How to activate spellCheck in C# Windows Form Application?

If you are using .net4 you can add the References System.Xaml and WindowsFormsIntegration to your Winforms project.

This allows you to find the ElementHost in you Toolbox. By using the ElementHost you can use WPF objects in your Winfroms project.

System.Windows.Forms.Integration.ElementHost elementHost1 = new System.Windows.Forms.Integration.ElementHost();
System.Windows.Controls.TextBox textBox = new System.Windows.Controls.TextBox();
textBox.SpellCheck.IsEnabled = true;
elementHost1.Child = textBox;

There is no built-in spellcheck capability on the Windows Forms textbox.

The best thing you can do is probably embed an WPF textbox in your form. Hans Passant gives a very thorough answer in this post on how to achieve that.