== Why is this an issue?

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

[source,csharp]
----
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"!
----


=== Compliant solution

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

chain23(); // will print "23"
----



ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Review this subtraction of a chain of delegates: it may not work as you expect.


'''
== Comments And Links
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=== on 30 Jun 2015, 16:12:38 Ann Campbell wrote:
\[~tamas.vajk] I've used a mashup of code samples. Please double-check my work

=== on 1 Jul 2015, 07:22:03 Tamas Vajk wrote:
\[~ann.campbell.2] Added a few more lines to the sample, but otherwise it looks good.


Note: delegate subtraction is not always a bug, so I'm not sure if we can apply the _bug_ label.

=== on 1 Jul 2015, 11:28:15 Ann Campbell wrote:
label updated [~tamas.vajk]

=== on 26 Jul 2016, 19:48:47 Ann Campbell wrote:
\[~tamas.vajk] I think we need to work on this one.

endif::env-github,rspecator-view[]
