Saturday, 7 January 2023

List patterns in C# 11 - And getting a compiler error on code that should work

I have tested out list patterns in .NET 7 and C#. I am using Linqpad 7 and .NET 7.0.1. List patterns are useful I guess to compared sequences and fun to test out. The '_' discard here means to ignore the number at a given position and the '..' range here is to match anything between a given set of value and one index and then a given value at a higher index with arbitrary values between. But in one of the samples, it says you can capture variables inside list patterns. I cannot make it work, I get a compiler error. I am getting a CS0165 'Use of unassigned local variable' error when I try to access the variable(s) captured. I tried checking the crashing code also inside VsCode, still getting the error, however if I debug inside Linqpad I can see the variables that are captured got values at least.

    var someOddNumbers = new int[] { 1, 3, 5, 7, 9, 11 };
    bool resultX = someOddNumbers is [1, 3, _, _, _, 11];
    resultX.Dump("The 'someOddNumbers' equals a sequence of numbers 1,3,then three arbitrary numbers, then 11?");

    bool isOdd = someOddNumbers is [1, .., 9, 11];
    isOdd.Dump("The 'someOddNumbers' equals a sequence of numbers 1, some arbitrary numbers, then ending with 9 and 11?");
    
    result = input is [var firstOddNumber,.. var lastOddNumber];

    if (result)
    {
        Console.WriteLine($"The captured variables are: {firstOddNumber} and {lastOddNumber}"); //this lines gives the CS0165 error
    }

If I comment out the if block I can run the code sample, and in the debugger I can see firstOddNumber and lastOddNumber being set to a value at runtime. But the C# 11 compiler seems to think this is illeagal code since it is using an unintialized variable. I expected to not get a compiler error and be able to also capture the variables defined in the list pattern. I cannot understand the usage of such variables if I cannot use them. I understand that these variables might not be captured if the list pattern does not match, but even when checking if a match was present, I got the compilation error. I can however run the code, just not access the variables.

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