## 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

    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

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

    chain23(); // will print "23"