.Net way to understand the tape out, ref signature and the signature of the difference between the ordinary method

Today there was a new colleague asked me .Net in with OUT , ref method signature and the signature of the common methods What is the difference? I think we can explain some of the key areas from the following example.

A , REF / OUT Modifier Description

For use ref / out described in the modifier MSDN detailed description, the following address:

http://msdn.microsoft.com/en-us/library/t3c3bfhx(VS.80).aspx。

 

Second, through the IL code is observed ref / out signature modification method ( a value type as an example )

1 , the sample code:

using  System;

namespace  ConsoleMain
{
     class  Program
    {
         static   void  Main()
        {
            Int32 p ;

            TestRef( out  p);                              // ①

             // TestRef(ref p)                         // ②
            
            TestRef(p);                          // ③

            Console.ReadKey();
        }

         static   void  TestRef(Int32 para)                  // ④
        {
            para  =   1 ;
        }

         static   void  TestRef( out  Int32 para)                 // ⑤
        {
            para  =   2 ;
        }

         /* static void TestRef(ref Int32 para)                //⑥
        {
            Para3 =  3;
        }  */
    }
}

2 , using the Reflector view the IL code is as follows:

(1) Main()

.method   private   hidebysig  static  void  Main()  cil   managed
{
     .entrypoint
     .maxstack   1
     .locals   init  (
        [ 0 ]  int32  p)
     L_0000:   ldloca.s  p
     L_0002:   call   void  ConsoleMain.Program::TestRef( int32 &)
     L_0007:   ldloc.0  
     L_0008:   call   void  ConsoleMain.Program::TestRef( int32 )
     L_000d:   call  valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()
     L_0012: pop L_0013: right }    
        

(2) TestRef(ref Int32 para)

.method   private   hidebysig  static  void  TestRef( int32 & para)  cil   managed
{
     .maxstack   8
     L_0000:   ldarg.0  
     L_0001:   ldc.i4.2  
     L_0002:   stind.i4  
     L_0003:   ret  
}

(3) TestRef (Int32 para)

.method   private   hidebysig  static  void  TestRef( int32  para)  cil   managed
{
     .maxstack   8
     L_0000:   ldc.i4.1  
     L_0001:   starg.s  para
     L_0003:   ret  
}

(4) TestRef(out Int32 para)

.method   private   hidebysig  static  void  TestRef([out]  int32 & para)  cil   managed
{
     .maxstack   8
     L_0000:   ldarg.0  
     L_0001:   ldc.i4.2  
     L_0002:   stind.i4  
     L_0003:   ret  
}

. 3 , of IL code analysis

A method creates is called Evaluation Stack , local variables, method parameters, area and other storage area is created, reference may be made MSIL experience article.

 

1) Main()

.entrypoint，

Current methods for the entry method;

.maxstack 1 ,

Created Evaluation Stack element is set to the maximum capacity of 1 ;

.locals init ([0] int32 p)

     pint32

L_0000: ldloca.s p

pEvaluation Stack

L_0002: call void ConsoleMain.Program ::TestRef (int32 &)

     callcall

    Call””(this)””

public   class  TestClass
{
     private   void  InvokeTest()
    {
        Test( 1 );
    }

     private   void  Test(Int32 i)
    {           
    }       
}   

InvokeTest方法的IL代码如下：

.method   private   hidebysig   instance   void  InvokeTest()  cil   managed
{
    .maxstack   8
     L_0000:   ldarg.0  
     L_0001:   ldc.i4.1  
     L_0002:   call   instance   void  ConsoleMain.TestClass::Test( int32 )
     L_0007:   ret  
}

L_0000: ldarg.0TestTestEvaluation Stackarg0TestClassthisStaticarg0

     CallVrtual tableCallCallvirt

     MainEvaluation Stack&pTestRef(int32&)””TestRef(int32&)MainEvaluation Stackref&

L_0007: ldloc.0

     Mainp

L_0008: call void ConsoleMain.Program::TestRef(int32)

     MainEvaluation StackpTestRef(int32)””TestRef(int32)

L_000d: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()

     mscorlib.dllReadKey(valuetype)System.ConsoleKeyInfoMainEvaluation Stack

L_0012: pop  L_0013: ret,

     MainEvaluation StackMain

 

2)TestRef(ref Int32 para)

 L_0000: ldarg.0

    0Evaluation Stack

L_0001: ldc.i4.2

    int322Evaluation Stack

L_0002: stind.i4

    Evaluation Stack(valueaddress)valueaddress

L_0003: ret

 

3)、TestRef(Int32 para)

L_0000:ldc.i4.1，

执行完成后的堆栈变化情况：

L_0001: starg.s para，

执行完成后的堆栈变化情况：

L_0003: ret，

方法返回，相应的Evaluation Stack、局部变量区、方法参数区等存储区被释放。

 

4)、TestRef(out Int32 para)

     从方法签名上看它只比TestRef(ref Int32 para)多一个[out]，其它内容完全一样。

     在代码中将②放开，会发现编译不通过，说明方法签名的区别如果仅仅是ref和out则无法实现方法的overload，也就是TestRef(ref Int32 para)和TestRef(out Int32 para)这两个方法不能同时存在于同一个类型中。

在代码中将①注释而将②放开，会发现编译不通过，因为不能将一个未初始化的变量传给ref修饰参数的方法，但是传给out修饰参数的方法是可以的，但是在方法返回前一定要给out修饰的参数赋值。借用MS的一句话：

the ref and out keywords are treated differently at run-time, but they are treated the same at compile time.

 

4、结论

     (1)、有ref和out修饰参数的方法和普通方法在调用前的数据准备是不一样的，由L_0000: ldloca.s p和L_0007: ldloc.0可以看到，前者是获取目标变量的内存地址，后者是获取目标变量的值，这就是所谓的传引用和传值。

     (2)、两个方法的区别仅仅是相同参数，一个使用的修饰符是ref，另一个是out，那么无法重载这两个方法，且分别编译它们得到的IL代码完全一样，只是方法签名中由out修饰的那个参数前会有个token[out]。

(3) use of out parameters of the role: I do not care what the initial value of the variable is or I do not know what the initial value should be assigned or I just want to know the state of my method execution is completed ( eg: success or error and to the cause of the error ) , because all with out modification of the process parameters in this parameter must be reassigned, as MS mentioned: a method for selectively allowing a return value.

Reproduced in: https: //www.cnblogs.com/vivounicorn/archive/2009/09/17/1568242.html