if
勿庸质疑，说到控制结构，首先会想到if。其实现应如下所示：
public class IfThen : ControlFlow{ public Expression.ExpressionNode Condition; public Block ThenClause { get { return Children.Count > 0 ? Children.First() as Block: null; } } public Block ElseClause { get { return Children.Count > 1 ? Children.Last() as Block: null; } }运行时处理则应该如下：
public override void Run(Context ctx){ Debug.WriteLine("if(" + Condition.ToString() + ")"); Expression.Operand.Operand condVal = Condition.Evaluate(this); Debug.WriteLine("Condition=" + condVal.GetValue(this).ToString()); if (condVal.GetValue(this).AsInt != 0) { if (ThenClause != null) { Debug.WriteLine("Then"); ThenClause.Run(this); } } else { if (ElseClause != null) { Debug.WriteLine("Else"); ElseClause.Run(this); } }}看起来相当简单。
loop
循环具有类似的结构：条件判断，循环体。其基类如下所示：
public class Loop : ControlFlow{ public Expression.ExpressionNode Condition; public virtual Block Body { get { return Children.Count > 0 ? Children.Last() as Block : null; } } public bool NeedBreak = false; public bool NeedContinue = false;最简单的循环莫过于while，或do..while。好了，已经将while循环都列举了。
先看while循环的运行代码：
public override void Run(Context ctx){ Debug.WriteLine("Begin While."); base.Run(ctx); while (true) { if (Condition != null) { Expression.Operand.Value condVal = Condition.Evaluate(this).GetValue(this); Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition.ToString(), condVal.ToString())); if (condVal.AsInt == 0) { break; } } Body.Run(this); if (NeedBreak) break; } // while Debug.WriteLine("End While.");}do...while与while不同的部分在于，先运行循环体，再运行判断：
public override void Run(Context ctx){ Debug.WriteLine("Begin Do...While."); base.Run(ctx); while(true) { Body.Run(this); if (NeedBreak) break; if (Condition != null) { Expression.Operand.Value val = Condition.Evaluate(this).GetValue(this); Debug.WriteLine(string.Format("Condition : {0} = {1}", Condition.ToString(), val.ToString())); if (val.AsInt == 0) { break; } } } // while Debug.WriteLine("End Do...While.");}复杂一点的是for循环，比其它循环结构多出初始化部分和迭代部分：
public class ForLoop : Loop{ public Context Initializer { get { return Children.Count > 0 ? Children.First() : null; } } public Expression.ExpressionNode Iterator;由于初始化部分可以定义循环变量，形如：for(int i = 0; ....)。所以，还需要重载以下两个方法：
public override bool HasDefined(string str){ if (FindByName(str) == null) return base.HasDefined(str); else return true;}public override Context FindByName(string str){ Context res = null; if (Initializer != null) { Initializer.Parent = null; res = Initializer.FindByName(str); Initializer.Parent = this; } if (res == null) return this.Parent.FindByName(str); else return res;}然后就可以确定for循环的运行代码：
public override void Run(Context ctx){ Debug.WriteLine("Begin For."); if (Initializer != null) Initializer.Run(this); base.Run(ctx); while (true) { if (Condition != null) { Expression.Operand.Value val = Condition.Evaluate(this).GetValue(this); Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition.ToString(), val.ToString())); if (val.AsInt == 0) { break; } } Body.Run(this); if (NeedBreak) break; if (Iterator != null) Iterator.Evaluate(this); } // while if (Initializer != null) { Initializer.FreeLocalVariables(); } Debug.WriteLine("End For.");}switch
鲜有解释器（或编译器）实现了switch控制。无它，唯复杂尔。SharpC将switch划分为两个部分实现：switch(表达式）和case。先看Case的实现，由条件值和执行部分组成：
public class Case : ControlFlow{ public Expression.Operand.Value Value; public Block Body { get { return Children.Count > 0 ? Children.First() as Block : null; } }}为简化实现，用Block作为case的执行部分。副作用是多了一层，略微影响效率。再看switch的实现：
