One way to do this is to make the TextBox either a public property or a public field. Then you will be able to access it through the instance of its parent form. So, if TextBox1 is a public member of FormA and myFormA is an instance of FormA, then you can use code such as

string strValue = myFormA.TextBox1.Text;

anywhere myFormA is known. Here is a VB project illustrating this technique.

Contributed from George Shepherd's Windows Forms FAQ