The LINQ expression “xxx“ could not be translated

Error example:

var A = B
.GroupBy(item => item.id)
.Select(groupedList => new
{
PlannerId = groupedList.Key,
RxList = groupedList
});

solution:

var A = B
.GroupBy(item => item.id)
.Select(groupedList => new
{
PlannerId = groupedList.Key,
RxList = groupedList.toList()
});

reason:

Your modification is correct and resolves the error. Let me explain why this modification works.

In the original code, you used the GroupByoperator to pair< based on item.id a i=4>The results are grouped. Then, you use the projection operator to create a new anonymous type that contains the grouping key (PlannerId) and the grouped list (RxList). BSelect

However, the problem is that LINQ expressions cannot directly translate theSelect operator with grouping parameters. Because the grouped list returns an object of type GroupedEnumerable instead of a standard collection type (such as List or Array).

By performing the .ToList() operation, we convert an object of type GroupedEnumerable into an object of type List . This way, in the Select projection operator, we will be able to operate on the grouping correctly because it has already been combined for translation.

So, by calling the  method in groupByPlannerIdQuaryable 's Select , you ensure that the grouped list has been combined Got up and successfully solved the error. .ToList()

When querying using LINQ expressions, the results can be grouped by the specified key by using the GroupBy operator. This returns an object of type GroupedEnumerable that represents the grouped results.

GroupedEnumerable is a class that implements the IEnumerable<T> interface, where T represents the type of elements in the original collection. Therefore, it can still be traversed and accessed, just organized in a different way.

Compared with standard collection types such as List or Array,GroupedEnumerable objects have some additional features. For example, it provides methods to directly access a group's key, count groups, and iterate over each group. This makes it easier to perform complex grouping operations on query results.

However, sinceGroupedEnumerable is not a standard collection type, some LINQ operators cannot be applied directly to it. One of these is the Select operator, since it expects a standard collection type as input.

To solve this problem, you can use the SelectMany operator to expand GroupedEnumerable into a flattened sequence, so that you can continue to apply other LINQ operator. Here's an example:

var students = new List<Student>
{
    new Student { Name = "Alice", Grade = "A" },
    new Student { Name = "Bob", Grade = "B" },
    new Student { Name = "Charlie", Grade = "A" },
    new Student { Name = "Dave", Grade = "C" }
};

var groupedStudents = students.GroupBy(s => s.Grade);

var flattenedStudents = groupedStudents.SelectMany(g => g);

foreach (var student in flattenedStudents)
{
    Console.WriteLine(student.Name);
}

In this example, we first group students according to their grades and get an GroupedEnumerable object. Then, it is expanded into a flattened sequence by using the SelectMany operator, and finally iterates through each student and outputs their name.