Question

我有一个我没有创建的数据库，我无法修改。我需要运行linq查询，但我需要在where子句中传入一个变量。

表格中的列：AdvMonAM（所有位），AdvMonAM，AdvTueAM，AdvTuePM，AdvWedAM等。

var column = "Adv" + dayOfWeek + time;
var employeesOnCall = from r in db.AdvOnCalls
                      where (variable column needed here) == true
                      select r.ChartEmployee;

如果我硬编码r.AdvTueAM它完美无缺，但r.column == true或column == true却没有。我觉得这应该很容易，但我很难过。

我正在努力寻找在一天中给定时间待命的员工。

Answer 1

糟糕的数据库设计导致了一个糟糕的解决方案：

where ( (column == "AdvMonAM" && AdvMonAM == 1)
        || (column == "AdvMonPM" && AdvMonPM == 1)
      )

依此类推，对于每个字段，您都会添加一个条件。（我不确定返回的数据类型。如果bool，请删除== 1）

Answer 2

如果数据库中的列不能为空：

string column = "Adv" + dayOfWeek + time;
var employeesOnCall = from r in db.AdvOnCalls
                      where (bool)r[column]
                      select r.ChartEmployee;

或：

string column = "Adv" + dayOfWeek + time;
var employeesOnCall = from r in db.AdvOnCalls
                      where r.Field<bool>(column)
                      select r.ChartEmployee;

如果column的值可以为null：

string column = "Adv" + dayOfWeek + time;
var employeesOnCall = from r in db.AdvOnCalls
                      where r.Field<bool?>(column)==true
                      select r.ChartEmployee;

Answer 3

您可以使用Dynamic LINQ来执行以下操作：

var column = "Adv" + dayOfWeek + time;
var employeesOnCall = db.AdvOnCalls
    .Where (column + " = 1")
    .Select(x => x.ChartEmployee);

...等

Answer 4

您可以构建表达式树来表示您想要手动执行的条件，而不是像您尝试的那样使用lambda：

public static IQueryable<T> WhereEquals<T>(
    this IQueryable<T> query,
    string property,
    object valueToCompare)
{
    var param = Expression.Parameter(typeof(T));
    var body = Expression.Equal(
        Expression.Property(param, property),
        Expression.Constant(valueToCompare));
    var lambda = Expression.Lambda<Func<T, bool>>(body, param);
    return query.Where(lambda);
}

这允许你写：

var employeesOnCall = db.AdvOnCalls.WhereEquals(column, true)
    .Select(adv => adv.ChartEmployee);

Answer 5

一个选项

var column = "Adv" + dayOfWeek + time;
var employeesOnCall = from r in db.AdvOnCalls
                      where GetPropByName(r, column)
                      select r.ChartEmployee;

使用此功能：

bool GetPropByName(AdvOnCall item, string column)
{
   return (bool)item.GetType().GetProperty(column).GetValue(item, null);
}

更好的选择:)：

var employeesOnCall = from r in db.AdvOnCalls
                      where columnSwitch(r, dayOfWeek, time) 
                      select r.ChartEmployee;

使用此功能：

bool columnSwitch(AdvOnCall item, string dayOfWeek, string time)
{
    if (time == "AM")
        if (dayOfWeek == "Mon")
            return item.AdvMonAM;
        else if (dayOfWeek == "Tue")
            return item.AdvTueAM;
        ...
     else // (time == "PM")
        if (dayOfWeek == "Mon")
            return item.AdvMonPM;
        else if (dayOfWeek == "Tue")
            return item.AdvTuePM;
        ...
}

LINQ where子句中的可变条件

5 个答案: