S3247 — Duplicate casts should not be made

Why is this an issue?

In C#, the is type testing operator can be used to check if the run-time type of an object is compatible with a given type. If the object is not null, then the is operator performs a cast, and so performing another cast following the check result is redundant.

This can impact:

• Performance: Performing the type check and cast separately can lead to minor performance issues. While this might not be noticeable in small applications, it can add up in larger, more complex systems.
• Readability: The code is less readable and less clean because it requires two lines (and two operations) to achieve something that could be done in one.

How to fix it

Use pattern macthing to perform the check and retrieve the cast result.

Code examples

Noncompliant code example

if (x is Fruit)  // Noncompliant
{
  var f = (Fruit)x; // or x as Fruit
  // ...
}

Compliant solution

if (x is Fruit fruit)
{
  // ...
}

Resources

Documentation

• Microsoft Learn - Type-testing operators and cast expressions - is, as, typeof and casts
• Microsoft Learn - is operator (C# reference)
• Microsoft Learn - Pattern matching overview

Benchmarks

Glossary

• Mean
• Standard Deviation

The results were generated by running the following snippet with BenchmarkDotNet:

private Random random = new Random(1);

private object ReturnSometimes<T>() where T : new() =>
    random.Next(2) switch
    {
        0 => new T(),
        1 => new object(),
    };

[BenchmarkCategory("ValueType"), Benchmark(Baseline = true)]
public int IsPattern_ValueType()
{
    var i = ReturnSometimes<int>();
    return i is int d
        ? d
        : default;
}

[BenchmarkCategory("ValueType"), Benchmark]
public int IsWithCast_ValueType()
{
    var i = ReturnSometimes<int>();
    return i is int
        ? (int)i
        : default;
}

[BenchmarkCategory("Class"), Benchmark(Baseline = true)]
public DuplicateCasts IsPattern_Class()
{
    var i = ReturnSometimes<DuplicateCasts>();
    return i is DuplicateCasts d
        ? d
        : default;
}

[BenchmarkCategory("Class"), Benchmark]
public DuplicateCasts IsWithCast_Class()
{
    var i = ReturnSometimes<DuplicateCasts>();
    return i is DuplicateCasts
        ? (DuplicateCasts)i
        : default;
}

[BenchmarkCategory("Interface"), Benchmark(Baseline = true)]
public IReadOnlyList<int> IsPattern_Interface()
{
    var i = ReturnSometimes<List<int>>();
    return i is IReadOnlyList<int> d
        ? d
        : default;
}

[BenchmarkCategory("Interface"), Benchmark]
public IReadOnlyList<int> IsWithCast_Interface()
{
    var i = ReturnSometimes<List<int>>();
    return i is IReadOnlyList<int>
        ? (IReadOnlyList<int>)i
        : default;
}

Hardware configuration:

BenchmarkDotNet v0.14.0, Windows 10 (10.0.19045.5247/22H2/2022Update)
Intel Core Ultra 7 165H, 1 CPU, 22 logical and 16 physical cores
  [Host]               : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256
  .NET 9.0             : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  .NET Framework 4.8.1 : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256