Allow Array.IndexOf to be Called as an Instance Method in C#

[ShieldOfSalvation]

Summary
This proposal suggests allowing commonly used static methods on arrays—such as Array.IndexOf—to be callable as instance methods directly on array objects in C#, improving readability, intuitiveness, and consistency with other .NET collection APIs.

Motivation
C# has long required calling certain operations on arrays through the static System.Array class:

int index = Array.IndexOf(myArray, value);

However, in modern C#, this feels unintuitive compared to how similar operations are performed on List or other collection types:

int index = myList.IndexOf(value); // intuitive, object-oriented

Why not allow this?

int index = myArray.IndexOf(value); // not currently allowed

This limitation leads to:

• Inconsistent API surface across collections
• Confusion for new developers
• Unnecessary reliance on utility classes or static extensions
• Reduced readability and elegance

Proposal
Allow Array to support instance method syntax for selected commonly used static methods, starting with:

• IndexOf
• LastIndexOf
• Possibly others (Sort, Reverse, etc.)

Example usage:

int[] data = { 1, 2, 3, 4, 5 };
int index = data.IndexOf(3);

Design Considerations

• Implementation: Internally could map to the same static call (Array.IndexOf(this, value))
• Source Compatibility: Would not break existing code
• Performance: No measurable performance impact expected
• Precedent: LINQ methods were extended similarly with extension methods

Alternatives

• Continue using static calls
• Write custom extensions (ArrayExtensions.IndexOf(this T[] array, T value))
• Migrate to List where instance methods exist

Drawbacks

• Slight increase in language complexity
• Potential confusion about which methods are instance-bound vs. static

Conclusion
This small but impactful change would improve C# ergonomics and bring consistency to working with arrays—especially for developers familiar with instance-based APIs. It aligns with the object-oriented principles of the language and reflects how developers naturally expect to interact with array-like structures.

[HaloFour]

Those methods are defined as static on the System.Array type as provided by the runtime. You could suggest that they add instance versions.

[huoyaoyuan]

There's no way to add instance method based on element type to array. This isn't actionable currently.

[HaloFour]

Extension methods sound like the better solution here then.