Question

我有以下查询：

db.Users.AsQueryable()
    .Where(u => u.Id = userResolver.LoggedUserId() && u.Packages.Where(p => 
            p.StatusId == (int)PackageStatus.InProgress ||
            p.StatusId == (int)PackageStatus.Delivered ||
            p.StatusId == (int)PackageStatus.Shipped ||
            p.StatusId == (int)PackageStatus.Waiting) 
        .Sum(p => p.Price) > u.MaxCredit)
    .ToList()

我要实现的目标是将所有程序包状态检查归为一个扩展方法。像这样：

db.Users.AsQueryable()
        .Where(u => u.Id = userResolver.LoggedUserId() &&
             u.Packages.Where(p => p.IsShippedOrInProgress())
            .Sum(p => p.Price) > u.MaxCredit)
        .ToList()


 //This is the extension method
 public static bool IsShippedOrInProgress(this Package p) {
    return p.StatusId == (int)PackageStatus.InProgress ||
           p.StatusId == (int)PackageStatus.Delivered ||
           p.StatusId == (int)PackageStatus.Shipped ||
           p.StatusId == (int)PackageStatus.Waiting)
 }

当我查看第一个示例中生成的sql查询时，一切似乎都不错，但是当我使用第二种方法时，检查状态的查询部分不存在。

Answer 1

尝试使用此方法将创建Expression，Package作为输入和bool作为输出：

public static System.Linq.Expressions.Expression<Func<Package, bool>> IsShippedOrInProgress()
{
    return p => p.StatusId == ( int )PackageStatus.InProgress ||
           p.StatusId == ( int )PackageStatus.Delivered ||
           p.StatusId == ( int )PackageStatus.Shipped ||
           p.StatusId == ( int )PackageStatus.Waiting);
}

这样称呼它：

private void SomeMethod()
{
    db.Users.AsQueryable()
    .Where(u => u.Id = userResolver.LoggedUserId() &&
         u.Packages.Where(IsShippedOrInProgress())
        .Sum(p => p.Price) > u.MaxCredit)
    .ToList()
}

Answer 2

我试图在记事本中写这个。因此可能存在错误。在评论中让我知道

public static Expression<Func<Package,bool>> IsShippedOrInProgress() {

    // Compose the expression tree that represents the parameter to the predicate.  
    ParameterExpression p = Expression.Parameter(typeof(Package), "p");  

    // Compose left side of the expression i.e `p.StatusId`
    Expression left = Expression.Call(p, typeof(Package).GetProperty("StatusId"));  

    // Compose right side of the expression i.e `(int)PackageStatus.InProgress` etc.
    Expression exprInProgress = Expression.Constant((int)PackageStatus.InProgress);  
    Expression exprDelivered = Expression.Constant((int)PackageStatus.Delivered);  
    Expression exprShipped = Expression.Constant((int)PackageStatus.Shipped);  
    Expression exprWaiting = Expression.Constant((int)PackageStatus.Waiting);  

    // Compose left equals right side
    Expression e1 = Expression.Equal(left, exprInProgress);  
    Expression e2 = Expression.Equal(left, exprDelivered);  
    Expression e3 = Expression.Equal(left, exprShipped);  
    Expression e4 = Expression.Equal(left, exprWaiting);  

    //Compose `p.StatusId == (int)PackageStatus.InProgress ||
    //       p.StatusId == (int)PackageStatus.Delivered ||
    //       p.StatusId == (int)PackageStatus.Shipped ||
    //       p.StatusId == (int)PackageStatus.Waiting`
    Expression orConditions = Expressions.OrElse(Expression.OrElse(Expression.OrElse(e1,e2),e3),e4);

    //Compose `p => 
    //        p.StatusId == (int)PackageStatus.InProgress ||
    //        p.StatusId == (int)PackageStatus.Delivered ||
    //        p.StatusId == (int)PackageStatus.Shipped ||
    //        p.StatusId == (int)PackageStatus.Waiting`
    return Expression.Lambda<Func<Package, bool>>(orConditions, new ParameterExpression[] { p })); 

}

更新

The C# compiler can generate expression trees from expression lambdas (or single-line lambdas)。请检查@Ankush答案

实体框架核心-在Queryable内部使用扩展方法

2 个答案: