What methods can be used to remove key restrictions in a dictionary data structure?

In C#, keys in a dictionary (Dictionary<TKey, TValue>) must be unique and immutable for correct hashing. To overcome key restrictions, the following approaches can be used:

1. Use a composite key — create a class or structure that combines multiple values, and override the Equals and GetHashCode methods for proper comparison.

2. Use a collection of values for a single key — for example, Dictionary<TKey, List<TValue>>, to store multiple values under one key.

3. Use Lookup or IGrouping — if you need to group data by key.

4. Use ConcurrentDictionary or other specialized collections — if thread safety is required.

5. Use ImmutableDictionary — if an immutable collection is needed.

Example of a composite key:

class CompositeKey
{
    public int Id { get; set; }
    public string Name { get; set; }

    public override bool Equals(object obj) => obj is CompositeKey other && Id == other.Id && Name == other.Name;
    public override int GetHashCode() => HashCode.Combine(Id, Name);
}

var dict = new Dictionary<CompositeKey, string>();
dict[new CompositeKey { Id = 1, Name = "A" }] = "Value1";