How to encrypt text, when my private key is in the XML file?

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C#

encryption

RSA

Hey,can you help me?
I want to encrypt text, when my private key is in the privat.xml file. My code doesn't work.

Thank you so much for help and your time.

What I have tried:

private void button2_Click(object sender, EventArgs e)
{
// convert the string into byte array
byte[] str = ASCIIEncoding.Unicode.GetBytes(textBox1.Text);

// read the private key
string privat = System.IO.File.ReadAllText(@"C:\Users\Gretaa\Desktop\RSA\privat.xml");

// compute the hash with any hash algorithm
SHA1Managed sha1hash = new SHA1Managed();
byte[] hashdata = sha1hash.ComputeHash(str);

// sign the hash data with private key
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.FromXmlString(privat);

// signature hold the sign data of plaintext , signed by private key
byte[] signature = rsa.SignData(str, "SHA1");

textBox14.Text = signature.ToString();
}