如果你只有少量的属性和操作符，那么阻力最小的方法就是将所有检查都编码为如下的特殊情况:

public bool ApplyRules(List<Rule> rules, User user)
{
    foreach (var rule in rules)
    {
        IComparable value = null;
        object limit = null;
        if (rule.objectProperty == "age")
        {
            value = user.age;
            limit = Convert.ToInt32(rule.TargetValue);
        }
        else if (rule.objectProperty == "username")
        {
            value = user.username;
            limit = rule.TargetValue;
        }
        else
            throw new InvalidOperationException("invalid property");

        int result = value.CompareTo(limit);

        if (rule.ComparisonOperator == "equal")
        {
            if (!(result == 0)) return false;
        }
        else if (rule.ComparisonOperator == "greater_than")
        {
            if (!(result > 0)) return false;
        }
        else
            throw new InvalidOperationException("invalid operator");
    }
    return true;
}

如果您有很多属性，您可能会发现表驱动方法更合适。在这种情况下，您将创建一个静态Dictionary，将属性名称映射到匹配的委托，例如Func<User，对象>。

如果在编译时不知道属性的名称，或者希望避免每个属性的特殊情况，并且不想使用表方法，则可以使用反射来获取属性。例如:

var value = user.GetType().GetProperty("age").GetValue(user, null);

但是由于TargetValue可能是一个字符串，如果有必要，您需要注意从规则表进行类型转换。

反思是最通用的答案。你有三列数据，它们需要以不同的方式处理:

字段名。反射是从编码字段名中获取值的方法。 比较运算符。它们的数量应该是有限的，因此case语句应该最容易处理它们。特别是其中一些(有一个或多个)稍微复杂一些。 你的比较值。如果这些都是直值，那么这很容易，尽管您需要将多个条目分开。但是，如果它们也是字段名，也可以使用反射。

我会采取这样的方法:

    var value = user.GetType().GetProperty("age").GetValue(user, null);
    //Thank you Rick! Saves me remembering it;
    switch(rule.ComparisonOperator)
        case "equals":
             return EqualComparison(value, rule.CompareTo)
        case "is_one_or_more_of"
             return IsInComparison(value, rule.CompareTo)

等等。

它为您提供了添加更多比较选项的灵活性。这也意味着您可以在compare方法中编写您可能想要的任何类型验证，并使它们尽可能复杂。这里还有一个选项，让CompareTo作为另一行的递归调用来计算，或者作为一个字段值，可以这样做:

             return IsInComparison(value, EvaluateComparison(rule.CompareTo))

这一切都取决于未来的可能性....

那么使用扩展方法的面向数据类型的方法呢?

public static class RoleExtension
{
    public static bool Match(this Role role, object obj )
    {
        var property = obj.GetType().GetProperty(role.objectProperty);
        if (property.PropertyType == typeof(int))
        {
            return ApplyIntOperation(role, (int)property.GetValue(obj, null));
        }
        if (property.PropertyType == typeof(string))
        {
            return ApplyStringOperation(role, (string)property.GetValue(obj, null));
        }
        if (property.PropertyType.GetInterface("IEnumerable<string>",false) != null)
        {
            return ApplyListOperation(role, (IEnumerable<string>)property.GetValue(obj, null));
        }
        throw new InvalidOperationException("Unknown PropertyType");
    }

    private static bool ApplyIntOperation(Role role, int value)
    {
        var targetValue = Convert.ToInt32(role.TargetValue);
        switch (role.ComparisonOperator)
        {
            case "greater_than":
                return value > targetValue;
            case "equal":
                return value == targetValue;
            //...
            default:
                throw new InvalidOperationException("Unknown ComparisonOperator");
        }
    }

    private static bool ApplyStringOperation(Role role, string value)
    {
        //...
        throw new InvalidOperationException("Unknown ComparisonOperator");
    }

    private static bool ApplyListOperation(Role role, IEnumerable<string> value)
    {
        var targetValues = role.TargetValue.Split(' ');
        switch (role.ComparisonOperator)
        {
            case "hasAtLeastOne":
                return value.Any(v => targetValues.Contains(v));
                //...
        }
        throw new InvalidOperationException("Unknown ComparisonOperator");
    }
}

