You can put it like this:
public partial class Form1: Form {
...
// textBox1 is private (we can't access in from Form2)
// so we'd rather create a public property
// in order to have an access to textBox1.Readonly
public Boolean IsLocked {
get {
return textBox1.Readonly;
}
set {
textBox1.Readonly = value;
}
}
}
...
public partial class Form2: Form {
...
private void checkBox1_CheckedChanged(object sender, EventArgs e) {
// When checkBox1 checked state changed,
// let's find out all Form1 instances and update their IsLocked state
foreach (Form fm in Application.OpenForms) {
Form1 f = fm as Form1;
if (!Object.RefrenceEquals(f, null))
f.IsLocked = checkBox1.Checked;
}
}
}