在 linq 表达式树中使用过滤器

Question

我的想法是公开一个api 控制器​/api​/Events​/{clientId}​/{projectId}/{eventTypeId} 这些树形参数在查询我的db.Events 表时将起到过滤器的作用。 如果我通过ClientId，而不通过其他两个过滤器，它应该只通过ClientId 进行过滤。

我试过这个方法：

public async Task<IEnumerable<Event>> Handle(GetEventsByClientIdAndProjectIdQuery query, CancellationToken cancellationToken)
{
    Expression<Func<Event, int?>> clientId = (s => query.ClientId);
    Expression<Func<Event, int?>> projectId = (s => query.ProjectId);
    Expression<Func<Event, int?>> eventTypeId = (s => query.EventTypeId);

    if (query.ProjectId.HasValue)
    {
        projectId = (p => p.ProjectId.Equals(query.ClientId)); //error: cannot implicitly convert type bool to int?
    }

    if (query.EventTypeId.HasValue)
    {
        eventTypeId = (e => e.EventTypeId == query.EventTypeId.Value);
    }

    var evts = await _context.Events.Where(clientId) //error: Argument 2: cannot convert from 'System.Linq.Expressions.Expression<System.Func<UNW.Domain.Entities.Event, int?>>' to 'System.Func<UNW.Domain.Entities.Event, bool>'
                                    .Where(projectId)
                                    .Where(eventTypeId)
                                    .ToListAsync();
                
                

    if (evts == null)
        return null;

    return evts.AsReadOnly();
}

还有我的GetEventsByClientIdAndProjectIdQuery 模特：

public int? ProjectId { get; set; }
public int? ClientId { get; set; }
public int? EventTypeId { get; set; }

我错过了什么？

Answer 1

你可以让它更简单

public async Task<IEnumerable<Event>> Handle(GetEventsByClientIdAndProjectIdQuery query, 
    CancellationToken cancellationToken)
{
    var dbQuery = _context.Events;
    if (query.ProjectId.HasValue)
    {
        dbQuery = dbQuery.Where(p => p.ProjectId.Equals(query.ClientId));
    }
    if (query.EventTypeId.HasValue)
    {
        dbQuery = dbQuery.Where(e => e.EventTypeId == query.EventTypeId.Value);
    }
    //same goes for projectID which is missing from your question

    var evts = await dbQuery.ToListAsync();
                
    //evts will nerver be null, you might want do something for evts.Count==0
    //but it is fine to return an empty list

    return evts.AsReadOnly();
}

Answer 2

我能想到的最简洁的解决方案

public async Task<IEnumerable<Event>> Handle(GetEventsByClientIdAndProjectIdQuery query, CancellationToken cancellationToken)
  => await _context.Events
        .Where(evt => evt.ClientId.Equals(query.ClientId))
        .Where(evt => query.ProjectId.HasValue ? evt.ProjectId.Equals(query.ProjectId.Value) : true)
        .Where(evt => query.EventTypeId.HasValue ? evt.EventTypeId.Equals(query.EventTypeId.Value) : true)
        .ToListAsync(cancellationToken)
        .AsReadOnly();             

如果提供了过滤器
那么使用它
否则不过滤掉元素

Answer 3

我喜欢并且喜欢@Magnetron 的简单回答，但要基于您现有的代码：

//error: cannot implicitly convert type bool to int?

问题 1：clientId、projectId 和 eventTypeId 的签名都设置为返回一个可为空的 int (Func.Equals() 返回一个布尔值。

假设我了解您想要完成的任务，您可以尝试以下更新：

// 1. Change the return values from int? to boolean.
// 2. Go ahead and set your expression to return Events where the ClientId is equal
//    to the ClientId passed in with your query parameter
Expression<Func<Event, bool>> clientId = 
                                 (s => s.ClientId.Equals(query.ClientId));

// 3. Similar to the above with ClientId, but we also include the guard clauses for the optional parameters (ProjectId and EventTypeId)
      
Expression<Func<Event, bool>> projectId = 
                                 (s => (!query.ProjectId.HasValue || query.ProjectId == 0) || s.ID.Equals(query.ProjectId));
Expression<Func<Event, bool>> eventTypeId = 
                                 (s => (!query.EventTypeId.HasValue || query.EventTypeId == 0) || s.ID.Equals(query.EventTypeId));

// (Issue 2 with clientId resolved by the updates made to resolve Issue 1)
var evts = await _context.Events.Where(clientId) 
                                .Where(projectId)
                                .Where(eventTypeId)
                                .ToListAsync();
            
            
if (evts == null)
    return null;

return evts.AsReadOnly();