ToString on null string
Solution 1
because you cannot call instance method ToString()
on a null
reference.
And MessageBox.Show()
is probably implemented to ignore null and print out empty message box.
Solution 2
It is because MessageBox.Show() is implemented with pinvoke, it calls the native Windows MessageBox() function. Which doesn't mind getting a NULL for the lpText argument. The C# language has much stricter rules for pure .NET instance methods (like ToString), it always emits code to verify that the object isn't null. There's some background info on that in this blog post.
Solution 3
As this question ranks quite high on Google for a search for "c# toString null", I would like to add that the Convert.ToString(null)
method would return an empty a null string, which is ignored by the messagebox.
However, just to reaffirm the other answers, you can use string.Concat("string", null)
in this example.
Edit - modified answer in line with HeyJude's comment below. As pointed out, a method like Convert.ToString(null).Length
will throw an exception.
Solution 4
Behind the scenes concat is being called in your follow up question / update E.g
string snull = null;
string msg = "hello" + snull;
// is equivalent to the line below and concat handles the null string for you.
string msg = String.Concat("hello", snull);
// second example fails because of the toString on the null object
string msg = String.Concat("hello", snull.ToString());
//String.Format, String.Convert, String.Concat all handle null objects nicely.
Solution 5
You are trying to execute the ToString() method on a null. You need a valid object in order to execute a method.
Comments
-
MartW over 3 years
Why does the second one of these produce an exception while the first one doesn't?
string s = null; MessageBox.Show(s); MessageBox.Show(s.ToString());
Updated - the exception I can understand, the puzzling bit (to me) is why the first part doesn't show an exception. This isn't anything to do with the Messagebox, as illustrated below.
Eg :
string s = null, msg; msg = "Message is " + s; //no error msg = "Message is " + s.ToString(); //error
The first part appears to be implicitly converting a null to a blank string.