Why would you use String.Equals over ==? [duplicate]

Answer 1

Both methods do the same functionally - they compare values.
As is written on MSDN:

• About String.Equals method - Determines whether this instance and another specified String object have the same value. (http://msdn.microsoft.com/en-us/library/858x0yyx.aspx)
• About == - Although string is a reference type, the equality operators (== and !=) are defined to compare the values of string objects, not references. This makes testing for string equality more intuitive. (http://msdn.microsoft.com/en-en/library/362314fe.aspx)

But if one of your string instances is null, these methods are working differently:

string x = null;
string y = "qq";
if (x == y) // returns false
    MessageBox.Show("true");
else
    MessageBox.Show("false");

if (x.Equals(y)) // returns System.NullReferenceException: Object reference not set to an instance of an object. - because x is null !!!
    MessageBox.Show("true");
else
    MessageBox.Show("false");

Answer 2

I've just been banging my head against a wall trying to solve a bug because I read this page and concluded there was no meaningful difference when in practice there is so I'll post this link here in case anyone else finds they get different results out of == and equals.

Object == equality fails, but .Equals succeeds. Does this make sense?

string a = "x";
string b = new String(new []{'x'});

Console.WriteLine("x == x " + (a == b));//True
Console.WriteLine("object x == x " + ((object)a == (object)b));//False
Console.WriteLine("x equals x " + (a.Equals(b)));//True
Console.WriteLine("object x equals x " + (((object)a).Equals((object)b)));//True

Answer 3

I want to add that there is another difference. It is related to what Andrew posts.

It is also related to a VERY annoying to find bug in our software. See the following simplified example (I also omitted the null check).

public const int SPECIAL_NUMBER = 213;

public bool IsSpecialNumberEntered(string numberTextBoxTextValue)
{
    return numberTextBoxTextValue.Equals(SPECIAL_NUMBER)
}

This will compile and always return false. While the following will give a compile error:

public const int SPECIAL_NUMBER = 213;

public bool IsSpecialNumberEntered(string numberTextBoxTextValue)
{
    return (numberTextBoxTextValue == SPECIAL_NUMBER);
}

We have had to solve a similar problem where someone compared enums of different type using Equals. You are going to read over this MANY times before realising it is the cause of the bug. Especially if the definition of SPECIAL_NUMBER is not near the problem area.

This is why I am really against the use of Equals in situations where is it not necessary. You lose a little bit of type-safety.

Answer 4

It's entirely likely that a large portion of the developer base comes from a Java background where using == to compare strings is wrong and doesn't work.

In C# there's no (practical) difference (for strings) as long as they are typed as string.

If they are typed as object or T then see other answers here that talk about generic methods or operator overloading as there you definitely want to use the Equals method.

Answer 5

There is practical difference between string.Equals and ==

bool result = false;

object obj = "String";    
string str2 = "String";
string str3 = typeof(string).Name;
string str4 = "String";
object obj2 = str3;

// Comparision between object obj and string str2 -- Com 1
result = string.Equals(obj, str2);// true
result = String.ReferenceEquals(obj, str2); // true
result = (obj == str2);// true

// Comparision between object obj and string str3 -- Com 2
result = string.Equals(obj, str3);// true
result = String.ReferenceEquals(obj, str3); // false
result = (obj == str3);// false

// Comparision between object obj and string str4 -- Com 3
result = string.Equals(obj, str4);// true
result = String.ReferenceEquals(obj, str4); // true
result = (obj == str4);// true

// Comparision between string str2 and string str3 -- Com 4
result = string.Equals(str2, str3);// true
result = String.ReferenceEquals(str2, str3); // false
result = (str2 == str3);// true

// Comparision between string str2 and string str4 -- Com 5
result = string.Equals(str2, str4);// true
result = String.ReferenceEquals(str2, str4); // true
result = (str2 == str4);// true

// Comparision between string str3 and string str4 -- Com 6
result = string.Equals(str3, str4);// true
result = String.ReferenceEquals(str3, str4); // false
result = (str3 == str4);// true

// Comparision between object obj and object obj2 -- Com 7
result = String.Equals(obj, obj2);// true
result = String.ReferenceEquals(obj, obj2); // false
result = (obj == obj2);// false

Adding Watch

obj     "String" {1#}   object {string}
str2    "String" {1#}   string
str3    "String" {5#}   string
str4    "String" {1#}   string
obj2    "String" {5#}   object {string}

Now look at {1#} and {5#}

obj, str2, str4 and obj2 references are same.

obj and obj2 are object type and others are string type

Conclusion:

1. com1: result = (obj == str2);// true
   • compares object and string so performs a reference equality check
   • obj and str2 point to the same reference so the result is true
2. com2: result = (obj == str3);// false
   • compares object and string so performs a reference equality check
   • obj and str3 point to the different references so the result is false
3. com3: result = (obj == str4);// true
   • compares object and string so performs a reference equality check
   • obj and str4 point to the same reference so the result is true
4. com4: result = (str2 == str3);// true
   • compares string and string so performs a string value check
   • str2 and str3 are both "String" so the result is true
5. com5: result = (str2 == str4);// true
   • compares string and string so performs a string value check
   • str2 and str4 are both "String" so the result is true
6. com6: result = (str3 == str4);// true
   • compares string and string so performs a string value check
   • str3 and str4 are both "String" so the result is true
7. com7: result = (obj == obj2);// false  - compares object and object so performs a reference equality check      - obj and obj2 point to the different references so the result is false

Answer 6

String.Equals does offer overloads to handle casing and culture-aware comparison. If your code doesn't make use of these, the devs may just be used to Java, where (as Matthew says), you must use the .Equals method to do content comparisons.