How to fix double to Int conversion of result of GetNumericValue , when using decrypting and encryption methods

Question

The goal of this code is to take in two text box fields and use the key to encrypt the original text box fields however I keep getting problems on char.GetNumericValue(orignalText[i]); it is saying can not convert type double to int are you missing a cast? I am not sure what this means, if anyone can provide insight and also if you see anything in my code that can be improved please let me know I love to learn more and more.

{
    public partial class Form1: Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        //We want to divide the plain texty into blocks of the key size.

    private void button1_Click(object sender, EventArgs e)
    {
        //Each char should be replaced by a number.


        int keySize = textBox2.Text.Length;
        int orginalSize = textBox1.Text.Length;


        char[] orignalText = textBox1.Text.ToCharArray();
        char[] keyText = textBox2.Text.ToCharArray();

        int[] encryptedText = new int[orginalSize];
        int[] encryptedKey = new int[keySize];
        int counter = 0;
        int[] combinedText = new int[orginalSize];
        for (int i = 0; i < keySize; i++)
        {
           
            encryptedKey[i] = char.GetNumericValue(keyText[i]);

        }


        while (orginalSize != counter)
        {

            int changeVal = 0;
            for (int i = counter; i < keySize; i++)
            {
                encryptedText[i] = char.GetNumericValue(orignalText[i]);



                combinedText[i] = encryptedText[i] + encryptedKey[changeVal];

                if (encryptedText[i] +encryptedKey[changeVal] > 27)
                {
                    combinedText[i] = (int)(encryptedText[i] + encryptedKey[changeVal]) - 27;
                }

                changeVal++;
                counter += 1;   
            }

            keySize += keySize;
            
        }

        string [] encryptedTextFinal = new string[textBox1.Text.Length];

        for (int j = 0; j < textBox1.Text.Length; j++)
        {
            encryptedTextFinal[j] = Convert.ToChar(combinedText[j]);

            MessageBox.Show(encryptedTextFinal[j].ToString());
        }
    }

}

}