Chapter157:ILGenerator

Section 157.1: Creates a DynamicAssembly that contains a UnixTimestamphelpermethod

ThisexampleshowstheusageoftheILGeneratorbygeneratingcodethatmakesuseofalreadyexistingandnew createdmembersaswellasbasicExceptionhandling.ThefollowingcodeemitsaDynamicAssemblythatcontains an equivalent to this c# code:

publicstaticclassUnixTimeHelper

{

privatereadonlystaticDateTimeEpochTime=newDateTime(1970,1,1);

publicstaticintUnixTimestamp(DateTimeinput)

{

inttotalSeconds; try

{

totalSeconds=checked((int)input.Subtract(UnixTimeHelper.EpochTime).TotalSeconds);

}

catch(OverflowExceptionoverflowException)

{

thrownewInvalidOperationException(“It’stolateforanInt32timestamp.”,overflowException);

}

returntotalSeconds;

}

}

//Gettherequiredmethods

vardateTimeCtor=typeof(DateTime)

.GetConstructor(new[]{typeof(int),typeof(int),typeof(int)});

vardateTimeSubstract=typeof(DateTime)

.GetMethod(nameof(DateTime.Subtract),new[]{typeof(DateTime)}); vartimeSpanSecondsGetter=typeof(TimeSpan)

.GetProperty(nameof(TimeSpan.TotalSeconds)).GetGetMethod(); varinvalidOperationCtor=typeof(InvalidOperationException)

.GetConstructor(new[]{typeof(string),typeof(Exception)});

if(dateTimeCtor==null||dateTimeSubstract==null|| timeSpanSecondsGetter==null||invalidOperationCtor==null)

{

thrownewException(“CouldnotfindarequiredMethod,cannotcreateAssembly.”);

}

//Setuptherequiredmembers

varan=newAssemblyName(“UnixTimeAsm”);

vardynAsm=AppDomain.CurrentDomain.DefineDynamicAssembly(an,AssemblyBuilderAccess.RunAndSave); vardynMod=dynAsm.DefineDynamicModule(an.Name,an.Name+”.dll”);

vardynType=dynMod.DefineType(“UnixTimeHelper”,

TypeAttributes.Abstract|TypeAttributes.Sealed|TypeAttributes.Public);

varepochTimeField=dynType.DefineField(“EpochStartTime”,typeof(DateTime), FieldAttributes.Private|FieldAttributes.Static|FieldAttributes.InitOnly);

varcctor=

dynType.DefineConstructor(

MethodAttributes.Private|MethodAttributes.HideBySig|MethodAttributes.SpecialName| MethodAttributes.RTSpecialName|MethodAttributes.Static, CallingConventions.Standard, Type.EmptyTypes);

varcctorGen=cctor.GetILGenerator();

cctorGen.Emit(OpCodes.Ldc_I4,1970);//LoadtheDateTimeconstructorargumentsontothestack

cctorGen.Emit(OpCodes.Ldc_I4_1);

cctorGen.Emit(OpCodes.Ldc_I4_1);

cctorGen.Emit(OpCodes.Newobj,dateTimeCtor);//Calltheconstructor cctorGen.Emit(OpCodes.Stsfld,epochTimeField);//StoretheobjectinthestaticfieldcctorGen.Emit(OpCodes.Ret);

varunixTimestampMethod=dynType.DefineMethod(“UnixTimestamp”, MethodAttributes.Public|MethodAttributes.HideBySig|MethodAttributes.Static, CallingConventions.Standard,typeof(int),new[]{typeof(DateTime)});

unixTimestampMethod.DefineParameter(1,ParameterAttributes.None,”input”);

varmethodGen=unixTimestampMethod.GetILGenerator();

methodGen.DeclareLocal(typeof(TimeSpan));

methodGen.DeclareLocal(typeof(int));

methodGen.DeclareLocal(typeof(OverflowException));

methodGen.BeginExceptionBlock();//Beginthetryblock

methodGen.Emit(OpCodes.Ldarga_S,(byte)0);//Tocallamethodonastructweneedtoloadthe addressof it

methodGen.Emit(OpCodes.Ldsfld,epochTimeField);

//Loadtheobjectofthestaticfieldwecreatedasargumentforthefollowingcall methodGen.Emit(OpCodes.Call,dateTimeSubstract);//CallthesubstractmethodontheinputDateTime methodGen.Emit(OpCodes.Stloc_0);//StoretheresultingTimeSpaninalocal methodGen.Emit(OpCodes.Ldloca_S,(byte)0);//Loadthelocalsaddresstocallamethodon it methodGen.Emit(OpCodes.Call,timeSpanSecondsGetter);//CalltheTotalSecondsGetmethodontheTimeSpan

methodGen.Emit(OpCodes.Conv_Ovf_I4);//ConverttheresulttoInt32;throwsanexceptiononoverflow

methodGen.Emit(OpCodes.Stloc_1);//storetheresultforreturninglater

//Theleaveinstructiontojumpbehindthecatchblockwillbeautomaticallyemitted

methodGen.BeginCatchBlock(typeof(OverflowException));//Beginthecatchblock

//Whenwearehere,anOverflowExceptionwasthrown,thatisnowonthestack methodGen.Emit(OpCodes.Stloc_2);//Storetheexceptioninalocal. methodGen.Emit(OpCodes.Ldstr,”It’stolateforanInt32timestamp.”);

//Loadourerrormessageontothestack

methodGen.Emit(OpCodes.Ldloc_2);//Loadtheexceptionagain methodGen.Emit(OpCodes.Newobj,invalidOperationCtor);

//CreateanInvalidOperationExceptionwithourmessageandinnerException methodGen.Emit(OpCodes.Throw);//Throwthecreatedexception methodGen.EndExceptionBlock();//Endthecatchblock

//Whenwearehere,everythingisfine

methodGen.Emit(OpCodes.Ldloc_1);//Loadtheresultvalue

methodGen.Emit(OpCodes.Ret);//Return it

dynType.CreateType();

dynAsm.Save(an.Name+”.dll”);

Section157.2:Createmethodoverride

ThisexampleshowshowtooverrideToStringmethodingeneratedclass

//createanAssemblyandnewtype

varname=newAssemblyName(“MethodOverriding”);

vardynAsm=AppDomain.CurrentDomain.DefineDynamicAssembly(name,AssemblyBuilderAccess.RunAndSave);

vardynModule=dynAsm.DefineDynamicModule(name.Name,$”{name.Name}.dll”);

vartypeBuilder=dynModule.DefineType(“MyClass”,TypeAttributes.Public|TypeAttributes.Class);

//defineanewmethod

vartoStr=typeBuilder.DefineMethod(

“ToString”,//name

MethodAttributes.Public|MethodAttributes.Virtual,//modifiers

typeof(string),//returntype

Type.EmptyTypes);//argumenttypes

varilGen=toStr.GetILGenerator();

ilGen.Emit(OpCodes.Ldstr,”Hello,world!”);

ilGen.Emit(OpCodes.Ret);

//setthismethodasoverrideofobject.ToString typeBuilder.DefineMethodOverride(toStr,typeof(object).GetMethod(“ToString”)); vartype=typeBuilder.CreateType();

//nowtestit:

varinstance=Activator.CreateInstance(type);

Console.WriteLine(instance.ToString());