convert string to bool error

Notice that in C#, '=' is the assignment operator and '==' is the equality operator.

Also, when you have code like txt1.Text = " " then, as well as assigning " " to txt1.Text, it's also regarded as an expression which has the same type and value (after the assignment) as the variable on the left hand side of the '='.

So, here it's a string expression with the value " ".

You can now see why you're getting the error because the 'if' statement requires a bool expression and you can't convert a string to a bool.

The solution, as Mukesh correctly said, is to use the equality operator '==' instead  of '='. This tests the L.H.S for equality with the R.H.S, yielding true or false, which is a bool expression.