C# Language Tutorial => Collection initializers in custom classes

Example

To make a class support collection initializers, it must implement IEnumerable interface and have at least one Add method. Since C# 6, any collection implementing IEnumerable can be extended with custom Add methods using extension methods.

class Program
{
    static void Main()
    {
        var col = new MyCollection {
            "foo",
            { "bar", 3 },
            "baz",
            123.45d,
        };
    }
}

class MyCollection : IEnumerable
{
    private IList list = new ArrayList();

    public void Add(string item)
    {
        list.Add(item)
    }

    public void Add(string item, int count)
    {
        for(int i=0;i< count;i++) {
            list.Add(item);
        }
    }

    public IEnumerator GetEnumerator()
    {
        return list.GetEnumerator();
    }
}

static class MyCollectionExtensions
{
    public static void Add(this MyCollection @this, double value) => 
        @this.Add(value.ToString());
}

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C# Language Collection Initializers Collection initializers in custom classes

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