Строка в двоичную в C #

I have a function to convert string to hex as this,

public static string ConvertToHex(string asciiString)
{
    string hex = "";
    foreach (char c in asciiString)
    {
         int tmp = c;
         hex += String.Format("{0:x2}", (uint)System.Convert.ToUInt32(tmp.ToString()));
    }
    return hex;
}

Could you please help me write another string to Binary function based on my sample function?

public static string ConvertToBin(string asciiString)
{
    string bin = "";
    foreach (char c in asciiString)
    {
        int tmp = c;
        bin += String.Format("{0:x2}", (uint)System.Convert.????(tmp.ToString()));
    }
    return bin;
}