c# - Expression.PropertyOrField 为字符串属性返回 "not a member of type ' System.String'"

Question

我正在尝试为规则引擎动态构建表达式并且一切进展顺利，直到我尝试允许将嵌套类型和属性指定为操作数。示例:

表达式生成器

public Expression BuildExpression<T>(string propertyName, Enums.Operator ruleOperator, object value, ParameterExpression parameterExpression)
    {
        ExpressionType expressionType = new ExpressionType();
        Expression body = parameterExpression;

        foreach (var member in propertyName.Split('.'))
        {
            body = MemberExpression.Property(body, member);
        }

        var leftOperand = MemberExpression.PropertyOrField(body, propertyName);
        var rightOperand = Expression.Constant(Convert.ChangeType(value, value.GetType()));
        FieldInfo fieldInfo = expressionType.GetType().GetField(Enum.GetName(typeof(Enums.Operator), ruleOperator));
        var expressionTypeValue = (ExpressionType)fieldInfo.GetValue(ruleOperator);

        return CastBuildExpression(expressionTypeValue, value, leftOperand, rightOperand);
    }

规则引擎

public Func<T, bool>[] CombineRules<T>(Criterion[] criteria)
    {
        List<Func<T, bool>> list = new List<Func<T, bool>>();
        foreach (var criterion in criteria)
        {                    
            ExpressionBuilder expressionBuilder = new ExpressionBuilder();
            var param = Expression.Parameter(typeof (T));
            Expression expression = expressionBuilder.BuildExpression<T>(criterion.PropertyName,
                    criterion.Operator_, criterion.Value, param);
            Func<T, bool> func = Expression.Lambda<Func<T, bool>>(expression, param).Compile();
            list.Add(func);
        }

        return list.ToArray();
    }

标准

public class Criterion
{
    private bool propertySet;
    public string PropertyName { get; set; }
    public Enums.Operator Operator_ { get; set; }
    public object Value { get; set; }

成员模型

public class MemberModel
{
    public string UserName{ get; set; }
    public PersonalDetailsModel PersonalDetails {get; set;}
}

PersonalDetailsModel

    public class PersonalDetailsModel
{
    public int PersonalDetailsId { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public string Middlename { get; set; }
    public string DateOfBirth { get; set; }
    public string GenderType { get; set; }
    public string SalutationType { get; set; }
}

当我尝试将嵌套属性作为左操作数传递时出现问题，即 PropertyName="PersonalDetails.FirstName"到 ruleEngine.CombineRules(criteria.ToArray());

我得到“‘PersonalDetails.Firstname’不是‘System.String’类型的成员”，尽管它显然是这样。我已经坚持了一段时间，知道是什么原因造成的吗？

如有任何帮助，我们将不胜感激。

Answer 1

这里

    Expression body = parameterExpression;

    foreach (var member in propertyName.Split('.'))
    {
        body = MemberExpression.Property(body, member);
    }

你已经处理了属性路径，所以这个

  var leftOperand = MemberExpression.PropertyOrField(body, propertyName);

没有意义，是异常的来源。在您的示例中，body 包含类似 p.PersonalDetails.FirstName (String) 的内容，上面的行试图构建类似 p.PersonalDetails.FirstName.PersonalDetails 的内容.FirstName.

改用 var leftOperand = body;。

您可以使用简单的方式缩短整个属性路径处理

var leftOperand = propertyName.Split('.')
    .Aggregate((Expression)parameterExpression, Expression.PropertyOrField);