评估复杂的表达式树

Question

我有一个基本规则引擎，我构建的方式与此处建议的路线非常相似：

How to implement a rule engine?

我已经根据进一步的要求对其进行了扩展，现在我需要评估复杂的类，例如

EvaluateRule("Transaction.IsOpen", "Equals", "true")  

最基本形式的代码是：

var param = inputMessageType;
left = Expression.Property(param, memberName);
tProp = typeof(T).GetProperty(r.MemberName).PropertyType;
right = Expression.Constant(Convert.ChangeType(r.TargetValue, tProp));
return Expression.MakeBinary(tBinary, left, right);    

为了评估复杂的类，我使用了类似于这里的方法：

https://stackoverflow.com/questions/16544672/dynamically-evaluating-a-property-string-with-expressions

我遇到的问题是，当我尝试使用类 (Transaction.IsOpen) 的属性评估规则时，我使用表达式右侧的根类型的类型得到它，但类型表达式左侧的复杂对象。

这会导致错误：

System.InvalidOperationException: The binary operator Equal is not defined for the types 'System.Func`2[Transaction,System.Boolean]' and 'System.Boolean'.

我该如何克服这个问题？我不是使用表达式树的专家，当示例偏离标准文档时，许多概念被证明难以掌握。

编辑：这是代码（我省略了一些特定于环境的内容，以便关注问题）

public Actions EvaluateRulesFromMessage(ClientEventQueueMessage message)
    {            
        var ruleGroups = _ruleRepository.GetRuleList();

    var actions = new Actions();

    foreach (var ruleGroup  in ruleGroups)
    {
        if (message.MessageType == "UI_UPDATE")
        {
            // clean up json object
            JObject dsPayload = (JObject.Parse(message.Payload));

            var msgParams = JsonConvert.DeserializeObject<UiTransactionUpdate>(message.Payload);                    
            msgParams.RulesCompleted = msgParams.RulesCompleted ?? new List<int>(); 

            var conditionsMet = false;
            // process the rules filtering out the rules that have already been evaluated                    
            var filteredRules = ruleGroup.Rules.Where(item =>
                !msgParams.RulesCompleted.Any(r => r.Equals(item.Id)));                    

            foreach (var rule in filteredRules)                                                
            {                        
                Func<UiTransactionUpdate, bool> compiledRule = CompileRule<UiTransactionUpdate>(rule, msgParams);
                if (compiledRule(msgParams))
                {
                    conditionsMet = true;

                }
                else
                {
                    conditionsMet = false;
                    break;
                }                        

            }
            if (conditionsMet) 
            {                        
                actions = AddAction(message, ruleGroup);
                break;
            }
        }
    }                
    return actions;
}

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

static Expression Eval(object root, string propertyString, out Type tProp)
{
    Type type = null;
    var propertyNames = propertyString.Split('.');
    ParameterExpression param = Expression.Parameter(root.GetType());
    Expression property = param;
    string propName = "";
    foreach (var prop in propertyNames)
    {                           
        property = MemberExpression.PropertyOrField(property, prop);
        type = property.Type;
        propName = prop;
    }

    tProp = Type.GetType(type.UnderlyingSystemType.AssemblyQualifiedName);

    var param2 = MemberExpression.Parameter(tProp);

    var e = Expression.Lambda(property, param);

    return e;
}

static Expression BuildExpr<T>(Rule r, ParameterExpression param, UiTransactionUpdate msg)
{
    Expression left;
    Type tProp;
    string memberName = r.MemberName;
    if (memberName.Contains("."))
    {
        left = Eval(msg, memberName, out tProp);            
    }
    else
    {
        left = Expression.Property(param, memberName);
        tProp = typeof(T).GetProperty(r.MemberName).PropertyType;
    }

    ExpressionType tBinary;            
    if (ExpressionType.TryParse(r.Operator, out tBinary))
    {
        Expression right=null;
        switch (r.ValueType)  ///todo: this needs to be refactored to be type independent
        {
            case TargetValueType.Value:
                right = Expression.Constant(Convert.ChangeType(r.TargetValue, tProp));
                break;                    
        }
        // use a binary operation ie true/false
        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);
    }
}

Answer 1

示例代码并未涵盖您的场景中使用的所有数据类型，因此很难准确判断它在哪里中断，但从异常 System.Func'2[Transaction,System.Boolean]' and 'System.Boolean 可以明显看出，在左侧您有一个接受 Transaction 的委托并返回bool (Func<Transaction, bool>)，而右手边只有bool。

无法将Func<Transaction, bool> 与bool 进行比较，但可以调用函数并比较其结果：

Func<Transaction, bool> func = ...;
bool comparand = ...;
Transaction transaction = ...;
if (func(transaction) == comparand) { ... }

翻译成表达式树的内容：

Expression funcExpression = ... /*LambdaExpression<Func<Transaction,bool>>*/;
Expression comparandExpression = Expression.Constant(true);
Expression transactionArg = /*e.g.*/Expression.Constant(transaction);
Expression funcResultExpression = Expression.Call(funcExpression, "Invoke", null, transactionArg);
Expression equalityTestExpression = Expression.Equal(funcResultExpression, comparandExpression);