Why is the result of adding two null strings not null? [duplicate]

Question

I was fiddling around in C# when I came across this weird behavior in .Net programming.

I have written this code:

  static void Main(string[] args)     {         string xyz = null;         xyz += xyz;         TestNullFunc(xyz);         Console.WriteLine(xyz);          Console.Read();      }      static void TestNullFunc(string abc)     {         if (abc == null)         {             Console.WriteLine("meow THERE ! ");         }         else         {             Console.WriteLine("No Meow ");         }     } 

I got the output as No meow, which means the string is not null. How is this possible? Why does adding two null strings, result in a non-null string?

On debugging when I check the value of xyz after adding it to itself, its value is "" (no characters).

Answer 1

From MSDN:

In string concatenation operations, the C# compiler treats a null string the same as an empty string,

Even though xyz is null, calling the += operator (which is converted to a call to the + operator (*)) on it does not throw a NullReferenceException because it's a static method. In pseudo code:

xyz = String.+(null, null); 

The implementation will then interpret this as if it was

xyz = String.+("", ""); 

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(*) Section §7.17.2 of the C# specification:

An operation of the form x op= y is processed by applying binary operator overload resolution (§7.3.4) as if the operation was written x op y.