《CLR Via C# 第3版》笔记之（六） IL中的call和callvirt

C#中调用一个函数时生成的IL代码有两种形式，分别为call 和 callvirt。

主要内容

• call和callvirt的区别
• call和callvirt的例子

1. call和callvirt的区别

call的callvirt的区别主要有两点：

1）call可以调用静态方法，实例方法和虚方法

     callvirt只能调用实例方法和虚方法，不能调用静态方法

2）call一般是以非虚的方式来调用函数的

     callvirt是以已多态的方式来调用函数的

2. call和callvirt的例子

示例代码如下：

using System;

namespace Test6
{
    public class CLRviaCSharp_6
    {
        static void Main(string[] args)
        {
            BaseClass.SShow();

            BaseClass b = new BaseClass();
            BaseClass s = new SubClass();
            b.VShow();
            s.VShow();
            Console.ReadKey(true);
        }
    }

    public class BaseClass
    {
        public static void SShow()
        {
            Console.WriteLine("Base class static method: SShow()!");
        }

        public virtual void VShow()
        {
            Console.WriteLine("Base class virtual method: VShow()!");
        }
    }

    public class SubClass : BaseClass
    {
        public override void VShow()
        {
            Console.WriteLine("Sub class virtual method: VShow()!");
        }        
    }
}

程序执行结果为：

利用ildasm.exe将上面代码生成的exe文件反编译为IL代码。

命令：ildasm .\Test6.exe /output:Test6.il

IL代码如下：

//  Microsoft (R) .NET Framework IL Disassembler.  Version 3.5.30729.1
//  Copyright (c) Microsoft Corporation.  All rights reserved.



// Metadata version: v4.0.30319
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 4:0:0:0
}
.assembly Test6
{
  .custom instance void [mscorlib]System.Reflection.AssemblyTitleAttribute::.ctor(string) = ( 01 00 0C 63 6E 62 6C 6F 67 5F 62 6F 77 65 6E 00   // ...cnblog_bowen.
                                                                                              00 ) 
  .custom instance void [mscorlib]System.Reflection.AssemblyDescriptionAttribute::.ctor(string) = ( 01 00 00 00 00 ) 
  .custom instance void [mscorlib]System.Reflection.AssemblyConfigurationAttribute::.ctor(string) = ( 01 00 00 00 00 ) 
  .custom instance void [mscorlib]System.Reflection.AssemblyCompanyAttribute::.ctor(string) = ( 01 00 18 4C 65 6E 6F 76 6F 20 28 42 65 69 6A 69   // ...Lenovo (Beiji
                                                                                                6E 67 29 20 4C 69 6D 69 74 65 64 00 00 )          // ng) Limited..
  .custom instance void [mscorlib]System.Reflection.AssemblyProductAttribute::.ctor(string) = ( 01 00 0C 63 6E 62 6C 6F 67 5F 62 6F 77 65 6E 00   // ...cnblog_bowen.
                                                                                                00 ) 
  .custom instance void [mscorlib]System.Reflection.AssemblyCopyrightAttribute::.ctor(string) = ( 01 00 2A 43 6F 70 79 72 69 67 68 74 20 C2 A9 20   // ..*Copyright .. 
                                                                                                  4C 65 6E 6F 76 6F 20 28 42 65 69 6A 69 6E 67 29   // Lenovo (Beijing)
                                                                                                  20 4C 69 6D 69 74 65 64 20 32 30 31 30 00 00 )    //  Limited 2010..
  .custom instance void [mscorlib]System.Reflection.AssemblyTrademarkAttribute::.ctor(string) = ( 01 00 00 00 00 ) 
  .custom instance void [mscorlib]System.Runtime.InteropServices.ComVisibleAttribute::.ctor(bool) = ( 01 00 00 00 00 ) 
  .custom instance void [mscorlib]System.Runtime.InteropServices.GuidAttribute::.ctor(string) = ( 01 00 24 61 64 61 62 34 66 63 33 2D 33 33 64 35   // ..$adab4fc3-33d5
                                                                                                  2D 34 62 64 30 2D 39 61 32 61 2D 39 35 35 61 65   // -4bd0-9a2a-955ae
                                                                                                  66 38 35 33 31 62 37 00 00 )                      // f8531b7..
  .custom instance void [mscorlib]System.Reflection.AssemblyFileVersionAttribute::.ctor(string) = ( 01 00 07 31 2E 30 2E 30 2E 30 00 00 )             // ...1.0.0.0..
  .custom instance void [mscorlib]System.Runtime.Versioning.TargetFrameworkAttribute::.ctor(string) = ( 01 00 29 2E 4E 45 54 46 72 61 6D 65 77 6F 72 6B   // ..).NETFramework
                                                                                                        2C 56 65 72 73 69 6F 6E 3D 76 34 2E 30 2C 50 72   // ,Version=v4.0,Pr
                                                                                                        6F 66 69 6C 65 3D 43 6C 69 65 6E 74 01 00 54 0E   // ofile=Client..T.
                                                                                                        14 46 72 61 6D 65 77 6F 72 6B 44 69 73 70 6C 61   // .FrameworkDispla
                                                                                                        79 4E 61 6D 65 1F 2E 4E 45 54 20 46 72 61 6D 65   // yName..NET Frame
                                                                                                        77 6F 72 6B 20 34 20 43 6C 69 65 6E 74 20 50 72   // work 4 Client Pr
                                                                                                        6F 66 69 6C 65 )                                  // ofile

