How Do C# Arrays Seemingly Implement IList Without an Explicit Count Property?

How Arrays Partially Implement IList, Despite the Missing Count Property

In C#, arrays implement IList, yet they lack the Count property explicitly declared in the interface. This has prompted questions about whether this behavior is a violation of interface implementation rules or simply a misunderstanding.

Delving into the Framework

While arrays do not implement IList directly, they do implement the non-generic IList. Furthermore, the CLR generates specialized array types dynamically, which may theoretically implement the generic IList interface. However, a direct implementation does not exist.

CLR's Quacks-Like-a-Duck Approach

The CLR implements special techniques for arrays, similar to those used for value types. The compiler recognizes casts to IList and instructs the CLR to implement it via the underlying array object. The CLR then delegates execution to the hidden System.SZArrayHelper class, which implements the necessary interfaces.

Count Property Implementation

Despite not being explicitly declared, the Count property can be accessed. However, it is implemented as follows:

internal int get_Count<T>() {
    // Warning: "this" is an array, not an SZArrayHelper
    T[] _this = JitHelpers.UnsafeCast<T[]>(this);
    return _this.Length;
}

As the comment suggests, this behavior may be considered a rule violation, but it facilitates efficient array handling.

Conclusion

Arrays partially implement IList through the CLR's quacks-like-a-duck approach. While the Count property is not explicitly defined, it is implemented internally by the CLR, allowing for a seamless integration between arrays and generic collections.

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