I will often use this code to compare a string:
if(!string.IsNullOrEmpty(str1) && str1.Equals(str2)){
//they are equal, do my thing
}
This handles the null case first etc.
Is there a cleaner way to do string comparison, perhaps with a single method call that will handle possible null values? I simply want to know that the strings are not equal if the testing value is null.
(I'm having dejavu that I may have asked this before, I apologize if so)
Update: In my case, the str2 is a known good string to compare to, so I don't need to check it for null. str1 is the "unknown" string which may be null, so I want to say "str1 does not equal str2" in the cases where str1 is null...
Unlike Java, C# strings override the == operator:
if (str1 == str2)
If you want a case-insensitive comparison:
if (string.Equals(str1, str2, StringComparison.OrdinalIgnoreCase))
If you do not want to treat two null strings as equal to each other, your code is optimal.
If, on the other hand, you want to treat null values as equal to each other, you can use
object.Equals(str1, str2)
for a more "symmetric" approach that also handles null values.
This is not the only approach to a symmetric check of string equality that treats null strings as equal: you could also use string.Equals(str1, str2) or even str1 == str2, which redirects to string.Equals.
I know this is some years old and I think the solution from dasblinkenlight is functionally perfect for what you asked for.
However I do prefer this code for readability reasons:
String.Equals(str1, str2)
There is no built in way to do this, but you could create an extension method to encapsulate this.
public static StringExtensions
{
public static Boolean IsNotNullAndEquals(this string str1, string str2)
{
return !string.IsNullOrEmpty(str1) && str1.Equals(str2)
}
}
then use it like this:
str1.IsNotNullAndEquals(str2);
Naming is going to be your hardest thing here IMO...since you need to convey that you are only null checking str1. When used as an extension method, it reads fairly well, but if used as a regular static, then it doesn't convey that as well.
You can use this code
object.Equals(str1, str2)
This will do it:
string.IsNullOrWhiteSpace(str1) ? string.IsNullOrWhiteSpace(str2) : str1.Equals(str2, StringComparison.OrdinalIgnoreCase);
I know this has been answer while ago and there's good answers, although as per Microsoft documentation (https://learn.microsoft.com/en-us/dotnet/api/system.string.equals?view=net-5.0) you can use the following to check both cases:
if(string.Equals(str1,str2)){
//they are equal with null handling
}
In more recent C# versions, we also have the null coalescing option now. This isn't an exact solution but can be a cleaner one for some purposes. It takes some liberties with "upgrading" str1 to an empty string for comparison purposes, in the instances where it actually is a null.
if((str1??"").Equals(str2))
{
//they are equal, do things here
}
Related
I have a localzation string which apparently if set to null is causing issues in the view. So my solution was to string it.
_localizer["Controller_Account_ResetPassword_ResetFailed"].ToString();
During code review my co-worker reminded me that .ToString() is not null safe. While its highly unlikely that a localization would be null I guess it could be. He suggested the following.
TempData["Message"] = $"{_localizer["Controller_Account_ResetPassword_ResetFailed"]}";
I have two questions.
Why is $"{var}" null safe and .ToString() is not.
Is there a better way of doing it because his solution seams ugly to me.
First of all, the reason is that it's basically decompiled as
var arg = _localizer["Controller_Account_ResetPassword_ResetFailed"];
string.Format("{0}", arg);
and string.Format does a null check on its arguments and automatically replaces them with string.Empty.
Second of all, this solution is beautiful and string interpolation is an ingenious tool. You should use it :)
EDIT:
I kinda overlooked the fact here string interpolation is used solely to fix the null problem. As some have pointed out that is unnecessary and as much as everyone should love string interpolation something like this:
_localizer["Controller_Account_ResetPassword_ResetFailed"]?.ToString() ?? "";
would be a better choice.
I like to use a ToStringSafe extension method for these kind of scenarios
public static class Extensions
{
public static string ToStringSafe(this object value)
{
// return empty string
if (value == null) {
return String.Empty;
}
return value.ToString();
}
}
You can use the ?? operator to substitute "" if needed:
var something = _localizer["Controller_Account_ResetPassword_ResetFailed"] ?? "";
I wouldn't use $"{value}" notation here. First of all it probably is unneeded, second it does extra work by calling string.Format() which internally creates a StringBuilder object.
It also is by no means clear that the only and actual reason for calling $"{value}" would be to deal with a potential null value, which is 100% clear when you use ??.
As it was stated above the string interpolation is null-safe as it's converted internally to the string.Format. However to answer the second question the information of how you're going to handle rare case with null localization is required. The solution suggested by your colleague is good (I'd only recommend to extract the variable instead of inlining for better readability), but you possibly may want to fall back to the non-localized version of the message instead of getting empty string in case of localization absence or handle the situation in any other way. In that case you'll have to do the explicit null verification possibly inside the _localizer type implementation so that it's always guaranteed to return non-null values or fail.
I am aware that I can set null like
string val = null;
But I am wondering the other ways I can set it to null. Is there a funcion like String.null that I can use.
I think you are looking far String.Empty
string val = String.Empty;
Update:
TheFuzzyGiggler comments are worth mentioning:
It's much better to set a string to empty rather than null. To avoid
exceptions later. If you have to read and write a lot of strings
you'll have null value exceptions all over the place. Or you'll have a
ton of if(!string.isNullorEmpty(string)) which get annoying
You can also use the default keyword.
string val = default(string);
Here is another SO question regarding the default keyword:
What is the use of `default` keyword in C#?
null is not a special string value (there's String.Empty for ""), but a general object literal for the empty reference.
