Question

我有一个基本的规则引擎，我建立的方式与这里建议的路线非常相似：

我已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，当我尝试用类的属性（Transaction.IsOpen）评估规则时，我得到了表达式右侧的根类型的类型但是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，很多概念都很难掌握。

编辑：这是代码（我省略了一些特定于环境的东西，以便继续关注问题）

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方案中使用的所有数据类型，因此很难确定它在哪里完全中断，但是从异常System.Func'2[Transaction,System.Boolean]' and 'System.Boolean开始，很明显左手边有一个代理接收{ {1}}并返回Transaction（bool），右侧只有Func<Transaction, bool>。

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

bool

翻译成表达式树的内容：

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

评估复杂表达式树

1 个答案: