Hi spopiela,
I'm afraid you need to provide more details for your question, what the type of your project is? Winform app? Or WPF? Or Web app?
If it is Winform application, which control are you using?
In fact, you just need to set the font with the following method:
YourControl.Font = new Font(YourControl.Font, YourControl.Font.Style | FontStyle.Bold);
Just like:
private void button1_Click(object sender, EventArgs e)
{
textBox5.Font = new Font(textBox5.Font, textBox5.Font.Style | FontStyle.Bold);
richTextBox1.Font= new Font(richTextBox1.Font, richTextBox1.Font.Style | FontStyle.Bold);
label5.Font = new Font(label5.Font, label5.Font.Style | FontStyle.Bold);
button2.Font = new Font(button2.Font, button2.Font.Style | FontStyle.Bold);
}

Hope this helps!
Regards,
Stanly
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