In C#, delegates can be added together to chain their execution, and subtracted to remove their execution from the chain.


Subtracting a chain of delegates from another one might yield unexpected results as shown hereunder - and is likely to be a bug.


== Noncompliant Code Example

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MyDelegate first, second, third, fourth;
first = () => Console.Write("1");
second = () => Console.Write("2");
third = () => Console.Write("3");
fourth = () => Console.Write("4");

MyDelegate chain1234 = first + second + third + fourth; // Compliant - chain sequence = "1234"
MyDelegate chain12 = chain1234 - third - fourth; // Compliant - chain sequence = "12"


MyDelegate chain14 = first + fourth; // creates a new MyDelegate instance which is a list under the covers
MyDelegate chain23 = chain1234 - chain14; // Noncompliant; (first + fourth) doesn't exist in chain1234


// The chain sequence of "chain23" will be "1234" instead of "23"!
// Indeed, the sequence "1234" does not contain the subsequence "14", so nothing is subtracted
// (but note that "1234" contains both the "1" and "4" subsequences)
chain23 = chain1234 - (first + fourth); // Noncompliant

chain23(); // will print "1234"!
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== Compliant Solution

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MyDelegate chain23 = chain1234 - first - fourth; // Compliant - "1" is first removed, followed by "4"

chain23(); // will print "23"
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