然后你可以这样评价:

var myResults = users.Where(u => roles.All(r => r.Match(u)));

这段代码将规则编译成快速可执行的代码(使用表达式树)，不需要任何复杂的switch语句:

(编辑:具有通用方法的完整工作示例)

public Func<User, bool> CompileRule(Rule r)
{
    var paramUser = Expression.Parameter(typeof(User));
    Expression expr = BuildExpr(r, paramUser);
    // build a lambda function User->bool and compile it
    return Expression.Lambda<Func<User, bool>>(expr, paramUser).Compile();
}

你可以这样写:

List<Rule> rules = new List<Rule> {
    new Rule ("Age", "GreaterThan", "21"),
    new Rule ( "Name", "Equal", "John"),
    new Rule ( "Tags", "Contains", "C#" )
};

// compile the rules once
var compiledRules = rules.Select(r => CompileRule(r)).ToList();

public bool MatchesAllRules(User user)
{
    return compiledRules.All(rule => rule(user));
}

下面是BuildExpr的实现:

Expression BuildExpr(Rule r, ParameterExpression param)
{
    var left = MemberExpression.Property(param, r.MemberName);
    var tProp = typeof(User).GetProperty(r.MemberName).PropertyType;
    ExpressionType tBinary;
    // is the operator a known .NET operator?
    if (ExpressionType.TryParse(r.Operator, out tBinary)) {
        var right = Expression.Constant(Convert.ChangeType(r.TargetValue, tProp));
        // use a binary operation, e.g. 'Equal' -> 'u.Age == 21'
        return Expression.MakeBinary(tBinary, left, right);
    } else {
        var method = tProp.GetMethod(r.Operator);
        var tParam = method.GetParameters()[0].ParameterType;
        var right = Expression.Constant(Convert.ChangeType(r.TargetValue, tParam));
        // use a method call, e.g. 'Contains' -> 'u.Tags.Contains(some_tag)'
        return Expression.Call(left, method, right);
    }
}

注意，我使用了'GreaterThan'而不是'greater_than'等等——这是因为'GreaterThan'是操作符的. net名称，因此我们不需要任何额外的映射。

如果你需要自定义名称，你可以创建一个非常简单的字典，在编译规则之前翻译所有操作符:

var nameMap = new Dictionary<string, string> {
    { "greater_than", "GreaterThan" },
    { "hasAtLeastOne", "Contains" }
};

为了简单起见，代码使用User类型。您可以用泛型类型T替换User，从而为任何类型的对象提供泛型规则编译器。此外，代码应该处理错误，比如未知的操作符名称。

请注意，即使在引入表达式树API之前，使用Reflection.Emit也可以动态生成代码。LambdaExpression.Compile()方法使用Reflection。在被子下发射(您可以使用ILSpy看到这一点)。

下面是一些按原样编译并完成这项工作的代码。 基本上使用两个字典，一个包含从操作符名称到布尔函数的映射，另一个包含从User类型的属性名称到用于调用属性getter(如果是公共的)的PropertyInfos的映射。 将User实例和表中的三个值传递给静态Apply方法。

class User
{
    public int Age { get; set; }
    public string UserName { get; set; }
}

class Operator
{
    private static Dictionary<string, Func<object, object, bool>> s_operators;
    private static Dictionary<string, PropertyInfo> s_properties;
    static Operator()
    {
        s_operators = new Dictionary<string, Func<object, object, bool>>();
        s_operators["greater_than"] = new Func<object, object, bool>(s_opGreaterThan);
        s_operators["equal"] = new Func<object, object, bool>(s_opEqual);

        s_properties = typeof(User).GetProperties().ToDictionary(propInfo => propInfo.Name);
    }

    public static bool Apply(User user, string op, string prop, object target)
    {
        return s_operators[op](GetPropValue(user, prop), target);
    }

    private static object GetPropValue(User user, string prop)
    {
        PropertyInfo propInfo = s_properties[prop];
        return propInfo.GetGetMethod(false).Invoke(user, null);
    }

