protected override Expression VisitTry(TryExpression node) { int startYields = _yields.Count; bool savedInTryWithFinally = _inTryWithFinally; if (node.Finally != null || node.Fault != null) { _inTryWithFinally = true; } Expression @try = Visit(node.Body); int tryYields = _yields.Count; IList <CatchBlock> handlers = Visit(node.Handlers, VisitCatchBlock); int catchYields = _yields.Count; // push a new return label in case the finally block yields _returnLabels.Push(Expression.Label()); // only one of these can be non-null Expression @finally = Visit(node.Finally); Expression fault = Visit(node.Fault); LabelTarget finallyReturn = _returnLabels.Pop(); int finallyYields = _yields.Count; _inTryWithFinally = savedInTryWithFinally; if (@try == node.Body && handlers == node.Handlers && @finally == node.Finally && fault == node.Fault) { return(node); } // No yields, just return if (startYields == _yields.Count) { return(Expression.MakeTry(null, @try, @finally, fault, handlers)); } if (fault != null && finallyYields != catchYields) { // No one needs this yet, and it's not clear how we should get back to // the fault throw new NotSupportedException("yield in fault block is not supported"); } // If try has yields, we need to build a new try body that // dispatches to the yield labels var tryStart = Expression.Label(); if (tryYields != startYields) { @try = Expression.Block(MakeYieldRouter(node.Body.Type, startYields, tryYields, tryStart), @try); } // Transform catches with yield to deferred handlers if (catchYields != tryYields) { var block = new List <Expression>(); block.Add(MakeYieldRouter(node.Body.Type, tryYields, catchYields, tryStart)); block.Add(null); // empty slot to fill in later for (int i = 0, n = handlers.Count; i < n; i++) { CatchBlock c = handlers[i]; if (c == node.Handlers[i]) { continue; } if (handlers.IsReadOnly) { handlers = handlers.ToArray(); } // the variable that will be scoped to the catch block var exceptionVar = Expression.Variable(c.Test, null); // the variable that the catch block body will use to // access the exception. We reuse the original variable if // the catch block had one. It needs to be hoisted because // the catch might contain yields. var deferredVar = c.Variable ?? Expression.Variable(c.Test, null); _vars.Add(deferredVar); // We need to ensure that filters can access the exception // variable Expression filter = c.Filter; if (filter != null && c.Variable != null) { filter = Expression.Block(new[] { c.Variable }, Expression.Assign(c.Variable, exceptionVar), filter); } // catch (ExceptionType exceptionVar) { // deferredVar = exceptionVar; // } handlers[i] = Expression.Catch( exceptionVar, Expression.Block( Expression.Assign(deferredVar, exceptionVar), Expression.Default(node.Body.Type) ), filter ); // We need to rewrite rethrows into "throw deferredVar" var catchBody = new RethrowRewriter { Exception = deferredVar }.Visit(c.Body); // if (deferredVar != null) { // ... catch body ... // } block.Add( Expression.Condition( Expression.NotEqual(deferredVar, AstUtils.Constant(null, deferredVar.Type)), catchBody, Expression.Default(node.Body.Type) ) ); } block[1] = Expression.MakeTry(null, @try, null, null, new ReadOnlyCollection <CatchBlock>(handlers)); @try = Expression.Block(block); handlers = EmptyArray <CatchBlock> .Instance; // so we don't reuse these } if (finallyYields != catchYields) { // We need to add a catch block to save the exception, so we // can rethrow in case there is a yield in the finally. Also, // add logic for returning. It looks like this: // // try { ... } catch (Exception all) { saved = all; } // finally { // if (_finallyReturnVar) goto finallyReturn; // ... // if (saved != null) throw saved; // finallyReturn: // } // if (_finallyReturnVar) goto _return; // We need to add a catch(Exception), so if we have catches, // wrap them in a try if (handlers.Count > 0) { @try = Expression.MakeTry(null, @try, null, null, handlers); handlers = EmptyArray <CatchBlock> .Instance; } // NOTE: the order of these routers is important // The first call changes the labels to all point at "tryEnd", // so the second router will jump to "tryEnd" var tryEnd = Expression.Label(); Expression inFinallyRouter = MakeYieldRouter(node.Body.Type, catchYields, finallyYields, tryEnd); Expression inTryRouter = MakeYieldRouter(node.Body.Type, catchYields, finallyYields, tryStart); var all = Expression.Variable(typeof(Exception), "e"); var saved = Expression.Variable(typeof(Exception), "$saved$" + _temps.Count); _temps.Add(saved); @try = Expression.Block( Expression.TryCatchFinally( Expression.Block( inTryRouter, @try, Expression.Assign(saved, AstUtils.Constant(null, saved.Type)), Expression.Label(tryEnd) ), Expression.Block( MakeSkipFinallyBlock(finallyReturn), inFinallyRouter, @finally, Expression.Condition( Expression.NotEqual(saved, AstUtils.Constant(null, saved.Type)), Expression.Throw(saved), Utils.Empty() ), Expression.Label(finallyReturn) ), Expression.Catch(all, Utils.Void(Expression.Assign(saved, all))) ), Expression.Condition( Expression.Equal(_gotoRouter, AstUtils.Constant(GotoRouterYielding)), Expression.Goto(_returnLabels.Peek()), Utils.Empty() ) ); @finally = null; } else if (@finally != null) { // try or catch had a yield, modify finally so we can skip over it @finally = Expression.Block( MakeSkipFinallyBlock(finallyReturn), @finally, Expression.Label(finallyReturn) ); } // Make the outer try, if needed if (handlers.Count > 0 || @finally != null || fault != null) { @try = Expression.MakeTry(null, @try, @finally, fault, handlers); } return(Expression.Block(Expression.Label(tryStart), @try)); }