jarhoads / dotnet-collections-example

Using the dotnet CLI tools to create an example of readonly collections

My Collections Example to Show a Subtle Bug

This is some code that I created to show that some collections can still be changed if they're marked as readonly, and to use the dotnet CLI tools to create a simple console based application with testing. I've been reaing about IReadOnlyList and IReadOnlyDictionary and it seems like they're usually used to keep this from happening.

##Restore - Build - Run the Console Program

• Move to the directory src\CollectionTest (I know this is a bad name for the code) in the command line
• dotnet restore to install dependencies
• dotnet build to build the source files
• dotnet run to run the executable created by Program.cs

##Restore - Build - Test to run the xUnit Tests

• Move to the directory test\CollectionTestTests (See why CollectionTest is a bad name) in the command line
• dotnet restore to install dependencies
• dotnet build to build the source files
• dotnet test to run the xUnit tests in CollectionExampleTests.cs

##The Subtle Bug When creating a readonly dictionary that contains a generic list and it's returned in a method, that list can be modified by a client that calls the method.

To illustrate I've created a readonly Dictionary<int, List<string>> that has the internal list returned. I've also created tests that show only one of them passing.

##Tests The tests are pretty short and probably do a better job of explaining it:

        [Fact]
        public void CanNotChangeTest()
        {
            CollectionExample ex = new CollectionExample();
            List<string> cantChange = ex.ClientCanNotChange(1);
            cantChange.Add("E");
            var expected = new List<string> {"A", "B", "C", "D"};
            var actual = ex.Dict[1];
            Assert.Equal(expected, actual);
        }

        [Fact]
        public void CanChangeTest()
        {
            CollectionExample ex = new CollectionExample();
            List<string> canChange = ex.ClientCanChange(1);
            canChange.Add("E");
            var expected = new List<string> {"A", "B", "C", "D"};
            var actual = ex.Dict[1];
            Assert.Equal(expected, actual);
        }