Question

我有一个名为User的类，其中有一个字段，用户可以发送任何数据类型。目前我们发送int，double和string。

由于字段是动态的，因此字段名称可以是我们使用表达式树的任何内容。我现在面临的问题是如果select字段或字段值中的字段为null，则会抛出错误。

以下是我的代码：

Expression<Func<User, bool>> comparison = null;

if (Value.GetType() == typeof(int))
    comparison = EvaluateRules<int>(attributeName);
else if (Value.GetType() == typeof(double))
    comparison = EvaluateRules<double>(attributeName);
else if (Value.GetType() == typeof(string))
    comparison = EvaluateRules<string>(attributeName);


private Expression<Func<User, bool>> EvaluateRules<T>(string attributeName)
{
    var attributeParameter = Expression.Parameter(typeof(User), "user");
    Expression<Func<User, bool>> comparison = null;
    var parseMethod = typeof(T).GetMethod("Parse", new[] { typeof(string) });

    switch (policyOperator)
    {
        case Operator.GreaterThanOrEqual:
            if (Value.GetType() != typeof(string))
                comparison = Expression.Lambda<Func<User, bool>>(
                                    Expression.GreaterThanOrEqual(
                                        Expression.Call(parseMethod, Expression.Property(attributeParameter, attributeName)),
                                        Expression.Constant(Value)),
                                        attributeParameter);
            break;
    }

    return comparison;
}

resultUsers = from user in users.AsQueryable().Where(comparison) select user

任何线索??

感谢您的时间。

Answer 1

感谢您的回复。我能够用下面的代码解决它。发布它，以防它帮助某人。

ParameterExpression attributeParameter = Expression.Parameter(typeof(User), "user");
MemberExpression attribute = Expression.Property(attributeParameter, attributeName);
BinaryExpression nullCheck = Expression.NotEqual(attribute, Expression.Constant(null, typeof(object)));
BinaryExpression condition = null;

 switch (policyOperator)
{
    case Operator.GreaterThanOrEqual:
    if (Value.GetType() != typeof(string))
        condition = Expression.GreaterThanOrEqual(Expression.Call(parseMethod, 
                        Expression.Property(attributeParameter, attributeName)), 
                            Expression.Constant(Value));

    .
    .
}

return Expression.Lambda<Func<User, bool>>(Expression.AndAlso(nullCheck, condition), attributeParameter);

C＃Null检查表达式树

1 个答案: