Develop Reference

C#, is this "if... else..." block equivalent to one line of code using null-coalescing operator

I'm curious if this block of code:
//value is an object, maybe null, maybe not
if (value == null)
item.PassageStimuliTitle = "";
else
item.PassageStimuliTitle = value.ToString().Trim();
is equivalent to this line:
item.PassageStimuliTitle = (string)(value ?? value.ToString().Trim());
I've used if... else... for a long time, but recently came across the null-coalescing operator in C#. I've used it to prevent null exception errors for params passed in method calls. I think using it in the example above is equivalent, and condenses the code from 4 lines to 1. Thanks for reading. Curious to hear feedback.
I'm expecting the two examples to be equivalent, but curious if there's something I'm missing.

No, the null-coalescing operator expression is not written correctly. a ?? b means "evaluate to b if a is null, otherwise evaluate to a". Therefore, your use of the null-coalescing operator will always produce a NullReferenceException if value is null - your code will try to evaluate value.ToString() when value is null.
Your use of ?? would translate to something like the following if statement, which I think you'd agree is quite non-sensical:
if (value == null) {
item.PassageStimuliTitle = (string)value.ToString().Trim();
} else {
item.PassageStimuliTitle = (string)value;
}
With certain assumptions, the if statement can be rewritten as:
item.PassageStimuliTitle = value?.ToString().Trim() ?? "";
This uses the null conditional operator ?.. If value is null, then the entire value?.ToString().Trim() expression is null, and hence the RHS of the ?? is evaluated. Otherwise, .ToString().Trim() is called on value.

item.PassageStimuliTitle = value != null ? value.ToString().Trim() : "";
From #Charles Mager comment:
item.PassageStimuliTitle = value?.ToString().Trim() ?? ""
Is a better and clearer one-liner.

You used ?? operator incorrectly.
In this example:
result = left_value ?? right_value;
?? operator returns left_value if it is NOT null.
If left_value is null, it returns right_value.
So, in your case, if variable value is of type object, it should be:
item.PassageStimuliTitle = (value ?? "").ToString().Trim();
Here is a successfully compiled code fiddle.

Is there a conditional operator for setting a variable in C# to the value of another variable if the latter isn't null?

Something like the ternary operator (?:) or the null coalescing operator (??). It seems silly to me to take up two lines and be so wordy when the other operators exist.
EDIT:
Since it's requested, here's two possible examples of what I hope that I can find
var variable ?= mightBeNull;
or
var variable = mightBeNull != null ? mightBeNull
Really, it's either something that can be used to assign a value, if it's not null, to a variable or an If without an Else packaged nicely in an operator.

The ??= operator is coming to C# 8.
int? i = null; // i = null
i ??= 0; // i = 0
i ??= 1; // i = 0
// different ways of writing 'i ??= 0;':
i = i ?? 0;
if (i == null) i = 0;

So you want this?
if (other != null)
someVariable = other;
You could do the following, but I'd argue that the above is better due to clarity and possible side effects:
someVariable = other ?? someVariable;
The above might cause side effects if someVariable is a property and either the get or set can cause side effects; this shouldn't be important if your property follows the ordinary guidelines. Or, as Servy points out, even if it's a field, it could created different semantics in a multithreaded app. I should note that in the first example, you read other twice, which also has the potential for complexity to enter (though a smaller potential than the latter example).

I bet this is what you are looking for.
Let's say we have a function, myFunction that says whether the argument passed input is null or not. If input is null, it will return "Input is null!", else it will return "Input is not null".
This is the normal approach:
public String myFunction(string input) {
if (input == null)
return "Input is null!";
else
return "Input is not null";
}
And this is the other approach (What you are looking for):
public String myFunction(string input) {
return (input == null) ? "Input is null!" : "Input is not null";
}
It is called (?: conditional operator).

To assign a value to a variable only if it is not null, you would need to use an if. Neither the conditinal nor the null coalesce operators would do that.
if(somethingElse != null) something = somethingElse;
There is no specific operator in C# that does exactly this.

