Problem

[HttpPost]
public string Book([FromBody] string test) // binding string always is null 
{
return test;
}

Root cause

When having [FromBody]attribute, the string sent should not be a raw string, but rather a JSON string as it includes the wrapping quotes:

'test string' or "test string"

So you can use JSON.stringify()

JSON.Stringify('test string') // will be => "test string"

But cannot do JSON.Stringify twice

let x= JSON.stringify(JSON.stringify('test string')) // will be => '"\\"test string\\""'

Avoid execute twice

export const isJsonString = (str: string) => {
  try {
    JSON.parse(str);
  } catch (e) {
    return false;
  }
  return true;
};