    #region Operators

    static bool s_opGreaterThan(object o1, object o2)
    {
        if (o1 == null || o2 == null || o1.GetType() != o2.GetType() || !(o1 is IComparable))
            return false;
        return (o1 as IComparable).CompareTo(o2) > 0;
    }

    static bool s_opEqual(object o1, object o2)
    {
        return o1 == o2;
    }

    //etc.

    #endregion

    public static void Main(string[] args)
    {
        User user = new User() { Age = 16, UserName = "John" };
        Console.WriteLine(Operator.Apply(user, "greater_than", "Age", 15));
        Console.WriteLine(Operator.Apply(user, "greater_than", "Age", 17));
        Console.WriteLine(Operator.Apply(user, "equal", "UserName", "John"));
        Console.WriteLine(Operator.Apply(user, "equal", "UserName", "Bob"));
    }
}

我构建了一个规则引擎，它采用的方法与您在问题中概述的方法不同，但我认为您会发现它比您当前的方法灵活得多。

您当前的方法似乎集中在单个实体“User”上，而您的持久规则标识“propertyname”、“operator”和“value”。相反，我的模式将谓词(Func<T, bool>)的c#代码存储在数据库中的“表达式”列中。在当前的设计中，使用代码生成，我正在从数据库中查询“规则”并编译具有“规则”类型的程序集，每个程序集都具有“测试”方法。下面是实现每个规则的接口的签名:

public interface IDataRule<TEntity> 
{
    /// <summary>
    /// Evaluates the validity of a rule given an instance of an entity
    /// </summary>
    /// <param name="entity">Entity to evaluate</param>
    /// <returns>result of the evaluation</returns>
    bool Test(TEntity entity);
    /// <summary>
    /// The unique indentifier for a rule.
    /// </summary>
     int RuleId { get; set; }
    /// <summary>
    /// Common name of the rule, not unique
    /// </summary>
     string RuleName { get; set; }
    /// <summary>
    /// Indicates the message used to notify the user if the rule fails
    /// </summary>
     string ValidationMessage { get; set; }   
     /// <summary>
     /// indicator of whether the rule is enabled or not
     /// </summary>
     bool IsEnabled { get; set; }
    /// <summary>
    /// Represents the order in which a rule should be executed relative to other rules
    /// </summary>
     int SortOrder { get; set; }
}

当应用程序第一次执行时，“Expression”被编译为“Test”方法的主体。如您所见，表中的其他列也作为规则上的一级属性显示，以便开发人员可以灵活地创建用户如何获得失败或成功通知的体验。

在应用程序中只生成一次内存程序集，在计算规则时不必使用反射，从而获得性能提升。在运行时检查表达式，因为如果属性名拼写错误，程序集将不能正确生成。

创建内存程序集的机制如下:

从DB加载规则 遍历规则和for-each，使用StringBuilder和一些字符串连接编写代表从IDataRule继承的类的Text 使用CodeDOM编译——更多信息

这实际上非常简单，因为对于大多数代码来说，这些代码是构造函数中的属性实现和值初始化。除此之外，唯一的其他代码是表达式。 注意:由于CodeDOM的限制，你的表达式必须是。net 2.0(没有lambdas或其他c# 3.0特性)。