In Visual Studio 2015 C# there is a new operator called the Null-Conditional that does what you are asking. It is the ?. or ? operator.
int? length = customers?.Length; // null if customers is null
Customer first = customers?[0]; // null if customers is null
int? count = customers?[0]?.Orders?.Count(); // null if customers, the first customer, or Orders is null
Currently useable in previous versions of C# but not an operator but rather a one liner would be to use the ? : statement
?: operator
condition ? first_expression : second_expression;
AssingningTO = (someVar == null) ? null: someVar; // if null assign null ELSE assign someVar
AssingningTO = (someVar == null) ? String.Empth(): someVar; // if null assign empty string else someVar

Nullable types can represent all the values of an underlying type, and an additional null value.
http://msdn.microsoft.com/en-us/library/2cf62fcy.aspx

Trim Textbox value by considering NULL

Can you tell me if TrimNull() is redundant and if I should be using an alternative?
For example:
string username = UsernameTextBox.Text.TrimNull();
I am told there is no definition or extension method. Perhaps there is a reference I am missing?
UPDATE:
What is the most readable way to return empty string if the value is NULL?

There's no such function as TrimNull(String) - it wouldn't do anything. A string is either a null or not null, it can't contain a mixture of both. If the string were null, a static function TrimNull(myString) would not be able to 'remove' anything from the string. If it weren't null, there would be no NULL to remove. Even worse, if TrimNull were an instance method myString.TrimNull() would simply cause an exception if myString were NULL - because you cannot invoke any method on a null reference.
If your goal is to trim whitespace characters surrounding the string, just use myString.Trim().
If your goal is to detect whether the string is null, use myString == NULL
If your goal is to detect whether the string is empty or null use String.IsNullOrEmpty(myString)
If your goal is to trim trailing null characters (\0) from data stream, try the following:
myString.TrimEnd(new char[] { '\0' } )
But as Frédéric Hamidi said, if you're referring to the latter, the user will have a hard time getting null characters into a TextBox, so you shouldn't worry about that scenario in your processing of their input.

I usually use String.IsNullOrWhiteSpace(), like this:
string username = (String.IsNullOrWhiteSpace(UsernameTextBox.Text) ?
null : UsernameTextBox.Text.Trim());
That way, if the .Text property is null, it doesn't cause an exception.

You could create your own extension-method for that, if you like:
public static class StringExtensions
{
public static string TrimNull(this string value)
{
return string.IsNullOrWhiteSpace(value) ? value : value.Trim();
}
}
Add this to your project and your code will work.
This is just an alternative.

Use null-coalescing operator as mentioned in #sixlettervariables answer in Negate the null-coalescing operator
string username = (UsernameTextBox.Text ?? String.Empty).Trim();

A string being NULL is not its value. its a state. It means it has not been assigned a memory (coz its a reference type). Had it been a value type datatype it woudld be assigned a default value automatically like for int its 0 and so on
u should use
if(!String.IsNullOrEmpty(UsernameTextBox.Text))
string username = UsernameTextBox.Text.Trim();

Negate the null-coalescing operator

I have a bunch of strings I need to use .Trim() on, but they can be null. It would be much more concise if I could do something like:
string endString = startString !?? startString.Trim();
Basically return the part on the right if the part on the left is NOT null, otherwise just return the null value. I just ended up using the ternary operator, but is there anyway to use the null-coalescing operator for this purpose?

You could create an extension method which returns null when it tries to trim the value.
public String TrimIfNotNull(this string item)
{
if(String.IsNullOrEmpty(item))
return item;
else
return item.Trim();
}
Note you can't name it Trim because extension methods can't override instance methods.

Not to spec: Not that I like it, but you could use:
string endString = (startString ?? String.Empty).Trim();
To spec, better as an Extension method like #Kevin's:
string endString = (startString == null ? null : startString.Trim());

string endString = string.IsNullOrEmpty(startString) ? startString : startString.Trim();
Though I've also gone the route of writing a string extension method called "safeTrim" which does what you're describing in one method instead of having to use this recipe every time. Check out Kevin's respone for the code.
EDIT: wow I had it all kinds of backwards, wrongly named variables and reversed ternary operators, all the more reason to write one extension method and code check it better than I did!