public class Switch : ControlFlow{ private Dictionary<int, int> m_caseDict; public Expression.ExpressionNode Condition; public Block Body { get { return Children.Count > 0 ? Children.First() as Block : null; } } public Case Default { get { return Body != null ? Body.Children.Last() as Case : null; } } public List<Context> CaseSet { get { return Body != null ? Body.Children.Count > 1 ? Body.Children.GetRange(0, Body.Children.Count - 1) : null : null; } }switch的执行部分亦为一个block。同时Default是最后一个Case。再看switch的执行部分：
public override void Run(Context ctx){ Debug.WriteLine("Begin Switch."); bool needBreak = false; if (IsFirstRunning) { m_caseDict = new Dictionary<int, int>(); int idx = 0; Body.OnReturn += delegate(Context stx, Return ret) { Debug.WriteLine("Return"); needBreak = true; Block parentBlock = this.ParentBlock; if (parentBlock != null && parentBlock.OnReturn != null) parentBlock.OnReturn(stx, ret); }; Body.OnBreak = delegate(Context stx) { Debug.WriteLine("Break."); needBreak = true; }; foreach (Context item in CaseSet) { Case stxCase = item as Case; m_caseDict.Add(stxCase.Value.AsInt, idx++); stxCase.Body.OnBreak += Body.OnBreak; } IsFirstRunning = false; } Expression.Operand.Value val = Condition.Evaluate(this).GetValue(ctx); Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition, val)); if (m_caseDict.ContainsKey(val.AsInt)) { int idx = m_caseDict[val.AsInt]; IEnumerator<Context> caseEnum = CaseSet.GetEnumerator(); int i = 0; while (caseEnum.MoveNext()) { if (i++ < idx) continue; Debug.WriteLine(string.Format("case {0}", (caseEnum.Current as Case).Value.AsInt)); (caseEnum.Current as Case).Body.Run(this); if (needBreak) break; } // while } else { Debug.WriteLine("Default"); Default.Run(this); }}在第一次执行的时候先建立case字典，希望下次运行的时候能够加快速度。然后，运行与表达式相匹配的case，并从该case开始，依次运行下一个case，直到运行到break为止。
Break, Continue and Return
这三种控制结构都必须结合上下文环境共同作用。看起来它们更像是一种标记。因此上，它们的实现也相当简单：
break
public class Break : ControlFlow{ public override void Print(int tabs = 0) { Debug.WriteLine(new string('\t', tabs) + "break;"); }}continue
public class Continue : ControlFlow{ public override void Print(int tabs = 0) { Debug.WriteLine(new string('\t', tabs) + "continue;"); }}return
return多了一个返回值的表达式：
public class Return : ControlFlow{ public Expression.ExpressionNode Expression; public override void Print(int tabs = 0) { if (Expression == null) Debug.WriteLine(new string('\t', tabs) + "return ;"); else { Debug.WriteLine(new string('\t', tabs) + "return " + Expression.ToString()); } }}一切需要回溯到Block，在Block运行时，会检测这三种控制结构：
public override void Run(Context ctx){ if (IsFirstRunning) { Block parentBlock = this.ParentBlock; if (parentBlock != null) { this.OnReturn += parentBlock.OnReturn; this.OnBreak += parentBlock.OnBreak; this.OnContinue += parentBlock.OnContinue; } IsFirstRunning = false; } foreach (Context stx in Children) { if (stx is ControlFlow.ControlFlow) { if (!ExecuteControlFlow(stx)) break;再看ExecuteControlFlow()：
private bool ExecuteControlFlow(Context stx){ if (stx is ControlFlow.Return) { if (OnReturn != null) OnReturn(this, stx as ControlFlow.Return); return false; } else { if (stx is ControlFlow.Break) { if (OnBreak != null) OnBreak(this); return false; } else if (stx is ControlFlow.Continue) { if (OnContinue != null) OnContinue(this); return false; } else stx.Run(this); } return true;}通过返回false来立即中止上层Block的运行循环。