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Discussion on: Pattern Matching Examples in C#

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uchitesting profile image
UchiTesting • • Edited

Hello.

Thanks for this article. I once found a bit of that syntax with switch and the when keyword. I was badly wishing there is such thing because this is handy but I didn't know it is so complete. I'm going through the codes and playing them in Visual Studio.

I have questions I note as I go so there may be one or more.

Variable Patterns :
There is apparently a small error in the article because the output gives "Hi" alone. Which makes sense to me when I read the code.

_ when greetWithName == false => $"Hi", applies.

Format a string
Modified the code so that phone is null.
Does not seem to work as expected and goes to the 1st pattern.
Any Idea why?

Regards.

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timdeschryver profile image
Tim Deschryver •

Thanks! I think you found some syntax mistakes in the snippets, and again thanks for pointing them out.

Variable pattern should be:

var greetWithName = true;
var output = "Mrs. Kim" switch
{
    _ when greetWithName == false => $"Hi",
    "Tim" => "Hi Tim!",
    var str when str.StartsWith("Mrs.") || str.StartsWith("Mr.") => $"Greetings {str}",
    var str => $"Hello ${str}",
};
// output: Greetings Mrs. Kim
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Format a string should use and instead of or:

var output = contactInfo switch
{
    { TelephoneNumber: not null } and { TelephoneNumber: not "" } => $"{contactInfo.FirstName} {contactInfo.LastName} ({contactInfo.TelephoneNumber})",
    _ => $"{contactInfo.FirstName} {contactInfo.LastName}"
};
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uchitesting profile image
UchiTesting •

Thanks for the fixes.
Tried them and they work.
I must admit that when reading the code this or looked legit.
But when you go back to boolean it makes sense.

TelephoneNumber is null
TelephoneNumber not null → False → 0
TelephoneNumber not "" → True → 1

0 or 1 is 1 so it executes the line.
0 and 1 is 0 so it filters accordingly.

Regards.