Starting with C# 6.0 (.NET Framework 4.6 / Visual Studio 2015) you can use null-conditional member access:
string? endString = startString?.Trim();

Sorry for the necromancy, but I was having this same problem and I solved this using a lambda operation. It isn't the prettiest, but it keeps my code succinct.
It's a shame C# doesn't support static imports or individual function imports, but anyway:
Define this function somewhere:
private static TResult N<TParent,TResult>(TParent parent, Func<TParent,TResult> operation) {
if( parent == null ) return default(TResult);
return operation( parent );
}
Then to use it in your example:
String endString = N(startString, s => s.Trim());
The N function returns null if the first argument is null, otherwise it will evaluate the specified lambda function with the value as the argument.
You can nest it, of course, like so. For example, to safely dereference a long chain, e.g.
String someValue = someObject.SomeProperty.SomeOtherProperty.SomeMethod().SomeFinalProperty;
if any of those properties or methods returns null then you have to insert null checks everywhere, or you could do this:
String someValue = N(N(N(N(someObject, o => o.SomeProperty), o => o.SomeOtherProperty), o => o.SomeMethod()), o => o.SomeFinalProperty);
As I said, it isn't the prettiest :)
You could simplify this by making N an extension method of System.Object, like so:
String someValue = someObject.N( o => o.SomeProperty ).N( o => o.SomeOtherProperty ).N( o => o.SomeMethod() ).N( o => o.SomeFinalProperty );
...which I think is a lot tidier.

Use
string endString = (startString ?? "").Trim();
This uses an empy string if startString is null. This, however, does not return null when endString is null.
Fast-forward to 2021:
10 years later startString?.Trim() is definitely the better option. And this does return null.

The following doesn't propagate null but it accepts null as a parameter and returns an empty string in that case.
using Microsoft.VisualBasic; // you need to add a reference to Microsoft.VisualBasic.dll
...
string endString = Strings.Trim(startString);
...
duck&run...

As as side note, if you're using .NET 4, there's a new convenient method String.IsNullOrWhiteSpace which you can use.

Shortest way to check for null and assign another value if not

I am pulling varchar values out of a DB and want to set the string I am assigning them to as "" if they are null. I'm currently doing it like this:
if (string.IsNullOrEmpty(planRec.approved_by) == true)
this.approved_by = "";
else
this.approved_by = planRec.approved_by.toString();
There seems like there should be a way to do this in a single line something like:
this.approved_by = "" || planRec.approved_by.toString();
However I can't find an optimal way to do this. Is there a better way or is what I have the best way to do it?

Try this:
this.approved_by = IsNullOrEmpty(planRec.approved_by) ? "" : planRec.approved_by.toString();
You can also use the null-coalescing operator as other have said - since no one has given an example that works with your code here is one:
this.approved_by = planRec.approved_by ?? planRec.approved_by.toString();
But this example only works since a possible value for this.approved_by is the same as one of the potential values that you wish to set it to. For all other cases you will need to use the conditional operator as I showed in my first example.

Starting with C# 8.0, you can use the ??= operator to replace the code of the form
if (variable is null)
{
variable = expression;
}
with the following code:
variable ??= expression;
More information is here

You are looking for the C# coalesce operator: ??. This operator takes a left and right argument. If the left hand side of the operator is null or a nullable with no value it will return the right argument. Otherwise it will return the left.
var x = somePossiblyNullValue ?? valueIfNull;

The coalesce operator (??) is what you want, I believe.

My guess is the best you can come up with is
this.approved_by = IsNullOrEmpty(planRec.approved_by) ? string.Empty
: planRec.approved_by.ToString();
Of course since you're hinting at the fact that approved_by is an object (which cannot equal ""), this would be rewritten as
this.approved_by = (planRec.approved_by ?? string.Empty).ToString();

With C#6 there is a slightly shorter way for the case where planRec.approved_by is not a string:
this.approved_by = planRec.approved_by?.ToString() ?? "";

Use the C# coalesce operator: ??
// if Value is not null, newValue = Value else if Value is null newValue is YournullValue
var newValue = Value ?? YourNullReplacement;

To extend #Dave's answer...if planRec.approved_by is already a string
this.approved_by = planRec.approved_by ?? "";

The accepted answer was correct in time, when it was given.
For people still finding this question:
Today you can use the ??= Operator.
e.g:
private string _test = null;
private void InitIfNull(){
_test ??= "Init";
}

To assign a non-empty variable without repeating the actual variable name (and without assigning anything if variable is null!), you can use a little helper method with a Action parameter:
public static void CallIfNonEmpty(string value, Action<string> action)
{
if (!string.IsNullOrEmpty(value))
action(value);
}
And then just use it:
CallIfNonEmpty(this.approved_by, (s) => planRec.approved_by = s);

You can also do it in your query, for instance in sql server, google ISNULL and CASE built-in functions.

I use extention method SelfChk
static class MyExt {
//Self Check
public static void SC(this string you,ref string me)
{
me = me ?? you;
}
}
Then use like
string a = null;
"A".SC(ref a);