Question

在System.Linq.Dynamic中，有一些方法可以动态地形成Select，Where和其他Linq语句。但SelectMany没有。

Select的方法如下：

    public static IQueryable Select(this IQueryable source, string selector, params object[] values)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (selector == null) throw new ArgumentNullException("selector");
        LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, null, selector, values);
        IQueryable result = source.Provider.CreateQuery(
            Expression.Call(
                typeof(Queryable), "Select",
                new Type[] { source.ElementType, lambda.Body.Type },
                source.Expression, Expression.Quote(lambda)));

        return result;
    }

我试着修改上面的代码，经过几个小时的工作，我找不到出路。

欢迎任何建议。

应

Answer 1

已经为我们的项目实施了这个，让我知道它是否适合您！

public static IQueryable SelectMany(this IQueryable source, string selector, params object[] values)
{
    if (source == null) 
        throw new ArgumentNullException("source");
    if (selector == null) 
        throw new ArgumentNullException("selector");

    // Parse the lambda
    LambdaExpression lambda = 
        DynamicExpression.ParseLambda(source.ElementType, null, selector, values);

    // Fix lambda by recreating to be of correct Func<> type in case 
    // the expression parsed to something other than IEnumerable<T>.
    // For instance, a expression evaluating to List<T> would result 
    // in a lambda of type Func<T, List<T>> when we need one of type
    // an Func<T, IEnumerable<T> in order to call SelectMany().
    Type inputType = source.Expression.Type.GetGenericArguments()[0];
    Type resultType = lambda.Body.Type.GetGenericArguments()[0];
    Type enumerableType = typeof(IEnumerable<>).MakeGenericType(resultType);
    Type delegateType = typeof(Func<,>).MakeGenericType(inputType, enumerableType);
    lambda = Expression.Lambda(delegateType, lambda.Body, lambda.Parameters);

    // Create the new query
    return source.Provider.CreateQuery(
        Expression.Call(
            typeof(Queryable), "SelectMany",
            new Type[] { source.ElementType, resultType },
            source.Expression, Expression.Quote(lambda)));
}

Answer 2

我添加了另一个返回匿名类型的SelectMany。

 public static IQueryable SelectMany(this IQueryable source, string selector, string resultsSelector, params object[] values)
    {
        if (source == null)
            throw new ArgumentNullException("source");
        if (selector == null)
            throw new ArgumentNullException("selector");

        // Parse the lambda 
        LambdaExpression lambda =
            DynamicExpression.ParseLambda(source.ElementType, null, selector, values);

        // Fix lambda by recreating to be of correct Func<> type in case  
        // the expression parsed to something other than IEnumerable<T>. 
        // For instance, a expression evaluating to List<T> would result  
        // in a lambda of type Func<T, List<T>> when we need one of type 
        // an Func<T, IEnumerable<T> in order to call SelectMany(). 
        Type inputType = source.Expression.Type.GetGenericArguments()[0];
        Type resultType = lambda.Body.Type.GetGenericArguments()[0];
        Type enumerableType = typeof(IEnumerable<>).MakeGenericType(resultType);
        Type delegateType = typeof(Func<,>).MakeGenericType(inputType, enumerableType);
        lambda = Expression.Lambda(delegateType, lambda.Body, lambda.Parameters);

        ParameterExpression[] parameters = new ParameterExpression[] { 
        Expression.Parameter(source.ElementType, "outer"), Expression.Parameter(resultType, "inner") };
        LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, resultsSelector, values);

        // Create the new query 
        return source.Provider.CreateQuery(
            Expression.Call(
                typeof(Queryable), "SelectMany",
                new Type[] { source.ElementType /*TSource*/, /*,TCollection*/resultType /*TResult*/, resultsSelectorLambda.Body.Type},
                source.Expression, Expression.Quote(lambda), Expression.Quote(resultsSelectorLambda)));
    }

我仍然需要弄清楚如何使用Dynamic执行以下操作，目标是返回一个新的结果对象。

        var customerandorderflat = db.Customers
            .SelectMany(c => c.Orders.SelectMany(o => o.Order_Details,
                    (ord, orddetail) => new
                        {
                            OrderID = ord.OrderID,
                            UnitPrice = orddetail.UnitPrice
                        }).DefaultIfEmpty(),
                (cus, ord) => new
                {
                    CustomerId = cus.CustomerID,
                    CompanyName = cus.CompanyName,
                    OrderId = ord.OrderID == null ? -1 : ord.OrderID,
                    UnitPrice = ord.UnitPrice
                });

Answer 3

我在尝试时使用NWDB：

var customerandorderquery = db.Customers .SelectMany(c => c.Orders.DefaultIfEmpty()).Select("new(CustomerId, CompanyName, OrderId)");

我收到错误，因为CompanyName位于Customers而不是Orders。所以它没有看到两个对象的组合。当我这样做时：

.SelectMany(c => c.Orders.DefaultIfEmpty(), (cus, ord) => new { CustomerId = cus.CustomerID, OrderId = ord.OrderID == null ? -1 : ord.OrderID });

返回所需的结果。

以System.Linq.Dynamic的方式动态调用SelectMany

3 个答案: