/ boxing

When doing string concatenation in C#, why don't you have to call ToString on non string variables?

To get the string representation of a variable, in this case we’ll use int, there is no implicit conversion from int to string so you can call the ToString method:

string s = myInt; //INVALID!!
string s = myInt.ToString(); // Valid!

When concatenating with another string, though, you don’t have to do so:

string s = "First string " + myInt; //No ToString??!?


When performing string concatenation like above, the code is compiled to actually use the string.Concat(object, object) method. So our example above actually compiles to:

string s = string.Concat((object)"First string ", (object)myInt);

Notice the casts to object for both the string and int variables. This is also known as boxing:

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object. Boxing is implicit; unboxing is explicit. The concept of boxing and unboxing underlies the C# unified view of the type system, in which a value of any type can be treated as an object.

The boxing and unboxing processes, as with anything, technically incur a performance penalty, but in the grand scheme of things they are nothing to worry about.

When it comes to string concatentation technically string s = "First string " + myInt.ToString(); is faster because it will use the string.Concat(string, string) overload. But the performance difference is so negligible that you should use whatever you find most readable.

comments powered by Disqus
David Zych

David Zych

Dave is a dad, husband, programmer, (amateur) photographer, half-Canadian, alumnus of www.csuci.edu, and an overall nice guy. Software Engineer at www.thetradedesk.com.

Read More