Question

Recently I have been informed that it is possible to concatenate integers (and other types) to string and vice versa, i.e.

// x == "1234"
// y == "7890"
string x = "123" + 4;
string y = 7 + "890";

For some reason I didn’t think this kind of thing was allowed, so I have always been using (since .NET 2) the form:

// x == "1234"
// y == "7890"
string x = "123" + 4.ToString();
string y = 7.ToString() + "890";

where the integers are converted to strings. Has the former version always been available, and I’ve missed it, or is it something that is new to C# 4 (which is what I am using now)?

Answer 1

This has always been there. The + is equivalent to string.Concat() if at least one of the operands is a string. string.Concat() has an overload that takes an object instance. Internally it will call the object’s ToString() method before concatenating.

Found the relevant section in the C# spec – section 7.7.4 Addition operator:

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
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.