There are a several of ways to do this. Here are a few:
1) Insert a carriage return-linefeed, ‘\r\n’, between your lines.
textBox1.Text = 'This is line 1.\r\nThis is line 2'; // or textBox1.Text = stringvar1 + '\r\n' + stringvar2;
2) Use the Environment.NewLine property. This property is normally set to ‘\r\n’.
textBox1.Text = 'This is line 1.' + Environment.NewLine + 'This is line 2';
3) Use the Lines property of the TextBox. Make sure you populate your array before you set the property.
string arr = new string; arr = 'this is line1'; arr = 'this is line 2'; textBox3.Lines = arr;