.ToString(), (string), or as String. When to use what?

Question

I ran into a bug that was bothering me. I had JObject that I thought would be fine with

obj["role"].ToString()

The string was there and everything. A final resort was to change to a

(string)obj["role"] 

just to see what happens and it works. My question is how do I know when to use a .ToString() as opposed to a (string) as opposed to an "as String".

Answer 1

If the object is a string, or there is an explicit cast operator to (string), then it is okay to say

string s = (string)obj["role"];

Otherwise, this will give you an InvalidCastException.

Note that here you could say

string s = obj["role"] as String;

which will set s to null if obj["role"] is not an instance of string. Note that for as, explicit cast operators are ignored.

If obj["role"] is neither a string nor an instance of a class that has an explicit cast operator to string, you have to say

string s = obj["role"].ToString();

But be careful, the latter can throw a NullReferenceException.

Answer 2

(string) casts the instance to string and it might throw an exception at runtime if the object you are trying to cast is not a string. The as operator also tries to cast but it doesn't throw an exception if the instance is not a string but returns null. And the ToString method of course comes from System.Object so it will depend on whether tyhe underlying type has overridden it.