下面是一些示例代码。

sb.AppendLine(string.Format("\tpublic class {0} : SomeCompany.ComponentModel.IDataRule<{1}>", className, typeName));
            sb.AppendLine("\t{");
            sb.AppendLine("\t\tprivate int _ruleId = -1;");
            sb.AppendLine("\t\tprivate string _ruleName = \"\";");
            sb.AppendLine("\t\tprivate string _ruleType = \"\";");
            sb.AppendLine("\t\tprivate string _validationMessage = \"\";");
            /// ... 
            sb.AppendLine("\t\tprivate bool _isenabled= false;");
            // constructor
            sb.AppendLine(string.Format("\t\tpublic {0}()", className));
            sb.AppendLine("\t\t{");
            sb.AppendLine(string.Format("\t\t\tRuleId = {0};", ruleId));
            sb.AppendLine(string.Format("\t\t\tRuleName = \"{0}\";", ruleName.TrimEnd()));
            sb.AppendLine(string.Format("\t\t\tRuleType = \"{0}\";", ruleType.TrimEnd()));                
            sb.AppendLine(string.Format("\t\t\tValidationMessage = \"{0}\";", validationMessage.TrimEnd()));
            // ...
            sb.AppendLine(string.Format("\t\t\tSortOrder = {0};", sortOrder));                

            sb.AppendLine("\t\t}");
            // properties
            sb.AppendLine("\t\tpublic int RuleId { get { return _ruleId; } set { _ruleId = value; } }");
            sb.AppendLine("\t\tpublic string RuleName { get { return _ruleName; } set { _ruleName = value; } }");
            sb.AppendLine("\t\tpublic string RuleType { get { return _ruleType; } set { _ruleType = value; } }");

            /// ... more properties -- omitted

            sb.AppendLine(string.Format("\t\tpublic bool Test({0} entity) ", typeName));
            sb.AppendLine("\t\t{");
            // #############################################################
            // NOTE: This is where the expression from the DB Column becomes
            // the body of the Test Method, such as: return "entity.Prop1 < 5"
            // #############################################################
            sb.AppendLine(string.Format("\t\t\treturn {0};", expressionText.TrimEnd()));
            sb.AppendLine("\t\t}");  // close method
            sb.AppendLine("\t}"); // close Class

除此之外，我做了一个我称为“DataRuleCollection”的类，它实现了ICollection>。这使我能够创建一个“TestAll”功能和一个索引器，用于按名称执行特定的规则。下面是这两个方法的实现。

    /// <summary>
    /// Indexer which enables accessing rules in the collection by name
    /// </summary>
    /// <param name="ruleName">a rule name</param>
    /// <returns>an instance of a data rule or null if the rule was not found.</returns>
    public IDataRule<TEntity, bool> this[string ruleName]
    {
        get { return Contains(ruleName) ? list[ruleName] : null; }
    }
    // in this case the implementation of the Rules Collection is: 
    // DataRulesCollection<IDataRule<User>> and that generic flows through to the rule.
    // there are also some supporting concepts here not otherwise outlined, such as a "FailedRules" IList
    public bool TestAllRules(User target) 
    {
        rules.FailedRules.Clear();
        var result = true;

        foreach (var rule in rules.Where(x => x.IsEnabled)) 
        {

            result = rule.Test(target);
            if (!result)
            {

                rules.FailedRules.Add(rule);
            }
        }

        return (rules.FailedRules.Count == 0);
    }

更多代码:有一个与代码生成相关的代码请求。我将这个功能封装在一个名为“RulesAssemblyGenerator”的类中，我在下面列出了这个类。

namespace Xxx.Services.Utils
    {
        public static class RulesAssemblyGenerator
        {
            static List<string> EntityTypesLoaded = new List<string>();

