我有一个问题,需要根据对象类型进行动态调度。我需要根据编译时已知的类型进行调度,在我的示例中它们是17。
我的初步想法是使用Dictionary<Type, Action<Object>>
进行调度,并使用obj.GetType()
查找适当的操作。但后来我决定使用BenchmarkDotNet来看看是否可以做得更好,以及调度查找的开销有多大。下面是我用于基准测试的代码。
public class Program
{
private static readonly Object Value = Guid.NewGuid();
private static readonly Dictionary<Type, Action<Object>> Dictionary = new Dictionary<Type, Action<Object>>()
{
[ typeof( Byte ) ] = x => Empty( (Byte)x ),
[ typeof( Byte[] ) ] = x => Empty( (Byte[])x ),
[ typeof( SByte ) ] = x => Empty( (SByte)x ),
[ typeof( Int16 ) ] = x => Empty( (Int16)x ),
[ typeof( UInt16 ) ] = x => Empty( (UInt16)x ),
[ typeof( Int32 ) ] = x => Empty( (Int32)x ),
[ typeof( UInt32 ) ] = x => Empty( (UInt32)x ),
[ typeof( Int64 ) ] = x => Empty( (Int64)x ),
[ typeof( UInt64 ) ] = x => Empty( (UInt64)x ),
[ typeof( Decimal ) ] = x => Empty( (Decimal)x ),
[ typeof( Single ) ] = x => Empty( (Single)x ),
[ typeof( Double ) ] = x => Empty( (Double)x ),
[ typeof( String ) ] = x => Empty( (String)x ),
[ typeof( DateTime ) ] = x => Empty( (DateTime)x ),
[ typeof( TimeSpan ) ] = x => Empty( (TimeSpan)x ),
[ typeof( Guid ) ] = x => Empty( (Guid)x ),
[ typeof( Char ) ] = x => Empty( (Char)x ),
};
[Benchmark]
public void Switch() => Switch( Value );
[Benchmark]
public void Lookup() => Lookup( Value );
private static void Switch( Object value )
{
if ( value is Byte ) goto L_Byte;
if ( value is SByte ) goto L_SByte;
if ( value is Int16 ) goto L_Int16;
if ( value is UInt16 ) goto L_UInt16;
if ( value is Int32 ) goto L_Int32;
if ( value is UInt32 ) goto L_UInt32;
if ( value is Int64 ) goto L_Int64;
if ( value is UInt64 ) goto L_UInt64;
if ( value is Decimal ) goto L_Decimal;
if ( value is Single ) goto L_Single;
if ( value is Double ) goto L_Double;
if ( value is DateTime ) goto L_DateTime;
if ( value is TimeSpan ) goto L_TimeSpan;
if ( value is DateTimeOffset ) goto L_DateTimeOffset;
if ( value is String ) goto L_String;
if ( value is Byte[] ) goto L_ByteArray;
if ( value is Char ) goto L_Char;
if ( value is Guid ) goto L_Guid;
return;
L_Byte: Empty( (Byte)value ); return;
L_SByte: Empty( (SByte)value ); return;
L_Int16: Empty( (Int16)value ); return;
L_UInt16: Empty( (UInt16)value ); return;
L_Int32: Empty( (Int32)value ); return;
L_UInt32: Empty( (UInt32)value ); return;
L_Int64: Empty( (Int64)value ); return;
L_UInt64: Empty( (UInt64)value ); return;
L_Decimal: Empty( (Decimal)value ); return;
L_Single: Empty( (Single)value ); return;
L_Double: Empty( (Double)value ); return;
L_DateTime: Empty( (DateTime)value ); return;
L_DateTimeOffset: Empty( (DateTimeOffset)value ); return;
L_TimeSpan: Empty( (TimeSpan)value ); return;
L_String: Empty( (String)value ); return;
L_ByteArray: Empty( (Byte[])value ); return;
L_Char: Empty( (Char)value ); return;
L_Guid: Empty( (Guid)value ); return;
}
private static void Lookup( Object value )
{
if ( Dictionary.TryGetValue( value.GetType(), out var action ) )
{
action( value );
}
}
[MethodImpl( MethodImplOptions.NoInlining )]
private static void Empty<T>( T value ) { }
static void Main( string[] args )
{
BenchmarkRunner.Run( typeof( Program ) );
Console.ReadLine();
}
}
在我的示例中,我使用了一个封装的Guid来运行测试,这是手工制作开关函数中最糟糕的情况。结果令人惊讶,可以这么说:
BenchmarkDotNet=v0.10.11, OS=Windows 10 Redstone 3 [1709, Fall Creators Update] (10.0.16299.125)
Processor=Intel Core i7-4790K CPU 4.00GHz (Haswell), ProcessorCount=8
Frequency=3903988 Hz, Resolution=256.1483 ns, Timer=TSC
[Host] : .NET Framework 4.7 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.2600.0
DefaultJob : .NET Framework 4.7 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.2600.0
Method | Mean | Error | StdDev |
------- |---------:|----------:|----------:|
Switch | 13.21 ns | 0.1057 ns | 0.0989 ns |
Lookup | 28.22 ns | 0.1082 ns | 0.1012 ns |
这个开关函数在最坏情况下运行速度快了两倍。如果我重新排列if语句,使最常见的类型排在前面,那么平均来说,我希望它能跑得更快3-5倍。
我的问题是为什么18次检查比单个字典查找要快这么多?我有没有漏掉一些明显的东西?
编辑:
原来的测试是在x64机器上的x86(优先32位)模式下进行的。我也在64版中进行了测试:
Method | Mean | Error | StdDev |
---------- |----------:|----------:|----------:|
Switch | 12.451 ns | 0.0600 ns | 0.0561 ns |
Lookup | 22.552 ns | 0.1108 ns | 0.1037 ns |