如何修改Expression>的类型参数？

我有一个以下的例子：

Expression<Func<IRequiredDate, bool>>

我希望将其转换为以下实例，以便它可以用于在实体框架中运行查询：

Expression<Func<TModel, bool>>

这将允许我对任何实现 IRequiredDate 的模型使用通用过滤查询，例如：

// In some repository function:
var query = DbContext.Set<Order>()
     .FilterByDateRange(DateTime.Today, DateTime.Today);

var query = DbContext.Set<Note>()
     .FilterByDateRange(DateTime.Today, DateTime.Today);

var query = DbContext.Set<Complaint>()
     .FilterByDateRange(DateTime.Today, DateTime.Today);


// The general purpose function, can filter for any model implementing IRequiredDate
public static IQueryable<TModel> FilterByDate<TModel>(IQueryable<TModel> query, DateTime startDate, DateTime endDate) where TModel : IRequiredDate
{
    // This will NOT WORK, as E/F won't accept an expression of type IRequiredDate, even though TModel implements IRequiredDate
    // Expression<Func<IRequiredDate, bool>> dateRangeFilter = x => x.Date >= startDate && x.Date <= endDate;
    // query = query.Where(dateRangeFilter);

    // This also WON'T WORK, x.Date is compiled into the expression as a member of IRequiredDate instead of TModel, so E/F knocks it back for the same reason:
    // Expression<Func<TModel, bool>> dateRangeFilter = x => x.Date >= startDate && x.Date <= endDate;
    // query = query.Where(dateRangeFilter);

    // All you need is lov.... uh... something like this:
    Expression<Func<IRequiredDate, bool>> dateRangeFilter = x => x.Date >= startDate && x.Date <= endDate;
    Expression<Func<TModel, bool>> dateRangeFilterForType = ConvertExpressionType<IRequiredDate, TModel>(dateRangeFilter); // Must convert the expression from one type to another
    query = query.Where(dateRangeFilterForType) // Ahhhh. this will work.

    return query;
}

public static ConvertExpressionType<TInterface, TModel>(Expression<Func<TInterface, bool>> expression)
where TModel : TInterface // It must implement the interface, since we're about to translate them
{
    Expression<Func<TModel, bool>> newExpression = null;

    // TODO: How to convert the contents of expression into newExpression, modifying the
    // generic type parameter along the way??

    return newExpression;
}

我知道它们是不同的类型，不能进行转换。不过我想知道是否有办法创建一个新的Expression<Func<TModel, bool>>，然后根据内容重建它Expression<Func<IRequiredDate, bool>>提供，切换任何类型引用IRequiredDate to TModel正在进行中。

这可以做到吗？

所以实际进行映射的方法不是that很难，但遗憾的是我没有找到一个很好的方法来概括它。这是一个方法，需要一个Func<T1, TResult>并将其映射到一个委托，其中参数比T1:

public static Expression<Func<NewParam, TResult>> Foo<NewParam, OldParam, TResult>(
    Expression<Func<OldParam, TResult>> expression)
    where NewParam : OldParam
{
    var param = Expression.Parameter(typeof(NewParam));
    return Expression.Lambda<Func<NewParam, TResult>>(
        expression.Body.Replace(expression.Parameters[0], param)
        , param);
}

这使用了Replace方法将一个表达式的所有实例替换为另一个表达式的所有实例。定义是：

internal class ReplaceVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public ReplaceVisitor(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
        return node == from ? to : base.Visit(node);
    }
}

public static Expression Replace(this Expression expression,
    Expression searchEx, Expression replaceEx)
{
    return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}

现在我们可以使用这个方法（应该给它一个更好的名字），如下所示：

Expression<Func<object, bool>> oldExpression = whatever;
Expression<Func<string, bool>> newExpression =
    Foo<string, object, bool>(oldExpression);

当然，自从Func实际上就其参数而言是协变的，我们可以确定对此方法的任何调用都会生成不会添加运行时故障点的表达式。

你可以简单地制作这个版本Func<T1, T2, TResult>，依此类推，直到 16 种不同类型Func如果您愿意，只需为每个参数创建一个参数表达式，然后用新的替换所有旧的。这会很乏味，但只要遵循模式即可。鉴于新旧参数类型都需要有一个通用参数，并且无法推断参数，这会变得……混乱。