benchmark result

Total time: 00:03:10 (190.47 sec)

// * Summary *

BenchmarkDotNet=v0.10.9, OS=Windows 10 Redstone 2 (10.0.15063)
Processor=Intel Core i5-4670 CPU 3.40GHz (Haswell), ProcessorCount=4
Frequency=3320320 Hz, Resolution=301.1758 ns, Timer=TSC
  [Host]     : .NET Framework 4.7 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.2101.1
  DefaultJob : .NET Framework 4.7 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.2101.1


                     Method |     Mean |     Error |    StdDev |
--------------------------- |---------:|----------:|----------:|
                     AsCast | 1.674 ns | 0.0337 ns | 0.0281 ns |
           AsCastFromObject | 1.870 ns | 0.0667 ns | 0.0794 ns |
                 DirectCast | 1.859 ns | 0.0671 ns | 0.1308 ns |
       DirectCastFromObject | 3.559 ns | 0.0997 ns | 0.1331 ns |
 DirectCastFromObjectWithIs | 3.424 ns | 0.0981 ns | 0.1050 ns |

// * Hints *
Outliers
  CastTest.AsCast: Default                     -> 2 outliers were removed
  CastTest.AsCastFromObject: Default           -> 1 outlier  was  removed
  CastTest.DirectCastFromObject: Default       -> 1 outlier  was  removed
  CastTest.DirectCastFromObjectWithIs: Default -> 1 outlier  was  removed

// * Legends *
  Mean   : Arithmetic mean of all measurements
  Error  : Half of 99.9% confidence interval
  StdDev : Standard deviation of all measurements
  1 ns   : 1 Nanosecond (0.000000001 sec)

// ***** BenchmarkRunner: End *****

code.cs

 public class A
    { }

    public class B : A
    { }

    public class CastTest
    {
        [Benchmark]
        public void AsCast()
        {
            var b = new B();
            var a = b as A;
        }

        [Benchmark]
        public void AsCastFromObject()
        {
            object b = new B();
            var a = b as A;
        }

        [Benchmark]
        public void DirectCast()
        {
            var b = new B();
            var a = (A)b;
        }

        [Benchmark]
        public void DirectCastFromObject()
        {
            object b = new B();
            var a = (A)b;
        }

        [Benchmark]
        public void DirectCastFromObjectWithIs()
        {
            object b = new B();
            if (b is A)
            {
                var a = (A)b;
            }
        }
    }