How does appending to a null string work in C#?

Question

I was surprised to see an example of a string being initialised to null and then having something appended to it in a production environment. It just smelt wrong.

I was sure it would have thrown a null object exception but this greatly reduced example also works:

string sample = null; sample += "test"; // sample equals "test" 

*Note the original code I found sets a string property to null and appends to it elsewhere so answers involving the compiler optimizing out the null at compile-time are irrelevant.

Can someone explain why this works without error?

Follow-up:

Based on Leppie's answer I used Reflector to see what is inside string.Concat. It is now really obvious why that conversion takes place (no magic at all):

public static string Concat(string str0, string str1) {     if (IsNullOrEmpty(str0))     {         if (IsNullOrEmpty(str1))         {             return Empty;         }         return str1;     }     if (IsNullOrEmpty(str1))     {         return str0;     }     int length = str0.Length;     string dest = FastAllocateString(length + str1.Length);     FillStringChecked(dest, 0, str0);     FillStringChecked(dest, length, str1);     return dest; } 

**Note: the specific implementation I was investigating (in the .Net library by Microsoft) does not convert to empty strings as is suggested by the C# standards and most of the answers, but uses a few tests to shortcut the process. The end result is the same as if it did but there you go :)

Answer 1

the + operator for strings are just shorthand for string.Concat which simply turns null arguments into empty strings before the concatenation.

Update:

The generalized version of string.Concat:

public static string Concat(params string[] values) {     int num = 0;     if (values == null)     {         throw new ArgumentNullException("values");     }     string[] array = new string[values.Length];     for (int i = 0; i < values.Length; i++)     {         string text = values[i];         array[i] = ((text == null) ? string.Empty : text);         num += array[i].Length;         if (num < 0)         {             throw new OutOfMemoryException();         }     }     return string.ConcatArray(array, num); } 

Answer 2

The relevant citation should be ECMA-334 §14.7.4:

String concatenation:

string operator +(string x, string y); string operator +(string x, object y); string operator +(object x, string y);   

The binary + operator performs string concatenation when one or both operands are of type string. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.