Obviously one more way to set something to null is to assign result of function that returns null:
string SomeFunctionThatReturnsNullString() {return null;}
string val = SomeFunctionThatReturnsNullString();
assign it to this value will make it a null ,(chose the right sql datatype , here its double )
System.Data.SqlTypes.SqlDouble.Null
Depending on your application you could use string.Empty. I'm always leery of initializing to null as the garbage collector is great, but not perfect, and could mark your string for garbage collection if used in some way that the garbage collector can't detect (this is very unlikely though).
Is there any difference in following two lines of code that compares the string values.
string str = "abc";
if(str.Equals("abc"))
and
if("abc".Equals(str))
in the first line I am calling the equals method on string variable to compare it with string literal. The second line is vice versa. Is it just the difference of coding style or there is a difference in the way these two statements are processed by the compiler.
The only difference is that, in the first case, when you do:
str.Equals("abc")
If str is null, you'll get an exception at runtime. By doing:
"abc".Equals(str)
If str is null, you'll get false.
The difference is that in the second example, you will never get a NullReferenceException because a literal can't be null.
To add to the other answers: the static string.Equals("abc", str) method always avoids triggering a null reference exception, regardless of which order you pass the two strings.
As mmyers said, you the second example will not throw a NullReferenceException and while allowing the program to "appear" to run error free, may lead to unintended results.
Yes, the way the compiler processed the statements is different. The function equals for String in most languages follows the same guidlines. Here is a semicode:
override def Equals(that:String):Boolean //Should override Object.Equals
if(that==null) return false
for i from 0 to this.length
if(!this(i).Equals(that(i))) return false
return true
Normally, the method will fisrt check that that IS a String, and that this and that have the same length.
You can see, as others pointed out, that if that is null the method returns false. On the other hand, the method is part of of String so it cannot be called on null. That is why in your exampleif str is null you will get a NullReferenceException.
That being said, if you know both variables are non-null Strings of the same length, both statements will evaluate to the same in the same time.
In C#, can you use a boolean predicate on its own as the parameter for an if statement?
eg:
string str = "HELLO";
if (str.Equals("HELLO"))
{
Console.WriteLine("HELLO");
}
Will this code output "HELLO", or does it need to be:
string str = "HELLO";
if (str.Equals("HELLO") == true)
{
Console.WriteLine("HELLO");
}
If there is anything else wrong with the above code segments, please point it out.
EDIT: double equals as per answers
Well, the latter snippet won't even compile because it's trying to assign true to str.Equals("Hello") (you've got a single = instead of ==) but either:
if (str.Equals("HELLO"))
or
if (str.Equals("HELLO") == true)
will work. The latter is pretty strongly discouraged though - it's pretty ugly.
Bear in mind that string overloads ==, so you can actually write:
if (str == "HELLO")
which is somewhat simpler.
The second version won't even compile, since you need ==, not =.
Seeing code like if (foo == true) makes bile rise up into my mouth.
Yes you can.
(Your second example needs 2 = (i.e. ==) to be correct).
can you use a boolean predicate on its own as the parameter for an if statement
Strictly speaking, the answer to this is 'no', because a predicate is a function (in the mathematical sense). What you can use for the conditional expression in an if is any boolean expression, including (as here) the result of the invocation of a predicate on a value.
To expand, the 'predicate' here is 'equals HELLO'. It is a function (in the mathematical sense) that operates on values of type string, and returns boolean values. Once you have obtained a boolean value (by applying this function to a particular string, str), you do not need to explicitly compare it to true: you can just use it.
As you will see from others' answers, code in which expressions of boolean type are explicitly compared to boolean literals will often cause code-style pain in the reader :) (My 'favourite' peeve is <boolean expression> ? true : false).
The code you have is correct, the if statement checks to see if the statement within the brackets evaluates to true.
Remember that comparisons in c# use == not = , so it should be if (x == true)
This can be used in other situations like:
bool isACat = true;
if (isACat)
{
}
It is good practice to put is or a similar identifier (Is/Has/Should/Will/Do/Must...) before boolean variable names, as it makes it easy to read when you are not using == true.
There's nothing wrong.
Just two different ways of picking up the same spoon to feed yourself ;)
This is a Java and C# question.
We all know that, Object Identity(==) tests whether two objects refer to the same location and Obejct Equality(Equals method) tests whether two different (non identical)objects have the same value .But In case of string object Object Identity and Object Equality are same.
For e.g Below two boolean expressions in if statements return true
string a="123";
string b="123";
if(a==b)
if(a.Equals(b))
Why is it so??
What is the rational behind this design decision?
Java and C# both use a memory-saving technique called string interning. Because strings are immutable in these languages, they can pool frequently-used strings (included hard-coded string literals, like in your example) and use multiple references to that one string in memory to save space.
As far as I know, in .net the == Operator for Strings is overloaded to use Equals() instead of object identity. See this explanation for details: http://www.dotnetperls.com/string-equals
For if you need to know if it's really the same object, use this:
Object.ReferenceEquals(string1, string2)
Actually, at least in Java, there is a caching mechanism on strings. A pitfall is that two strings that are equal will sometimes, but not always return true when applying the identity operator. the following code prints false:
String a="123";
String b="12";
b=b+"3";
System.out.println(a==b);
If you really want to make sure, that a.equals(b) == true but (a==b) == false evaluates to false for two String a and b, then you can use the completely undervalued (^^) String constructor:
String a = new String("abc");
String b = new String("abc");
if (a.equals(b)) {
doTheyAreEqual();
if (a != b) {
doButNotTheSame();
}
}