Exception Handling In Microsoft

Exception Handling Exception handling in IL is a big let down. We expected a significant amount of complexity, but were proved wrong, right from the beginning. IL cannot be termed as a machine level assembler. It actually has a number of directives like try and catch, that work like their higher level counterparts. a.cs class zzz { public static void Main() { try { abc(); System.Console.WriteLine("Bye"); } catch (System.Exception e) { System.Console.WriteLine("In Exception"); } System.Console.WriteLine("After Exception"); } public static void abc() { throw new System.Exception(); System.Console.WriteLine("abc"); } } a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed

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{ newobj instance void [mscorlib]System.Exception::.ctor() throw } }

Output In Exception After Exception

In the above example, the function abc first creates an object that looks like Exception using newobj and places it on the stack. Thereafter the throw instruction throws an exception. This Exception is placed on the stack, hence the catch instruction is called. In the catch instruction, e, a local varaible, is an instance of Exception. The next instruction, leave.s jumps to label IL_001e, the label is beyond the catch. To exit off from a try or a catch block, instead of the branch instruction br, leave is used. The reason is that we are dealing with exceptions, which are to be handled in a special way in IL. Exception handling in IL is done using higher level instructions. a.cs class zzz { public static void Main() { yyy a; a=new yyy(); try { a.abc(); System.Console.WriteLine("Bye"); } catch { System.Console.WriteLine("In Exception"); } finally { System.Console.WriteLine("In finally"); } } } class yyy { public void abc() { throw new System.Exception(); } } a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint

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.locals (class yyy V_0) newobj instance void yyy::.ctor() stloc.0 .try { .try { ldloc.0 call instance void yyy::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_0025 } catch [mscorlib]System.Object { pop ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_0025 } IL_0025: leave.s IL_0032 } finally { ldstr "In finally" call void [mscorlib]System.Console::WriteLine(class System.String) endfinally } IL_0032: ret } } .class private auto ansi yyy extends [mscorlib]System.Object { .method public hidebysig instance void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output In Exception In finally

The above program has utilised a try catch without a parameter and a finally clause. Adding a finally clause associates the same try with a catch, and a finally. In a sense, two copies of try are created, one for catch and the other for finally. If the catch directive is not supplied with an Exception object, it will take an object that looks like System.Object. In the catch, the item is popped of the stack, as its value holds no significance. The string is printed before the leave. Also, along with try-catch is the finally clause which is the next to be executed. A finally is executed as a separate try finally directive and it can only be exited using the endfinally instruction. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed

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{ .entrypoint .try { leave.s IL_0032 } finally { ldstr "In finally" call void [mscorlib]System.Console::WriteLine(class System.String) endfinally } IL_0032: ret } } Output In finally

Nowhere is it specified that a try must have a catch. A finally will ultimately be called at the end of the try. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output Exception occurred: System.Exception: An exception of type System.Exception was thrown. at zzz.vijay()

In the absence of a try catch block, if function abc throws an exception, it will not get caught. Instead, a runtime error is generated. A try catch clause is recommended to proactively catch the exception, otherwise when an exception is thrown and the program will come to a grinding halt. a.cs public class zzz { public static void Main() { int i = 1; for ( i = 1; i<= 10 ; i++) { try { System.Console.WriteLine("1 try");

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try { System.Console.WriteLine("2 try"); break; } finally { System.Console.WriteLine("2 finally"); } } finally { System.Console.WriteLine("1 finally"); } } System.Console.WriteLine(i); } } a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (int32 V_0) ldc.i4.1 stloc.0 ldc.i4.1 stloc.0 br.s IL_0032 .try { IL_0006: ldstr "1 try" call void [mscorlib]System.Console::WriteLine(class System.String) .try { ldstr "2 try" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_0037 } finally { ldstr "2 finally" call void [mscorlib]System.Console::WriteLine(class System.String) endfinally } } finally { ldstr "1 finally" call void [mscorlib]System.Console::WriteLine(class System.String) endfinally } IL_0032: ldloc.0 ldc.i4.s 10 ble.s IL_0006 IL_0037: ldloc.0

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call void [mscorlib]System.Console::WriteLine(int32) ret } } Output 1 try 2 try 2 finally 1 finally 1

