利用DynamicILInfo類動態產生方法

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　　ILGenerator與DynamicILInfo類是不同的。它直接使用位元組數組設定IL代碼,異常和本地的簽名。那麼,我們怎樣才能獲得這些位元組數組從ILGenerator 與DynamicILInfo類是不同的。它直接使用位元組數組設定IL代碼,異常和本地的簽名。那麼,我們怎樣才能獲得這些位元組數組，請看下面的代碼:

     

using System;using System.Reflection;using System.Reflection.Emit;class Demo{    static void Main()    {        DynamicMethod dm = new DynamicMethod("HelloWorld", typeof(void), Type.EmptyTypes, typeof(Demo), false);        DynamicILInfo il = dm.GetDynamicILInfo();        SignatureHelper sigHelper = SignatureHelper.GetLocalVarSigHelper();        il.SetLocalSignature(sigHelper.GetSignature());        byte[] code = { 0x00, 0x72, 0x01, 0x00, 0x00, 0x70, 0x28, 0x04, 0x00, 0x00, 0x0a, 0x00, 0x2a };        int token0 = il.GetTokenFor("Hello world");        int token1 = il.GetTokenFor(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }).MethodHandle);        PutInteger4(token0, 0x0002, code);        PutInteger4(token1, 0x0007, code);        il.SetCode(code, 8);        dm.Invoke(null, null);
　　     Console.Read();    }    static void PutInteger4(int value, int startPos, byte[] array)    {        array[startPos++] = (byte)value;        array[startPos++] = (byte)(value >> 8);        array[startPos++] = (byte)(value >> 16);        array[startPos++] = (byte)(value >> 24);    }}

以下是Microsoft enginee 的解釋：

If the dynamic method we are going to create is based on a static method in some existing assembly, we can view the IL code (bytes) of the static method via ildasm or by using MethodBody.GetILByteArray. We can then build the byte array for SetCode from there. For all tokens used in the original static method, be sure to replace them (bold, as shown above and below in this example) with those generated from GetTokenFor. 

.method public hidebysig static void HelloWorld() cil managed
// SIG: 00 00 01
{
  // Method begins at RVA 0x2050
  // Code size 13 (0xd)
  .maxstack 8
  IL_0000: /* 00 |             */ nop
  IL_0001: /* 72 | (70)000001 */ ldstr "Hello world"
  IL_0006: /* 28 | (0A)000004 */ call void [mscorlib]System.Console::WriteLine(string)
  IL_000b: /* 00 |             */ nop
  IL_000c: /* 2A |             */ ret
}

To build the byte array for SetException, I‘d suggest taking a couple of minutes to read ECMA spec partiton II (25.4.6) first. The Flags/TryOffset/TryLength/HandlerOffset/HandlerLength ... can be retrieved from MethodBody.ExceptionHandlingClauses programatically. Be aware the exception type token (ClassToken) has to be obtained from GetTokenFor as well. Do you know Ildasm can show the raw exception information? I did not know this until I learned about it recently from Peli.:) Uncheck View->Expand try/catch and check show bytes; we will see something like the following at the bottom of the method IL:

  // Exception count 1
  .try IL_002e to IL_0051 finally handler IL_0051 to IL_0065
  // HEX: 02 00 00 00 2E 00 00 00 23 00 00 00 51 00 00 00 14 00 00 00 00 00 00 00
}

Class SignatureHelper offers a convenient way to construct the local variable signature, if each local variable type is known. We can just make calls of SignatureHelper.AddArgument.

Are there any scenarios in your mind where using DynamicILInfo could be better than using DynamicILGenerator? How‘d you like to construct those byte arrays? I am thinking about writing and sharing a tool, which, based on an existing method, generates C# code to define the equivalent dynamic method (using DynamicILInfo) (if this turns out to be a popular scenario).

利用DynamicILInfo類動態產生方法