Implicit string cast when attempting to add int to string

[DukeOfSuffix]

I was demonstrating static typing to a friend who is trying to learn C#. When I tried to add a string to an integer, it seems to implicitly cast the int to a string and perform a string concatenation:

Was this added to C# recently, or is it a bug?

[CyrusNajmabadi]

Was this added to C# recently,

Yes. Though 'recent' only relative to the age of the universe. This functionality was available in C#1.0

or is it a bug?

It is not a bug. (Practically) all types in .Net extend from System.Object and thus have the ability to convert themselves to strings with .ToString(). String concatenation is thus supported in C# for all of these types through the use of combining the actual strings, and the stringified values into one final string.

I was demonstrating static typing to a friend who is trying to learn C#

Note: C# is statically typed. And in this scenario static typing is in effect. Static typing does not mean that strings cannot be concatenated to non-string values.

[DukeOfSuffix]

Regarding your final point, I didn't mean to imply that C# wasn't statically typed. I just thought that the behavior may not be consistent with the spirit of static typing (i.e. the integration of two fundamentally different data types).

[CyrusNajmabadi]

(i.e. the integration of two fundamentally different data types).

Note: static typing has no problem with this :)

Static typing just means that the types are known and checked at compile time prior to running your program. Here the C# compiler knew the types here and checked that what you were doing was ok. It determined that it's 100% ok and it allowed this code and compiled it into something sensible :)

[mburbea]

The + operator in C# when applied to string as an operand is sugar for string.Concat. The compiler will call ToString on all non-strings in the invocation, and the appropriate overload of string.Concat will be called. See this sharplab:
https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA+ABATARgLABQGAzAATakDCpA3oaQ+WRjgAykCyAFAJS32NBGAOykARJADuYgNQ4sAbgEMAvoRVA