private void decrypt_Click(object sender, EventArgs e)
        {
            v = Convert.ToDouble (txtencrypt.Text);
            t = Math.Pow(v, d);

            MessageBox.Show(Convert.ToString(t));
            MessageBox.Show(""+t.ToString());
            MessageBox.Show("" + Convert.ToDecimal(n));
            msg = Convert.ToInt16(Math.Pow(v,d) % Convert.ToDouble(n));
            txtdecrypt.Text = Convert.ToString(msg);

        }

this is the code im working now for rsa decryption. the problem im getting is the pow function
return in exponential format, but i need the non exponential format of output.
can any one please help me to find out solution?

The Format function should do the trick. Here's the MSDN article

You can use the double.ToString(string format) to specify the way a double is formatted. See Standard Numeric Format Strings and Custom Numeric Format Strings. For example, 1e64 with the "custom" format string 0 returns the string 10000000000000000000000000000000000000000000000000000000000000000. This example doesn't account for decimal digits, but you could use the standard f format specifier, if you have an idea of how many digits you need.

If you're running into issues with decimal digits (eg. 0.000000000000000002929), then you'll want to have a look at this thread.

I want actual value when perform power operation
ex:11pow23=895430243255237372246531
but i got 895430243255237000000000 value when i perform power operation after convert into decimal.
please help me.

A double has a precision of 15-16 digits. Look up the article in say MSDN.

Perhaps this is what you really need.