  // --- The following custom attribute is added automatically, do not uncomment -------
  //  .custom instance void [mscorlib]System.Diagnostics.DebuggableAttribute::.ctor(valuetype [mscorlib]System.Diagnostics.DebuggableAttribute/DebuggingModes) = ( 01 00 07 01 00 00 00 00 ) 

  .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
  .custom instance void [mscorlib]System.Runtime.CompilerServices.RuntimeCompatibilityAttribute::.ctor() = ( 01 00 01 00 54 02 16 57 72 61 70 4E 6F 6E 45 78   // ....T..WrapNonEx
                                                                                                             63 65 70 74 69 6F 6E 54 68 72 6F 77 73 01 )       // ceptionThrows.
  .hash algorithm 0x00008004
  .ver 1:0:0:0
}
.module Test6.exe
// MVID: {7020C5F0-2D89-4214-AD2C-E6BFBEC57CA1}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       // WINDOWS_CUI
.corflags 0x00000003    //  ILONLY 32BITREQUIRED
// Image base: 0x030F0000


// =============== CLASS MEMBERS DECLARATION ===================

.class public auto ansi beforefieldinit Test6.CLRviaCSharp_6
       extends [mscorlib]System.Object
{
  .method private hidebysig static void  Main(string[] args) cil managed
  {
    .entrypoint
    // Code size       41 (0x29)
    .maxstack  1
    .locals init ([0] class Test6.BaseClass b,
             [1] class Test6.BaseClass s)
    IL_0000:  nop
    IL_0001:  call       void Test6.BaseClass::SShow()
    IL_0006:  nop
    IL_0007:  newobj     instance void Test6.BaseClass::.ctor()
    IL_000c:  stloc.0
    IL_000d:  newobj     instance void Test6.SubClass::.ctor()
    IL_0012:  stloc.1
    IL_0013:  ldloc.0
    IL_0014:  callvirt   instance void Test6.BaseClass::VShow()
    IL_0019:  nop
    IL_001a:  ldloc.1
    IL_001b:  callvirt   instance void Test6.BaseClass::VShow()
    IL_0020:  nop
    IL_0021:  ldc.i4.1
    IL_0022:  call       valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey(bool)
    IL_0027:  pop
    IL_0028:  ret
  } // end of method CLRviaCSharp_6::Main

  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  ret
  } // end of method CLRviaCSharp_6::.ctor

} // end of class Test6.CLRviaCSharp_6

.class public auto ansi beforefieldinit Test6.BaseClass
       extends [mscorlib]System.Object
{
  .method public hidebysig static void  SShow() cil managed
  {
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "Base class static method: SShow()!"
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method BaseClass::SShow

  .method public hidebysig newslot virtual 
          instance void  VShow() cil managed
  {
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "Base class virtual method: VShow()!"
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method BaseClass::VShow

  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  ret
  } // end of method BaseClass::.ctor

} // end of class Test6.BaseClass

.class public auto ansi beforefieldinit Test6.SubClass
       extends Test6.BaseClass
{
  .method public hidebysig virtual instance void 
          VShow() cil managed
  {
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "Sub class virtual method: VShow()!"
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method SubClass::VShow

  .method public hidebysig specialname rtspecialname 
          instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void Test6.BaseClass::.ctor()
    IL_0006:  ret
  } // end of method SubClass::.ctor

} // end of class Test6.SubClass


// =============================================================

// *********** DISASSEMBLY COMPLETE ***********************
// WARNING: Created Win32 resource file Test6.res

其中Main函数从63行开始。

第71行的静态方法由call来调用的，78行和81行的虚方法则是由callvirt来调用的。

为了验证call使用非虚的方式来调用方法的，我们将生成的IL文件Test6.il中81行的callvirt改为call。

修改Test6.il后保存，然后用ilasm.exe来编译此IL文件为新的exe文件。

命令：ilasm .\Test6.il /res:Test6.res /output:Test6_new.exe

然后执行Test6_new.exe，发现最后一步也是调用基类的方法，无法表现出多态性。

其实我们在写C#代码时并不用关心call和callvirt，c#编译器会帮助我们选择合适的调用方法。

只是在分析IL时经常会遇到callvirt，这里记录下来方便以后查阅。