CSL1011.md

CSL1011: Implement params collection.

(This feature is not available in .NET Framework or .NET Standard 2.0 and below.)

In C# 13 one can use a collection type instead of an array for open parameters in a method.

// Before
public void Foo<T>(params T[] items)
{
}

// After
using System;

public void Foo<T>(params ReadOnlySpan<T> items)
{
}

The benefits of this change is a better handling of null for the parameter, and performance.

By replacing params params T[] with params ReadOnlySpan<T> one ensures null is not a concern anymore, since ReadOnlySpan<T> is a ref struct, and the burden of the check is moved to the caller.

If the method must not be changed because it's a public API, create an override using params ReadOnlySpan<T>, and unify the two methods. The compiler will choose the best override seamlessly.

Diagnostic CSL1011 is emitted under the following conditions:

• The params parameter is an array.
• The method does not already have an override with params ReadOnlySpan<T>.
• The method is not async (until the language supports it).
• This parameter is used in the method.

Sample code

public static int Foo<T>(params T[] items) // CSL1011: Implement params collection.
{
    return items.Length;
}