CA1833: Use AsSpan or AsMemory instead of Range-based indexers for getting Span or Memory portion of an array

#Cause

When using a range-indexer on an array and implicitly assigning the value to System.Span or System.Memory.

#Rule description

The range indexer on a System.Span is a non-copying System.Span.Slice operation. But for the range indexer on an array, the method System.Runtime.CompilerServices.RuntimeHelpers.GetSubArray will be used instead of System.Span.Slice, which produces a copy of the requested portion of the array. This copy is usually unnecessary when it's implicitly used as a System.Span or System.Memory value. If a copy isn't intended, use the System.MemoryExtensions.AsSpan or System.MemoryExtensions.AsMemory method to avoid the unnecessary copy. If the copy is intended, either assign it to a local variable first or add an explicit cast. The analyzer only reports when an implicit cast is used on the result of the range indexer operation.

#Detects

Implicit conversions:

• Span<SomeT> slice = arr[a..b];
• Memory<SomeT> slice = arr[a..b];

#Does not detect

Explicit conversions:

• Span<SomeT> slice = (Span<SomeT>)arr[a..b];
• Memory<SomeT> slice = (Memory<SomeT>)arr[a..b];

#How to fix violations

To fix a violation of this rule, use the System.MemoryExtensions.AsSpan or System.MemoryExtensions.AsMemory extension method to avoid creating unnecessary data copies.

class C
{
    public void TestMethod(byte[] arr)
    {
        // The violation occurs for both statements below
        Span<byte> tmp2 = arr[0..5];
        Memory<byte> tmp4 = arr[5..10];
        ...
    }
}

class C
{
    public void TestMethod(byte[] arr)
    {
        // The violations fixed with AsSpan or AsMemory accordingly
        Span<byte> tmp2 = arr.AsSpan()[0..5];
        Memory<byte> tmp4 = arr.AsMemory()[5..10];
        ...
    }
}

You can also avoid this warning by adding an explicit cast.

class C
{
    public void TestMethod(byte[] arr)
    {
        // The violation occurs
        Span<byte> tmp1 = arr[0..5];
        Memory<byte> tmp2 = arr[5..10];
        ...
    }
}

class C
{
    public void TestMethod(byte[] arr)
    {
        // The violation fixed with explicit casting
        Span<byte> tmp1 = (Span<byte>)arr[0..5];
        Memory<byte> tmp2 = (Memory<byte>)arr[5..10];
        ...
    }
}

#When to suppress warnings

It's safe to suppress a violation of this rule if creating a copy is intended.

#Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA1833
// The code that's violating the rule is on this line.
#pragma warning restore CA1833

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA1833.severity = none

For more information, see How to suppress code analysis warnings.

#Related rules

• CA1831: Use AsSpan instead of Range-based indexers for string when appropriate
• CA1832: Use AsSpan or AsMemory instead of Range-based indexers for getting ReadOnlySpan or ReadOnlyMemory portion of an array

#See also

• Performance rules