如何将字符串转换为等价的LINQ表达式树？
如何将字符串转换为等价的LINQ表达式树？
提问于 2009-05-04 18:48:00
这是原始问题的简化版本。
我有一个名为Person的类：
public class Person {
public string Name { get; set; }
public int Age { get; set; }
public int Weight { get; set; }
public DateTime FavouriteDay { get; set; }
}...and假设有一个实例：
var bob = new Person {
Name = "Bob",
Age = 30,
Weight = 213,
FavouriteDay = '1/1/2000'
}我想在我最喜欢的文本编辑器中将以下内容写成字符串...
(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3我想获取这个字符串和我的对象实例，并计算一个TRUE或FALSE -即计算对象实例上的Func。
以下是我目前的想法：
- 在ANTLR中实现了一个基本语法，以支持基本的比较和逻辑运算符。我正在考虑在这里复制Visual Basic的优先级和一些特性集：http://msdn.microsoft.com/en-us/library/fw84t893(VS.80).aspx
- Have ANTLR从提供的字符串中创建一个合适的AST。bool>
- Evaluate
- 遍历AST并使用Predicate Builder框架动态创建Func
我的问题是我是不是做得太过火了？有其他选择吗？
编辑:选择的解决方案
我决定使用Dynamic Linq Library，特别是LINQSamples中提供的动态查询类。
代码如下：
using System;
using System.Linq.Expressions;
using System.Linq.Dynamic;
namespace ExpressionParser
{
class Program
{
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
public int Weight { get; set; }
public DateTime FavouriteDay { get; set; }
}
static void Main()
{
const string exp = @"(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3";
var p = Expression.Parameter(typeof(Person), "Person");
var e = System.Linq.Dynamic.DynamicExpression.ParseLambda(new[] { p }, null, exp);
var bob = new Person
{
Name = "Bob",
Age = 30,
Weight = 213,
FavouriteDay = new DateTime(2000,1,1)
};
var result = e.Compile().DynamicInvoke(bob);
Console.WriteLine(result);
Console.ReadKey();
}
}
}结果的类型为System.Boolean，在本例中为TRUE。
非常感谢Marc Gravell。
包括System.Linq.Dynamic nuget包、documentation here
回答 7
Stack Overflow用户
回答已采纳
发布于 2009-05-04 19:34:37
dynamic linq library在这方面会有帮助吗？具体地说，我认为这是一个Where子句。如有必要，将其放入列表/数组中，以便对其调用.Where(string)！即
var people = new List<Person> { person };
int match = people.Where(filter).Any();如果没有，那么编写一个解析器(在幕后使用Expression )并不是很麻烦--我在圣诞节前的火车上写了一个类似的解析器(尽管我不认为我有源码)……
Stack Overflow用户
发布于 2010-02-08 19:04:26
另一个这样的库是Flee
我对Dynamic Linq Library和Flee进行了快速比较，发现expression "(Name == \"Johan\" AND Salary > 500) OR (Name != \"Johan\" AND Salary > 300)"的速度要快10倍
这就是你用Flee编写代码的方法。
static void Main(string[] args)
{
var context = new ExpressionContext();
const string exp = @"(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3";
context.Variables.DefineVariable("Person", typeof(Person));
var e = context.CompileDynamic(exp);
var bob = new Person
{
Name = "Bob",
Age = 30,
Weight = 213,
FavouriteDay = new DateTime(2000, 1, 1)
};
context.Variables["Person"] = bob;
var result = e.Evaluate();
Console.WriteLine(result);
Console.ReadKey();
}Stack Overflow用户
发布于 2013-11-01 19:08:15
void Main()
{
var testdata = new List<Ownr> {
//new Ownr{Name = "abc", Qty = 20}, // uncomment this to see it getting filtered out
new Ownr{Name = "abc", Qty = 2},
new Ownr{Name = "abcd", Qty = 11},
new Ownr{Name = "xyz", Qty = 40},
new Ownr{Name = "ok", Qty = 5},
};
Expression<Func<Ownr, bool>> func = Extentions.strToFunc<Ownr>("Qty", "<=", "10");
func = Extentions.strToFunc<Ownr>("Name", "==", "abc", func);
var result = testdata.Where(func.ExpressionToFunc()).ToList();
result.Dump();
}
public class Ownr
{
public string Name { get; set; }
public int Qty { get; set; }
}
public static class Extentions
{
public static Expression<Func<T, bool>> strToFunc<T>(string propName, string opr, string value, Expression<Func<T, bool>> expr = null)
{
Expression<Func<T, bool>> func = null;
try
{
var type = typeof(T);
var prop = type.GetProperty(propName);
ParameterExpression tpe = Expression.Parameter(typeof(T));
Expression left = Expression.Property(tpe, prop);
Expression right = Expression.Convert(ToExprConstant(prop, value), prop.PropertyType);
Expression<Func<T, bool>> innerExpr = Expression.Lambda<Func<T, bool>>(ApplyFilter(opr, left, right), tpe);
if (expr != null)
innerExpr = innerExpr.And(expr);
func = innerExpr;
}
catch (Exception ex)
{
ex.Dump();
}
return func;
}
private static Expression ToExprConstant(PropertyInfo prop, string value)
{
object val = null;
try
{
switch (prop.Name)
{
case "System.Guid":
val = Guid.NewGuid();
break;
default:
{
val = Convert.ChangeType(value, prop.PropertyType);
break;
}
}
}
catch (Exception ex)
{
ex.Dump();
}
return Expression.Constant(val);
}
private static BinaryExpression ApplyFilter(string opr, Expression left, Expression right)
{
BinaryExpression InnerLambda = null;
switch (opr)
{
case "==":
case "=":
InnerLambda = Expression.Equal(left, right);
break;
case "<":
InnerLambda = Expression.LessThan(left, right);
break;
case ">":
InnerLambda = Expression.GreaterThan(left, right);
break;
case ">=":
InnerLambda = Expression.GreaterThanOrEqual(left, right);
break;
case "<=":
InnerLambda = Expression.LessThanOrEqual(left, right);
break;
case "!=":
InnerLambda = Expression.NotEqual(left, right);
break;
case "&&":
InnerLambda = Expression.And(left, right);
break;
case "||":
InnerLambda = Expression.Or(left, right);
break;
}
return InnerLambda;
}
public static Expression<Func<T, TResult>> And<T, TResult>(this Expression<Func<T, TResult>> expr1, Expression<Func<T, TResult>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, TResult>>(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
}
public static Func<T, TResult> ExpressionToFunc<T, TResult>(this Expression<Func<T, TResult>> expr)
{
var res = expr.Compile();
return res;
}
}LinqPad具有Dump()方法
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原文链接：
https://stackoverflow.com/questions/821365
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