Deleting Contents in TextBox crashes program

Question

I just  began learning C# last week and I'm practicing by creating a WPF Application that converts units.
I'm converting the Celsius textbox's input to double in order to calculate the temperatures for Farenheit and Kelvin, then I output those results and append the (°F or °K) to the string. The problem that I have is that if I try to delete the text in Celsius
to change it to another number, then the program crashes and I get the following error: 
An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll
Additional information: Input string was not in a correct format.
This is the code I have from the celsius textBox (c_textBox) method:
private void c_textBox_TextChanged(object sender, TextChangedEventArgs e) //Text Entered in Celsius
string C_in = c_textBox.Text;
double C = double.Parse(C_in);
double C_to_F = C * (9 / 5) + 32;
double C_to_K = C + 273.15;
f_textBox.Text = String.Format("{0}°F", C_to_F.ToString());
k_textBox.Text = String.Format("{0}°K", C_to_K.ToString());
Any ideas why this is happening?
I have another question: See how when I output the result I append the °F or °K. I would like to have the textBox setup to add °C,F,K or any other unit whenever the user enters input, but only read the numeric values, not the unit symbol. 
Thanks,

Answer 1

That's because you are trying to convert a blank string to a number, which is just as un-legit as trying to convert letters to a number. The simplest way to deal with it is to test for a blank string.
if(C_in == "")
// Do whatever you want for a blank string here.
else
// Get the number from the string here.
But a more thorough approach would be to use TryParse instead of Parse to check if the string cannot be converted for any reason. See the following link.
https://msdn.microsoft.com/en-us/library/994c0zb1%28v=vs.110%29.aspx