            public static void Execute(string typeName, string scriptCode)
            {
                if (EntityTypesLoaded.Contains(typeName)) { return; } 
                // only allow the assembly to load once per entityType per execution session
                Compile(new CSharpCodeProvider(), scriptCode);
                EntityTypesLoaded.Add(typeName);
            }
            private static void Compile(CodeDom.CodeDomProvider provider, string source)
            {
                var param = new CodeDom.CompilerParameters()
                {
                    GenerateExecutable = false,
                    IncludeDebugInformation = false,
                    GenerateInMemory = true
                };
                var path = System.Reflection.Assembly.GetExecutingAssembly().Location;
                var root_Dir = System.IO.Path.Combine(System.AppDomain.CurrentDomain.BaseDirectory, "Bin");
                param.ReferencedAssemblies.Add(path);
                // Note: This dependencies list are included as assembly reference and they should list out all dependencies
                // That you may reference in your Rules or that your entity depends on.
                // some assembly names were changed... clearly.
                var dependencies = new string[] { "yyyyyy.dll", "xxxxxx.dll", "NHibernate.dll", "ABC.Helper.Rules.dll" };
                foreach (var dependency in dependencies)
                {
                    var assemblypath = System.IO.Path.Combine(root_Dir, dependency);
                    param.ReferencedAssemblies.Add(assemblypath);
                }
                // reference .NET basics for C# 2.0 and C#3.0
                param.ReferencedAssemblies.Add(@"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\System.dll");
                param.ReferencedAssemblies.Add(@"C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll");
                var compileResults = provider.CompileAssemblyFromSource(param, source);
                var output = compileResults.Output;
                if (compileResults.Errors.Count != 0)
                {
                    CodeDom.CompilerErrorCollection es = compileResults.Errors;
                    var edList = new List<DataRuleLoadExceptionDetails>();
                    foreach (CodeDom.CompilerError s in es)
                        edList.Add(new DataRuleLoadExceptionDetails() { Message = s.ErrorText, LineNumber = s.Line });
                    var rde = new RuleDefinitionException(source, edList.ToArray());
                    throw rde;
                }
            }
        }
    }

如果对进一步的代码示例有任何其他问题、评论或请求，请告诉我。

使用工作流规则引擎怎么样?

您可以在没有工作流的情况下执行Windows工作流规则 请参阅Guy Burstein的博客:http://blogs.microsoft.co.il/blogs/bursteg/archive/2006/10/11/RuleExecutionWithoutWorkflow.aspx

以编程方式创建规则，请参阅Stephen Kaufman的WebLog

http://blogs.msdn.com/b/skaufman/archive/2006/05/15/programmatically-create-windows-workflow-rules.aspx

尽管回答“如何实现规则引擎?”(在c#中)的问题是按顺序执行给定的一组规则，这通常被认为是naïve实现(并不意味着它不起作用:-)

在你的情况下，它似乎“足够好”，因为你的问题似乎更多的是“如何按顺序运行一组规则”，如果你有最新的c#版本，lambda/expression树(Martin的答案)肯定是最优雅的方式。

然而，对于更高级的场景，这里有一个到Rete算法的链接，它实际上在许多商业规则引擎系统中实现，还有一个到NRuler的链接，它是该算法在c#中的实现。

马丁的回答很好。实际上，我做了一个规则引擎，与他的想法相同。我很惊讶，这几乎是一样的。我加入了他的一些代码，以在一定程度上改进它。尽管我已经让它处理更复杂的规则。

你可以看看Yare。网

或者在Nuget下载

我为和或规则之间添加了实现 我添加了RuleExpression类，表示树的根，它可以是叶子，可以是简单规则，也可以是和，或者二进制表达式，因为它们没有规则和表达式:

public class RuleExpression
{
    public NodeOperator NodeOperator { get; set; }
    public List<RuleExpression> Expressions { get; set; }
    public Rule Rule { get; set; }

    public RuleExpression()
    {

    }
    public RuleExpression(Rule rule)
    {
        NodeOperator = NodeOperator.Leaf;
        Rule = rule;
    }

    public RuleExpression(NodeOperator nodeOperator, List<RuleExpression> expressions, Rule rule)
    {
        this.NodeOperator = nodeOperator;
        this.Expressions = expressions;
        this.Rule = rule;
    }
}


public enum NodeOperator
{
    And,
    Or,
    Leaf
}

我有另一个类编译ruleExpression到一个Func<T, bool>:

