起因

今天在更新.Net Runtime(2021-05-31)源码的时候,在is not null的用法,我们先看看如何使用.

is not null

由于个人习惯,在项目中不怎么使用is进行判断,需要类型转换的话,通常会使用as,具体原因是在 写更好的CSharp代码 ,有提到.这里就不多说了,看看新语法 is no null如何使用.

using System;

namespace CharpLearning.Syntax
{
    /// <summary>
    /// is not null 语法糖
    /// 在C# 9 加入
    /// </summary>
    public class LearnIsNotNull
    {
        public void IsNotNull(object obj)
        {
            //之前
            if (obj != null)
            {
                Console.WriteLine("obj != null");
            }

            //现在可以用,is not null只是语法糖,在编译时,会替换成"!=null"
            if (obj is not null)
            {
                Console.WriteLine("obj is not null");
            }

            //obj is not null 反编译 
            /*
             if (obj != null)
	     			 {
								  	Console.WriteLine("obj is not null");
	     				}
             */
        }
    }
}

is no null语法,只是语法糖,在编译时,会替换为!=null,以后看到语法不会茫然了.

is和模式匹配组合

namespace CharpLearning.Syntax
{
	/// <summary>
	/// 增强的模式匹配 和is一起使用
	/// c# 9 加入,其实也是语法糖
	/// </summary>
    public class LearnPatternMatch
	{
		public bool IsAlpha(char c)
		{
			return c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';
            //JustDecompile反编译结果:
            /*						
							 bool flag;
                bool flag1;
                if (c >= 'a')
                {
                    if (c <= 'z')
                    {
                        flag = true;
                        flag1 = flag;
                        return flag1;
                    }
                }
                else if (c >= 'A')
                {
                    if (c <= 'Z')
                    {
                        flag = true;
                        flag1 = flag;
                        return flag1;
                    }
                }
                flag = false;
                flag1 = flag;
                return flag1;
							*/
        }
    }
}

又是一个新的语法糖,这个使用就有点香,还是可以提高生产力,不过使用反编译工具ILSpy看的结果不是上面反编译的代码段,而是这样的.

/ CharpLearning.Syntax.LearnPatternMatch
public class LearnPatternMatch
{
	public bool IsAlpha(char c)
	{
		switch (c)
		{
		case 'A':
		case 'B':
		case 'C':
		case 'D':
		case 'E':
		case 'F':
		case 'G':
		case 'H':
		case 'I':
		case 'J':
		case 'K':
		case 'L':
		case 'M':
		case 'N':
		case 'O':
		case 'P':
		case 'Q':
		case 'R':
		case 'S':
		case 'T':
		case 'U':
		case 'V':
		case 'W':
		case 'X':
		case 'Y':
		case 'Z':
		case 'a':
		case 'b':
		case 'c':
		case 'd':
		case 'e':
		case 'f':
		case 'g':
		case 'h':
		case 'i':
		case 'j':
		case 'k':
		case 'l':
		case 'm':
		case 'n':
		case 'o':
		case 'p':
		case 'q':
		case 'r':
		case 's':
		case 't':
		case 'u':
		case 'v':
		case 'w':
		case 'x':
		case 'y':
		case 'z':
			return true;
		default:
			return false;
		}
	}
}

这个从IL代码推测应该是JustDecomplie还原的更准确.我们看一下IL代码:

.class public auto ansi beforefieldinit CharpLearning.Syntax.LearnPatternMatch
    extends [System.Runtime]System.Object
{
    .method public hidebysig specialname rtspecialname instance void .ctor () cil managed 
    {
        IL_0000: ldarg.0
        IL_0001: call instance void [System.Runtime]System.Object::.ctor()
        IL_0006: nop
        IL_0007: ret
    }

    .method public hidebysig instance bool IsAlpha (
            char c
        ) cil managed 
    {
        .locals init (
            [0] bool V_0,
            [1] bool V_1
        )

        //判断>='a' a=97
        IL_0000: nop
        IL_0001: ldarg.1
        IL_0002: ldc.i4.s 97
        IL_0004: blt.s IL_000d

        //判断<='z' 122
        IL_0006: ldarg.1
        IL_0007: ldc.i4.s 122
        IL_0009: ble.s IL_0019

        IL_000b: br.s IL_001d

        //判断>='A' A=65
        IL_000d: ldarg.1
        IL_000e: ldc.i4.s 65
        IL_0010: blt.s IL_001d

        //判断<='Z' Z=90
        IL_0012: ldarg.1
        IL_0013: ldc.i4.s 90
        IL_0015: ble.s IL_0019

        IL_0017: br.s IL_001d

        IL_0019: ldc.i4.1
        IL_001a: stloc.0
        IL_001b: br.s IL_001f

        IL_001d: ldc.i4.0
        IL_001e: stloc.0

        IL_001f: ldloc.0
        IL_0020: stloc.1
        IL_0021: br.s IL_0023

        IL_0023: ldloc.1
        IL_0024: ret
    }
}

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