The above program is quite lengthy, but very simple. It proves the fact that code placed in a finally block is always executed. Like death and taxes, a finally cannot be avoided. The for statement branches to label IL_0032 where we first check for the value to be less than or equalto10. If it results in TRUE, the code at label IL_0006 is executed. A we learnt in one of the earlier chapters, the condition check for the for statement is always placed at the bottom in IL. In the first attempt, string "1 try" is printed . Thereafter the code within the second try is executed, where "2 try" is printed. The break statement in C# gets converted to a leave to label IL_0037 in IL. This label signifies the end of the for statement. The leave instruction is smart enough to realize that it is located within two trys with a finally clause, hence it calls the code with the finally instruction. Under normal circumstances, break becomes a branch instruction if not placed within try-catch. a.cs public class zzz { public void abc() { lock(this) { } } public static void Main() { } } a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint ret } .method public hidebysig instance void abc() il managed { .locals (class zzz V_0) ldarg.0 dup stloc.0 call void [mscorlib]System.Threading.Monitor::Enter(class System.Object) .try { leave.s IL_0011 } finally

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{ ldloc.0 call void [mscorlib]System.Threading.Monitor::Exit(class System.Object) endfinally } IL_0011: ret } }

The lock keyword ensures that while one thread executes a function, the other threads remain suspended. This keyword gets translated into a large amount of IL code. In fact, it generates the maximum amount of code amongst all the keywords. The C# compiler first calls the static function Enter from the Monitor class. Then, it executes the code located in a try. The try block here contains no code. On encountering the leave instruction, the program enters the finally which calls the Exit function from the Monitor class. This initiates another thread that is waiting at the Enter function. Whenever an exception occurs, an object representing the exception must be created. This exception object has to be a class derived from Exception and cannot be a value type or a pointer. A Structured Exception Handling (SEH) block is made up of a try and one or more handlers. A try directive is used to declare a protected block. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { ret } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } }

We cannot exit from a protected block with a ret but, on using it, no errors are generated at assemble time or run time. As a rule, only a leave or a throw to another exception is acceptable to exit from a protected block. A leave statement is permitted in the try and not in the catch. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint

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.locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) .try { call void zzz::pqr() ldstr "Bye1" call void [mscorlib]System.Console::WriteLine(class System.String) leave aa1 } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception1" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } aa1: leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { ret } .method public hidebysig static void pqr() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output Bye In Exception1 After Exception

We can nest as many trys or protected blocks within each other. A leave is required at the end of every try to avoid all errors. We can have four types of handlers for a try or a protected block. They are:

• • •

finally catch fault

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•

filter

Only one of it can be used at a time. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } finally { ldstr "In finally" call void [mscorlib]System.Console::WriteLine(class System.String) endfinally } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output In Exception After Exception

If we comment out the code where the function abc has been called, we get the following output: Output Bye In finally After Exception

As mentioned earlier, the protected block can only be handled by a single handler. In the above example, when an exception is thrown, the catch is called and not finally as it does not have its own try directive. The runtime does not give us any error, but it ignores the finally handler.

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If, however, no exception is thrown, as is the case when we comment out the call of the function abc, then the finally gets called. It is quaint that the try is a directive, but the handler is not. We have two classes of handlers:

• •

exception resolving handlers exception observing handlers.

The finally and fault handlers are exception observing handlers as they do not resolve the exception. In an exception resolving handler, we try and resolve the exception so that normal program control continues. The catch and filter handlers are examples of such handlers. The deepest handler will be visited first, followed by the next enclosing one and so on, until we find an appropriate handler. A handler has its own instructions, using which, the program can exit a handler. It is illegal to use any other instruction for this purpose, but at times the assembler is unable to detect this misfit. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { } catch [mscorlib]System.Exception { br a1 } ret } .method public hidebysig static void pqr() il managed { a1: ret } } Error ***** FAILURE *****

We are not allowed to jump off a catch handler. It is essential to leave the handler in an orderly manner only. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class zzz V_0) newobj instance void zzz::.ctor()

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stloc.0 startTry: ldstr "in try" call void System.Console::WriteLine(class System.String) //call instance void zzz::abc() ldstr "after function call" call void System.Console::WriteLine(class System.String) leave exitSEH endTry: startFault: ldstr "in fault" call void System.Console::WriteLine(class System.String) endfault endFault: .try startTry to endTry fault handler startFault to endFault exitSEH: ldstr "over" call void System.Console::WriteLine(class System.String) ret } .method public hidebysig instance void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output in try after function call over

