C# - Convert ASCII String To Hex

C# method to convert an ascii string to hex:

public 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;
}