Implementación el método IsNumeric de Visual Basic .Net para C#.

/// Determina si obj es numérico
public static bool IsNumeric(object obj)
{
	double retNum;
	bool isNum = Double.TryParse(Convert.ToString(obj),
								 System.Globalization.NumberStyles.Any,
								 System.Globalization.NumberFormatInfo.InvariantInfo,
								 out retNum);
	return isNum;
}

Método para validación del RUT de Chile.

Los parámetros out long rut y out string dv permiten obtener ambas partes del RUT en cada variable.

/// 

/// Determina si un rut es válido
/// 
public static string VerificarRut(string sRut, out long rut, out string dv)
{
	rut = 0;
	dv = "";
	if (string.IsNullOrEmpty(sRut))
		return null;
	sRut = sRut.Replace("-", "");
	sRut = sRut.Replace(".", "");
	sRut = sRut.Replace(" ", "");
	if(sRut.Length < 2)
		return null;
	string sdv = sRut.Substring(sRut.Length - 1, 1).ToUpper();
	sRut = sRut.Substring(0, sRut.Length - 1);
	if(!MyUtil.IsNumeric(sRut))
		return null;

	int m = 2;
	int total = 0;
	for(int i = sRut.Length - 1; i >= 0; i--)
	{
		total = total + Convert.ToInt32(sRut.Substring(i, 1)) * m;
		if(m == 7)
		m = 1;
		m++;
	}
	int res = 11 - (total % 11);
	string dvr = "0";
	if(res == 10) {
		dvr = "K";
	} else if(res == 11) {
		dvr = "0";
	} else {
		dvr = res.ToString();
	}
	if(sdv.Equals(dvr)) {
		rut = Convert.ToInt64(sRut);
		dv = sdv;
		return sRut + "-" + sdv;
	}
	return null;
}