I can't put a if else for saying if user input put string then say you must use double. ( textbox) [duplicate]

I have 1 day searching and not finding how to do this. Im actually building a windows forms in c# and 90% finished it but im not finding a code for a textbox if you put string to say the error message till you put double. Any idea please? here is the code. ( i didn't add anything for this because i can't understand the way_)

    private void button1_Click(object sender, EventArgs e)
    {
        string Processing = "";
        
        if (_Username.Text.Equals("John") || _Username.Text.Equals("Rick"))
        {
            usernameError.Text = ("This username is taken! Please use another one!");
        }
        else
        {
            usernameError.Text = null;
        }
        if (_RepeatPassword.Text != _Password.Text)
        {
            repeatPassword.Text = ("Password doesn't match together!");
        }
        else
        {
            repeatPassword.Text = null;
        }
        
    }