Direct casting vs 'as' operator?

Question

Consider the following code:

void Handler(object o, EventArgs e) {    // I swear o is a string    string s = (string)o; // 1    //-OR-    string s = o as string; // 2    // -OR-    string s = o.ToString(); // 3 } 

What is the difference between the three types of casting (okay, the 3rd one is not a casting, but you get the intent). Which one should be preferred?

Answer 1

string s = (string)o; // 1 

Throws InvalidCastException if o is not a string. Otherwise, assigns o to s, even if o is null.

string s = o as string; // 2 

Assigns null to s if o is not a string or if o is null. For this reason, you cannot use it with value types (the operator could never return null in that case). Otherwise, assigns o to s.

string s = o.ToString(); // 3 

Causes a NullReferenceException if o is null. Assigns whatever o.ToString() returns to s, no matter what type o is.

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Use 1 for most conversions - it's simple and straightforward. I tend to almost never use 2 since if something is not the right type, I usually expect an exception to occur. I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes (e.g. return null = error, instead of using exceptions).

3 is not a cast and is just a method invocation. Use it for when you need the string representation of a non-string object.