C# object looks like dynamic type

Question

Have been playing around with the 4.0 DLR and was comparing dynamic to object and came across this:

Code:

object x = 10;
            Console.WriteLine("x = {0} and is a {1}.\n", x, x.GetType());
            x = (int)x + 3;
            Console.WriteLine("x = {0} and is a {1}.\n", x, x.GetType());
            x = x + "a";
            Console.WriteLine("x = {0} and is a {1}.\n", x, x.GetType());

Result:

x = 10 and is a System.Int32.

x = 13 and is a System.Int32.

x = 13a and is a System.String.

To me, it looks like object tries to fit the object to a type at runtime (dynamic). However if I don't cast x to an int on the 3rd line, it gives me a compiler area which seems correct for static typing. But then it allows me to add an "a" to x and now it recognizes it as a string.

What am I missing?

Answer 1

You are missing the fact that the + operator, when applied to strings, does an automatic conversion (by calling the .ToString() method on the operand that is not an instance of the String type).

Answer 2

From the C# Language Specification:

7.8.4 Addition operator

When one or both operands are of type string, the predefined addition operators concatenate the string representation of the operands.

• String concatenation:

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

  These overloads of the binary + operator perform string concatenation. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument 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.