TextBox output please ?

Question

Hi friends 

Well i do believe that string , object are reference types :) so there default values when not initialized should be NULL right ?? 

Humm now the problem ..

I have a textbox where when applied the text property on it te output should be in STRING FORMAT am i right here ??

Well now i applied like this 

int a = Convert.toInt32(TextBox1.text);

messagebox.show(a);

But i am getting exception : Input string was not in a correct format.

I given no input to textbox i.e i hoped it will take NULL by default but its not happening .. instead its taking this "" .. Y Y Y .

I hope i am clear with my doubt ??

Ty for help

Have  a nice day