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转自:http://blog.163.com/redtomato/blog/static/48648149200831202846410/
      有时候需要表达式运算，如

String strExpression="-12 * ( - 2.2 + 7.7 ) - 44 * 2";

网上找的，一般是利用CodeDom，见

http://www.codeproject.com/csharp/runtime_eval.asp

简化为：

System.CodeDom.Compiler.ICodeCompiler comp = (new Microsoft.CSharp.CSharpCodeProvider().CreateCompiler());

System.CodeDom.Compiler.CompilerParameters cp = new System.CodeDom.Compiler.CompilerParameters();

object qswhEval2(string Expression){

StringBuilder code = new StringBuilder();

code.Append("using System; \n");

code.Append("namespace ADOGuy { \n");

code.Append(" public class _Evaluator { \n");

code.Append(" public object __foo() ");

code.Append("{ ");

code.AppendFormat(" return ({0}); ", Expression);

code.Append("}\n");

code.Append("} }");

System.CodeDom.Compiler.CompilerResults cr = comp.CompileAssemblyFromSource(cp, code.ToString());

System.Reflection.Assembly a = cr.CompiledAssembly;

object _Compiled = a.CreateInstance("ADOGuy._Evaluator");

System.Reflection.MethodInfo mi = _Compiled.GetType().GetMethod("__foo");

return mi.Invoke(_Compiled, null);

}

但用起来感觉很慢，毕竟需要实时编译。

于是，就自己照数据结构书上写了一种算法：

string Precede(string p, string q){

switch(p){

case "+":

case "-":return ("*/(".IndexOf(q)!=-1)?"<":">";

case "*":

case "/":return (q=="(")?"<":">";

case "(":return (q==")")?"=":"<";

case ")":return (q=="(")?"?":">";

case "#":return (q=="#")?"=":"<";

}

return "?";

}

Double Operate(Double a,char o,Double b)

{

switch(o)

{

case '+':return a+b;

case '-':return a-b;

case '*':return a*b;

case '/':return a/b;

}

return 0;

}

Object qswhEval1(string Expression){

/*************(qiushuiwuhen 2002-12-14)****************/

Stack nArr=new Stack(),oArr=new Stack();

int j=0;

Double a=0,b=0;

string w="";

char o;

MatchCollection arr=Regex.Matches(Expression.Replace(" ","")+"#",@"(((?<=(^|\())-)?\d+(\.\d+)?|\D)");

oArr.Push('#');

w=Convert.ToString(arr[j++]);

while(!(w=="#"&&Convert.ToString(oArr.Peek())=="#")){

if("+-*/()#".IndexOf(w)!=-1){

switch(Precede(oArr.Peek().ToString(),w)){

case "<":

oArr.Push(w);

w=Convert.ToString(arr[j++]);

break;

case "=":

oArr.Pop();

w=Convert.ToString(arr[j++]);

break;

case ">":

o=Convert.ToChar(oArr.Pop());

b=Convert.ToDouble(nArr.Pop());

a=Convert.ToDouble(nArr.Pop());

nArr.Push(Operate(a,o,b));

break;

default:

return "Error";

break;

}

}else{

nArr.Push(w);

w=Convert.ToString(arr[j++]);

}

}

return nArr.Pop();

}

还有利用JScript的Eval的两种算法

Microsoft.JScript.Vsa.VsaEngine ve=Microsoft.JScript.Vsa.VsaEngine.CreateEngine();

object qswhEval3(string Expression){

return Microsoft.JScript.Eval.JScriptEvaluate(Expression,ve);

}

object qswhEval4(string Expression){

return qswhJs.qswhEval.Eval(Expression);

}

第四种需先建立一js编译为dll,如下代码

import System;

package qswhJs {

class qswhEval {

static function Eval(Expression):Object { return eval(Expression); }

}

}

测试代码如下：

void Page_Load(Object o,EventArgs ea){

String strExpression="-12 * ( - 2.2 + 7.7 ) - 44 * 2";

int i=0,c=100;

DateTime d1,d2;

cp.GenerateExecutable = false;

cp.GenerateInMemory = true;

d1=DateTime.Now;

for(i=0;i d2=DateTime.Now;

Response.Write("方法一：对表达式分析 "+d2.Subtract(d1)+"

");

d1=DateTime.Now;

for(i=0;i d2=DateTime.Now;

Response.Write("方法二：利用CodeCom "+d2.Subtract(d1)+"

");

d1=DateTime.Now;

for(i=0;i d2=DateTime.Now;

Response.Write("方法三：利用Jscript+Vsa "+d2.Subtract(d1)+"

");

d1=DateTime.Now;

for(i=0;i d2=DateTime.Now;

Response.Write("方法四：利用Jsc+Dll "+d2.Subtract(d1)+"

");

}

测试结果：

方法一：对表达式分析 00:00:00.1702448

方法二：利用CodeCom 00:00:23.7942144

方法三：利用Jscript+Vsa 00:00:00.1902736

方法四：利用Jsc+Dll 00:00:00.2403456

在此推荐第一种（如果要纯CSharp的话）

和第三种（代码简单，功能更多）