If we comment out the call of the function abc, we get an error, a Windows error, which is incomprehensible. The purpose of the above program is to demonstrate that we can use labels to delimit code in a protected block. Thereafter, we can use the try directive, indicating to it the start and end label of our code and also the start and end of the fault handler. This is another way of utilising the try directive. The catch keyword creates a type filtered exception. Whenever an exception occurs in a protected block, the EE checks whether the exception that occurred earlier, is equal to or a sub-type of the error that the catch expects. If the type matches, the code of the catch is called. If the type does not match, the EE will continue to search for another handler. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() a1: ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception

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{ stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s a1 } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output In Exception Bye After Exception

The program is not allowed to resume execution after an exception occurs. This means that, we cannot go back to the protected block where the exception took place. In this case, we are allowed to do so, but keep in mind that we were using a beta copy of the assembler. Whenever the EE sees a leave in a catch block, it knows that the exception is done with, and the system returns to a normal state.

a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) rethrow } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed {

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newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output In Exception Exception occurred: System.Exception: An exception of type System.Exception was thrown. at zzz.vijay()

Here, at the end of the catch is the rethrow instruction, which rethrows the same exception again. As there is no other catch block to catch the exception, the exception is thrown at run time. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) rethrow } } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception1" call void [mscorlib]System.Console::WriteLine(class System.String) } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } } Output In Exception In Exception1

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After Exception

Here, we placed another try directive with a separate catch. The exception that is thrown by the inner catch, cannot be caught by another catch at the same level. It needs to be caught by the catch at the higher level. Thus one more catch is needed. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public hidebysig static void vijay() il managed { .entrypoint .locals (class [mscorlib]System.Exception V_0) .try { call void zzz::abc() ldstr "Bye" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.Exception { stloc.0 ldstr "In Exception" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } catch [mscorlib]System.IOException { stloc.0 ldstr "In Exception1" call void [mscorlib]System.Console::WriteLine(class System.String) leave.s IL_001e } IL_001e: ldstr "After Exception" call void [mscorlib]System.Console::WriteLine(class System.String) ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.IOException::.ctor() throw } } Output In Exception After Exception

The C# compiler watches our code like a hawk. On the other hand, the assembler is a blind bat. There are two exception handlers:

• •

IOException: This handles Input/Output Exceptions.

Exception: This is a generic handler that handles generic exceptions since all exceptions are derived from exception.

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Consciously, we have placed the generic Exception handler first. Therefore, irrespective of the exception thrown, the second handler will never get called. The function abc now throws a IOException. The generic Exception handler, which is placed earlier in the code, is encountered first, and therefore, it deals with the exception. The C# compiler would have generated an error in this situation, but the assembler assumes what you are conscious of your deeds. Output Exception occurred: System.ExecutionEngineException: An exception of type System.ExecutionEngineException was thrown. at zzz.vijay()

The above exception is thrown when there is no leave in a catch and there is another catch following this one. In one of our earlier programs above, we have used only a single catch. a.il .assembly mukhi {} .class private auto ansi zzz extends [mscorlib]System.Object { .method public static void vijay() { .entrypoint ldstr "start" call void System.Console::WriteLine(class System.String) br start a2: ldstr "in a2" call void System.Console::WriteLine(class System.String) pop ldc.i4.0 endfilter start: .try { ldstr "in try" call void System.Console::WriteLine(class System.String) call void zzz::abc() ldstr "after function call" call void System.Console::WriteLine(class System.String) leave a1 } filter a2 { ldstr "filter" call void System.Console::WriteLine(class System.String) pop leave a1 } a1: ret } .method public hidebysig static void abc() il managed { newobj instance void [mscorlib]System.Exception::.ctor() throw } }

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Output start in try Exception occurred: System.Exception: An exception of type System.Exception was thrown. at zzz.vijay()

The last type of fault handler, which is the most generic, either does not seem to work or it could also mean that we have done something wrong. We use the keyword filter with a label, which denotes the start point of some code. This code checks whether the exception must be handled or not. This is triggered off by placing either the number 0 or 1 on the stack. In our case, none of the code dealing with the filter gets called.

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