Question

I’m trying to reverse a string through the Windows form but for any reason for loop is giving an exception when I’m executing the code.

System.NullReferenceException: Object reference is not set to an instance of an object

Button_Click event

string input=textBox1.Text;
input=Convert.ToString(Console.ReadLine());
string output="";

if(textBox1.Text=="")
{
    MessageBox.Show("Sorry! You have not given any input for perform action");
}
else
{
    try{

        for(int i=input.Length-1; i>=0; i--)
        {
            output= output+input[i];
        }

            textBox2.Text=output;
            }

        catch(Exception ex)
        {
            MessageBox.Show(""+ex);
        }
    }
 }

While same logic is working perfectly with console application. I know I’m missing very basic thing but at the moment I’m out of ideas.

Answer 1

In a winform, Console.ReadLine() returns null (because there is no console). Likewise, Convert.ToString((string)null) returns null. And you can’t call .Length (in the for) on a null. Key pieces of code:

input=Convert.ToString(Console.ReadLine());
...
for(int i=input.Length-1; i>=0; i--)

Also, as a minor point: building up a string via concatenation is very inefficient.