 public static Func<T, bool> CompileRuleExpression<T>(RuleExpression ruleExpression)
    {
        //Input parameter
        var genericType = Expression.Parameter(typeof(T));
        var binaryExpression = RuleExpressionToOneExpression<T>(ruleExpression, genericType);
        var lambdaFunc = Expression.Lambda<Func<T, bool>>(binaryExpression, genericType);
        return lambdaFunc.Compile();
    }

    private static Expression RuleExpressionToOneExpression<T>(RuleExpression ruleExpression, ParameterExpression genericType)
    {
        if (ruleExpression == null)
        {
            throw new ArgumentNullException();
        }
        Expression finalExpression;
        //check if node is leaf
        if (ruleExpression.NodeOperator == NodeOperator.Leaf)
        {
            return RuleToExpression<T>(ruleExpression.Rule, genericType);
        }
        //check if node is NodeOperator.And
        if (ruleExpression.NodeOperator.Equals(NodeOperator.And))
        {
            finalExpression = Expression.Constant(true);
            ruleExpression.Expressions.ForEach(expression =>
            {
                finalExpression = Expression.AndAlso(finalExpression, expression.NodeOperator.Equals(NodeOperator.Leaf) ? 
                    RuleToExpression<T>(expression.Rule, genericType) :
                    RuleExpressionToOneExpression<T>(expression, genericType));
            });
            return finalExpression;
        }
        //check if node is NodeOperator.Or
        else
        {
            finalExpression = Expression.Constant(false);
            ruleExpression.Expressions.ForEach(expression =>
            {
                finalExpression = Expression.Or(finalExpression, expression.NodeOperator.Equals(NodeOperator.Leaf) ?
                    RuleToExpression<T>(expression.Rule, genericType) :
                    RuleExpressionToOneExpression<T>(expression, genericType));
            });
            return finalExpression;

        }      
    }      

    public static BinaryExpression RuleToExpression<T>(Rule rule, ParameterExpression genericType)
    {
        try
        {
            Expression value = null;
            //Get Comparison property
            var key = Expression.Property(genericType, rule.ComparisonPredicate);
            Type propertyType = typeof(T).GetProperty(rule.ComparisonPredicate).PropertyType;
            //convert case is it DateTimeOffset property
            if (propertyType == typeof(DateTimeOffset))
            {
                var converter = TypeDescriptor.GetConverter(propertyType);
                value = Expression.Constant((DateTimeOffset)converter.ConvertFromString(rule.ComparisonValue));
            }
            else
            {
                value = Expression.Constant(Convert.ChangeType(rule.ComparisonValue, propertyType));
            }
            BinaryExpression binaryExpression = Expression.MakeBinary(rule.ComparisonOperator, key, value);
            return binaryExpression;
        }
        catch (FormatException)
        {
            throw new Exception("Exception in RuleToExpression trying to convert rule Comparison Value");
        }
        catch (Exception e)
        {
            throw new Exception(e.Message);
        }

    }

我已经创建了一个用dotnet编写的丰富而高性能的规则引擎包，查看此repo以获取更多信息。 一旦安装，你可以简单地使用它:

var engine = new RulesService<TestModel>(new RulesCompiler(), new LazyCache.Mocks.MockCachingService());
            
    var matchingRules = engine.GetMatchingRules(
        new TestModel { NumericField = 5 },
        new[] {
            new RulesConfig {
                Id = Guid.NewGuid(),
                RulesOperator = Rule.InterRuleOperatorType.And,
                RulesGroups = new RulesGroup[] {
                    new RulesGroup {
                        RulesOperator = Rule.InterRuleOperatorType.And,
                        Rules = new[] {
                            new Rule { 
                            ComparisonOperator = Rule.ComparisonOperatorType.Equal,
                            ComparisonValue = 5.ToString(),
                            ComparisonPredicate = nameof(TestModel.NumericField) 
                            }
                        }
                    }
                }
            }
        });

我想添加微软的规则引擎库，以供将来访问;

https://github.com/microsoft/RulesEngine