Validation.cs

[HttpGet]
[Route("add/{x}/{y}")]
public ObjectResult Add(string x, string y)
{
    try
    {
        ValidateInputs(x, y);

        var xVal = decimal.Parse(x);
        var yVal = decimal.Parse(y);

        return Ok(xVal + yVal);
    }
    catch (ValidationException ex)
    {
        return BadRequest(ex.Message);
    }
}

private void ValidateInputs(string x, string y)
{
    if (!decimal.TryParse(x, out _))
    {
        throw new ValidationException($"Cannot parse '{x}' to a number");
    }

    if (!decimal.TryParse(y, out _))
    {
        throw new ValidationException($"Cannot parse '{y}' to a number");
    }
}