使用表达式树在linq中创建实体的自定义订单

问题描述：

我有一个已映射的表，但是在编译后，可以在该表中添加或删除其他列.我正在尝试提出一个linq查询，该查询将考虑这些新列.在这种情况下，我想按那些动态列之一进行排序.这就是我到目前为止所拥有的.

I have a table that's mapped, but after compile additional columns can be added or removed from the table. I'm trying to come up with a linq query that will take those new columns into account. In this scenario, I want to order by one of those dynamic columns. This is what I have so far.

var queryableData = dc.wf_task_ext_attributes.AsQueryable();
ParameterExpression pe = Expression.Parameter(typeof(DateTime), "ExtValue105");

// The next line is where it fails
MethodCallExpression orderByCallExpression = Expression.Call(
      typeof(Queryable),
       "OrderBy",
       new Type[] { queryableData.ElementType, queryableData.ElementType },
       queryableData.Expression,
       Expression.Lambda<Func<DateTime, DateTime>>(pe, new ParameterExpression[] { pe }));

IQueryable<string> results = queryableData.Provider.CreateQuery<string>
                         (orderByCallExpression);

失败，并显示以下消息:

It's failing with the following message:

类型为'System.Linq.Queryable'的通用方法'OrderBy'与提供的类型实参和实参不兼容.如果该方法是非泛型的，则不应该提供任何类型的参数.

No generic method 'OrderBy' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

我在做什么错了?

答

您的代码尝试创建类似Queryable.OrderBy(queryableData.Expression, ExtValue105 => ExtValue105)的代码.我不知道您为什么希望它能起作用.

Your code tries to create something like Queryable.OrderBy(queryableData.Expression, ExtValue105 => ExtValue105). I have no idea why would you expect that to work.

如果我正确理解了您的问题，则需要动态创建一个类似attribute => attribute.ExtValue105的表达式，然后可以使用它来调用OrderBy().

If I understand your question correctly, you need to dynamically create an expression like attribute => attribute.ExtValue105 and then you can use that to call OrderBy().

代码可能看起来像这样(假设queryableData是IQueryable<Attribute>):

The code could look something like this (assuming queryableData is IQueryable<Attribute>):

var parameter = Expression.Parameter(typeof(Attribute), "attribute");
var property = Expression.Property(parameter, "ExtValue105");
var lambda = Expression.Lambda(property, parameter);

IQueryable<Attribute> results =
    Queryable.OrderBy(queryableData, (dynamic)lambda);

您可以手动使用queryableData.Provider.CreateQuery()来避免dynamic调用，但这会更加复杂.

You could use queryableData.Provider.CreateQuery() manually to avoid the dynamic call, but that would be more complicated.

使用表达式树在linq中创建实体的自定义订单

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