[EC87] Use collection indexer

[Djoums]

Category : Performance

Severity : Warning

Why is this an issue ?

Linq methods like First(), Last() and ElementAt() can be necessary on enumerable types that don't have an indexer. But for those that do, direct index access is a lot cheaper at runtime than their Linq counterpart and should be used instead.

When can it be ignored ?

This rule shouldn't be ignored.

Examples

public static void Test(int[] arr)
{
    int first = arr.First(); // Non-compliant, use arr[0]
    int last = arr.Last(); // Non-compliant, use arr[^1], or arr[arr.Length - 1] if C# < 8
    int third = arr.ElementAt(2); // Non-compliant, use arr[2]
}

Asm code analysis

Using this C# code :

public static int FirstNonCompliant(int[] arr) => arr.First();
public static int FirstCompliant(int[] arr) => arr[0];

We get the following compiled asm code, showing the clear difference between the 2.

FirstNonCompliant(Int32[])
    L0000: push ebp
    L0001: mov ebp, esp
    L0003: push eax
    L0004: lea edx, [ebp-4]
    L0007: call dword ptr [0x258beb50]
    L000d: cmp byte ptr [ebp-4], 0
    L0011: je short L0017
    L0013: mov esp, ebp
    L0015: pop ebp
    L0016: ret
    L0017: call dword ptr [0xc9677c8]
    L001d: int3

FirstCompliant(Int32[])
    L0000: cmp dword ptr [ecx+4], 0
    L0004: jbe short L000a
    L0006: mov eax, [ecx+8]
    L0009: ret
    L000a: call 0x73794810
    L000f: int3