Possible Duplicate:
C#: String.Equals vs. ==
Are string.Equals() and == operator really same?
sometimes in a condition between two strings, I write:
if(string1==string2) //Do something
and sometimes I write:
if(string1.Equals(string2)) //Do something
The problem is sometimes the first one doesn't work, or miswork, is there any difference between the two expressions?
The first one will always work so long as the compile-time type of both operands is string.
If the compile-time type of either operand is anything other than string, it will use the normal reference identity comparison, rather than comparing strings for equality. Basically you want to call the ==(string, string) overload instead of the normal ==(object, object) overload.
Note that the first will succeed even if string1 is null, whereas the second will throw NullReferenceException in that case. An alternative in order to preserve the Equals call but avoiding this problem is to call the static object.Equals(object, object) method:
if (object.Equals(string1, string2))
Personally I'd just use == in cases where the compile-time types are appropriate though.
object.== could be called in scenarios other than the one you mention, e.g. if one of the variables is typed as IEnumerable<char>.The use of == on two string types will perform a reference identity check, meaning it will only return true if both references point to the same object. Equals on the other hand is expected to perform a value comparison, and will return true if the references point to objects that are equivalent.
==(string, string) then that will perform a value equality comparison, not a reference identity check. It's not clear what you mean by a "simple type" but I suspect you misunderstand something about the string type. It's entirely possible to have two equal but distinct instances of string, and if you use the ==(object, object) overload, that will return false."cat" == "cat" will return true even though they are not the same object. The real difference is == will call the static String method and myStr.Equals() will call the instance method (which is why it fails if the instance is null.
== on a string type". If x and y are string variables, then x == y will perform the same test as Equals.
string1is null, aNullReferenceExceptionwill occur in the second one.