CA2009: Do not call ToImmutableCollection on an ImmutableCollection value

#Cause

ToImmutable method was unnecessarily called on an immutable collection from System.Collections.Immutable namespace.

#Rule description

System.Collections.Immutable namespace contains types that define immutable collections. This rule analyzes the following immutable collection types:

• System.Collections.Immutable.ImmutableArray
• System.Collections.Immutable.ImmutableList
• System.Collections.Immutable.ImmutableHashSet
• System.Collections.Immutable.ImmutableSortedSet
• System.Collections.Immutable.ImmutableDictionary
• System.Collections.Immutable.ImmutableSortedDictionary

These types define extension methods that create a new immutable collection from an existing System.Collections.Generic.IEnumerable collection.

• System.Collections.Immutable.ImmutableArray defines System.Collections.Immutable.ImmutableArray.ToImmutableArray.
• System.Collections.Immutable.ImmutableList defines System.Collections.Immutable.ImmutableList.ToImmutableList.
• System.Collections.Immutable.ImmutableHashSet defines System.Collections.Immutable.ImmutableHashSet.ToImmutableHashSet.
• System.Collections.Immutable.ImmutableSortedSet defines System.Collections.Immutable.ImmutableSortedSet.ToImmutableSortedSet.
• System.Collections.Immutable.ImmutableDictionary defines System.Collections.Immutable.ImmutableDictionary.ToImmutableDictionary.
• System.Collections.Immutable.ImmutableSortedDictionary defines System.Collections.Immutable.ImmutableSortedDictionary.ToImmutableSortedDictionary.

These extension methods are designed to convert a mutable collection to an immutable collection. However, the caller might accidentally pass in an immutable collection as input to these methods. This can represent a performance and/or a functional issue.

• Performance issue: Unnecessary boxing, unboxing, and/or runtime type checks on an immutable collection.
• Potential functional issue: Caller assumed to be operating on a mutable collection, when it actually had an immutable collection.

#How to fix violations

To fix violations, remove the redundant ToImmutable call on an immutable collection. For example, the following two code snippets show a violation of the rule and how to fix them:

using System;
using System.Collections.Generic;
using System.Collections.Immutable;

public class C
{
    public void M(IEnumerable<int> collection, ImmutableArray<int> immutableArray)
    {
        // This is fine.
        M2(collection.ToImmutableArray());

        // This leads to CA2009.
        M2(immutableArray.ToImmutableArray());
    }

    private void M2(ImmutableArray<int> immutableArray)
    {
        Console.WriteLine(immutableArray.Length);
    }
}

using System;
using System.Collections.Generic;
using System.Collections.Immutable;

public class C
{
    public void M(IEnumerable<int> collection, ImmutableArray<int> immutableArray)
    {
        // This is fine.
        M2(collection.ToImmutableArray());

        // This is now fine.
        M2(immutableArray);
    }

    private void M2(ImmutableArray<int> immutableArray)
    {
        Console.WriteLine(immutableArray.Length);
    }
}

#When to suppress warnings

Do not suppress violations from this rule, unless you're not concerned about the performance impact from unnecessary allocations of immutable collections.

#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 CA2009
// The code that's violating the rule is on this line.
#pragma warning restore CA2009

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

[*.{cs,vb}]
dotnet_diagnostic.CA2009.severity = none

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

#See also

• Reliability rules
• Performance rules