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

Answer 1

When you use the += operator, you are actually calling the string.Concat method, that, as stated in the documentation:

The method concatenates str0 and str1; it does not add any delimiters. An Empty string is used in place of any null argument.

In fact this code:

string xyz = null;
xyz += xyz;

will be compiled in:

IL_0000:  ldnull      
IL_0001:  stloc.0     // xyz
IL_0002:  ldloc.0     // xyz
IL_0003:  ldloc.0     // xyz
IL_0004:  call        System.String.Concat
IL_0009:  stloc.0     // xyz