Error while decrypting - RSA decryption

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C#

.NET

I am able to encrypt successfully but am unable to decrypt it.
My code goes like this:

public string DecryptRSA(string data, string privatestring)
    {
        string rsaprivate = System.Web.HttpContext.Current.Server.MapPath(privatestring);
        string clearData = null;
      //  try
       // {
            CspParameters param = new CspParameters();
            param.Flags = CspProviderFlags.UseMachineKeyStore;
            RSACryptoServiceProvider rsaProvider = new RSACryptoServiceProvider(param);

            //string merchantRsaPublic = GetXmlRsaKey(rsaprivate);
            StreamReader reader = new StreamReader(rsaprivate);
            string privatekeyXml = reader.ReadToEnd();
            rsaProvider.FromXmlString(privatekeyXml);
            byte[] asciiString = Encoding.ASCII.GetBytes(data);

           #1 byte[] decryptedData = rsaProvider.Decrypt(asciiString, true);
            string resultString = Convert.ToBase64String(decryptedData);

           // clearData = Convert.ToString(decryptedData);
       // }
       //catch (CryptographicException ex)
       // {
            Console.Write("asasa");
            //Log.Error("A cryptographic error occured trying to decrypt a value for " + privatestring, ex);

       //}
        return resultString;
    }

Now, with the try catch block there is no runtime error and the desired ouput is also received.
However with debugging & no try catch block the line #1 gives a error which says:
"The data to be decrypted exceeds the maximum for this modulus of 256 bytes."-> I have knowledge about this but don't know, how to solve this error. What would be the right code?

Please help.