private void decrypt_Click(object sender, EventArgs e)
        v = Convert.ToDouble (txtencrypt.Text);            
        t = Math.Pow(v, d);   
        msg = Convert.ToInt16(Math.Pow(v,d) % Convert.ToDouble(n));
        txtdecrypt.Text = Convert.ToString(msg);

 This is the code im doing for decryption in rsa algorithm but i got a problem in power operation.
 For a small power value,the power operation performed well but when im using highest value as a power i got exponential value as a output. 

Can we have some example values for v, n and d?

Please explain what exactly are using v,n and d for. are they IV's and Keys ?
Did you know you can use .NET building Encryption and Decryption algorithms ?

.NET RSA security

Hope you find it useful.

try this, (long)Math.Pow(v, d)

the value is

When running those figures I recieve:
t = 8.95430243255237E+23
msg = 149

I presume the issue is coming from the result of t?

@tinstaafl using your suggestion just results in the two results above being flipped to negatives and values changed, i.e:
t = -9.22337203685478E+18
msg = -162

Tested using the code:

            double v = 11;
            double d = 23;
            double n = 187;
            double t = (long)Math.Pow(v, d);
            Int16 msg = Convert.ToInt16(t % Convert.ToDouble(n));

Note the line Convert.ToInt16(t % Convert.ToDouble(n)); where I have removed the second power function and used the result of your variable t as your just getting the same value twice when it isn't needed.

Using your original code and the numbers you gave, by changing t to decimal and casting the power function to decimal I get 895430243255237000000000 for t and 58 for msg

            double v = 11;
            decimal t = (decimal)Math.Pow(v, 23);
            double msg = Convert.ToInt16(t % Convert.ToDecimal(187));
            Console.WriteLine(Convert.ToString(msg)+" "+t.ToString());

plain text is 88
cipher text is 11(i.e. "v" value)
but while doing the power operation with long data type,im getting the plain text as -162

Thanks tinstaafl for your suggestion
im also getting the value 895430243255237000000000 but i need an exact values instead of 000000000. But the normal calculation the answer is 895430243255237372246531

Good idea ddanbe, this produces the results you want:

First you have to be targeting atleast Framework 4, which means VS 2010 or later, then you have to add a reference to System.Numerics.dll, then you have include it using System.Numerics;

            BigInteger v = 11;
            BigInteger t = BigInteger.Pow(v,23);
            BigInteger msg = (BigInteger.Remainder(t,187));
            Console.WriteLine(Convert.ToString(msg)+" "+t.ToString());

I think the problem with using the Math.Pow method is, it only returns a double, which limits it's accuracy on really large number like this.

Please give some other suggestion.

If you can live with the 28 digit accuracy of the decimal type for the result of the exponentiation, then you can implement your own power function.

    private void test()
        Int32 v = 11;
        Int32 d = 23;
        Int32 n = 187;

        decimal exp = DecPower(v, d);

        decimal m = decimal.Remainder(exp, n);
        string msg = m.ToString();


    private decimal DecPower(decimal Num, Int32 power)
        decimal ret = 1;
        for (Int32 i = 1; i <= power; i++)
            ret = decimal.Multiply(ret, Num);
        return ret;