chore: add plan for issue 116

Author: feO2x

Code/Plans/issue-116-must-not-contain-for-immutable-array.md

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+# Issue 116 - MustNotContain for ImmutableArray
+
+## Context
+
+The .NET library Light.GuardClauses already has several assertions for collections. They often rely on `IEnumerable<T>` or `IEnumerable`. However, these assertions would result in `ImmutableArray<T>` being boxed - we want to avoid that by providing dedicated assertions for this type which avoids boxing. For this issue, we implement the `MustNotContain` assertion for `ImmutableArray<T>`.
+
+## Tasks for this issue
+
+- [ ] The production code should be placed in the Light.GuardClauses project. The file `Check.MustNotContain.cs` already exists in the root folder of the project - extend this existing file.
+- [ ] In this file, create two extension method overloads called `MustNotContain` for `ImmutableArray<T>`. It should be placed in the class `Check` which is marked as `partial`.
+- [ ] Each assertion in Light.GuardClauses has two overloads - the first one takes the optional `parameterName` and `message` arguments and throw the default exception. The actual exception is thrown in the `Throw` class - use the existing `Throw.ExistingItem` method which is located in `ExceptionFactory/Throw.ExistingItem.cs`.
+- [ ] The other overload takes a delegate which allows the caller to provide their own custom exceptions. Use the existing `Throw.CustomException` method and pass the delegate, the erroneous `ImmutableArray<T>` instance and the unwanted item.
+- [ ] Create unit tests for both overloads. The corresponding tests should be placed in Light.GuardClauses.Tests project, in the existing file 'CollectionAssertions/MustNotContainTests.cs'. Please follow conventions of the existing tests (e.g. use FluentAssertions' `Should()` for assertions).
+
+## Notes
+
+- There are already plenty of other assertions and tests in this library. All overloads are placed in the same file in the production code project. The test projects has top-level folders for different groups of assertions, like `CollectionAssertions`, `StringAssertions`, `DateTimeAssertions` and so on. Please take a look at them to follow a similar structure and code style.
+- This assertion specifically targets `ImmutableArray<T>` to avoid boxing that would occur with generic `IEnumerable<T>` extensions.
+- The assertion should verify that the `ImmutableArray<T>` does not contain the specified item.
+- If you have any questions or suggestions, please ask me about them.