WPF TextBox.Text VS Variable

There is a textbox myTextBox in my WPF application.

The myTextBox.Text value is used by a button function for a few times.

I would like to know is there any difference in PERFORMANCE and MEMORY USAGE for the following cases:

a) Directly use myTextBox.Text for a few times

Call functionA(myTextBox.Text);

Call functionB(myTextBox.Text);

or

b) Declare a variable first then use it in the latter part

string str = myTextBox.Text;

Call functionA(str);

Call functionB(str);

Thanks.