Like Richard, I tried your code, namely
#include "stdafx.h"
using namespace System;
int main(array<System::String ^> ^args)
{
String^ str1 = "OXF";
String^ str2 = "@" + str1;
Console::WriteLine(str2);
return 0;
}
compiles and runs fine on my PC.
Possibly there is something wrong in your compilation environment.