C# regarding Null Conditional Operator & Null Propagation

Question

C# regarding Null Conditional Operator & Null Propagation

T.Zacks 3,996

See the sample code first

class NullCheckOperator  
{  
    public static void Main()  
    {  
        person p = new person();  
        var name = p?.Name ?? "value cannot be null";  
        System.Console.WriteLine($"{name}");  
    }  
}  
class person  
{  
    public string Name { get; set; }  
}

what is the meaning of this line p?.Name ?? "value cannot be null"; i know the meaning of null coalescing. if p,Name is null then show that message "value cannot be null"

but p? meaning not clear. please help me to understand it. thanks

Answer accepted by question author

2 additional answers

Your answer

Answer 1

Let me translate the p?.Name ?? "value cannot be null" for you:

p?.Name translated as below:

if (p == null)  
{  
    return null;  
}  
else  
{  
    return p.Name;  
}

if the return result is null, the p?.Name ?? "value cannot be null" expression with return "value cannot be null".

So, sum up, the below code maybe help you for understanding:

if (p == null || (p != null && p.Name == null))  
{  
    return "value cannot be null";  
}  
else  
{  
    return p.Name;  
}

If right, please Accept.
Enjoy Programming!!!

Answer 2

?. is the null-conditional operator, so in this case if p is null then the overall result of p?.Name is null:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/member-access-operators#null-conditional-operators--and-

?. effectively short-circuits the property accessor expression to do an "else" condition in the event that p is null.

Once p?.Name has resolved to null the null-coalescing operator (??) will result in the fallback value of "value cannot be null" ending up as the value of name:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator

Answer 3

Karen Payne MVP 35,591 Volunteer Moderator

A side note, when I see var name = p?.Name ?? "value cannot be null"; my first thought is why are you not using data annotations or fluent validation?

public class Person  
{  
    [Required]  
    public string Name { get; set; }  
}

Some simple validating.

And for not going with typical validations there is another way to check for null

Person person = new();  
  
Console.WriteLine("person.FirstName is { } firstName");  
  
if (person.FirstName is { } firstName)  
{  
	Console.WriteLine($"\tFirst Name: {firstName}");  
}  
else  
{  
	Console.WriteLine("\tFirst name is null");  
}

T.Zacks 3,996 Reputation points

2022-07-26T10:36:40.17+00:00

Thanks for your reply what is the meaning of this code

person.FirstName is { } firstName ? { } is used to check null ? if yes then why we use firstName after this {} ? please guide me.

thanks

Karen Payne MVP 35,591 Volunteer Moderator

It's a pattern matching example

private static void Ambiguous1()  
{  
  
    Person person = new();  
  
    if (person.FirstName is { } )  
    {  
        Console.WriteLine($"\tFirst Name: {person.FirstName}");  
    }  
    else  
    {  
        Console.WriteLine("\tFirst name is null");  
    }  
      
    Console.WriteLine(!string.IsNullOrWhiteSpace(person.FirstName) ?   
        $"\tFirst Name: {person.FirstName}" : "\tFirst name is null");  
  
    Console.WriteLine(person.FirstName is { Length: > 0 } ?   
        $"\tFirst Name: {person.FirstName}" : "\tFirst name is null");  
  
  
    Console.WriteLine(person!.FirstName is not null ?   
        "\tFirst name is null" : $"\tFirst Name: {person.FirstName}");  
  
    person.FirstName = "Karen";  
    Console.WriteLine(person.FirstName is { Length: > 0 } ?  
        $"\tFirst Name: {person.FirstName}" :  
        "\tFirst name is null");  
  
    if (person.FirstName is { })  
    {  
        Console.WriteLine($"\tFirst Name: {person.FirstName}");  
    }  
    else  
    {  
        Console.WriteLine("\tFirst name is null");  
    }  
}

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C# regarding Null Conditional Operator & Null Propagation

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