LINQ Sum()中的IsNull

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问题描述

我在 LINQ 中遇到 IsNull 问题:

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

p.LoadingsDetails.Sum(n => n.Quantity) 可能性 NULL.我想要这样的结果:

select Id, IsNull(Sum(Quantity),0) as Quantity from LoadingsDetails

我尝试过类似的方法:

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.LoadingDetails.First() == null ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

它返回错误:

<块引用>

System.NotSupportedException:方法"First"只能用作最终查询操作.考虑在而是这个实例.

我尝试过这样的事情:

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = (p.LoadingsDetails.Sum(n => n.Quantity) == DBNull.Value) ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

或

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.LoadingsDetails.First().Quantity == DBNull.Value ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

返回错误:

<块引用>

运算符"=="不能应用于"十进制"和"DBNull"类型的操作数

推荐答案

您可以在这里使用 Any() 来检查 LoadingDetails 是否有任何行，如果有则将 Quantity 与 LoadingDetails 相加:

RemainingQuantity = p.LoadingDetails.Any()  ? 
                p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity) : 
                p.Quantity

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问题描述

I have problem with IsNull in LINQ :

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

p.LoadingsDetails.Sum(n => n.Quantity) possibility NULL. I want to have result like this :

select Id, IsNull(Sum(Quantity),0) as Quantity from LoadingsDetails

I have tried something like :

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.LoadingDetails.First() == null ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

It return errors :

System.NotSupportedException: The method 'First' can only be used as a final query operation. Consider using the method 'FirstOrDefault' in this instance instead.

I have tried something like this :

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = (p.LoadingsDetails.Sum(n => n.Quantity) == DBNull.Value) ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

db.WarehouseInputsDetails
    .Select(p => new WarehouseInputDetailsViewModel
    {
        Id = p.Id
        RemainingQuantity = p.LoadingsDetails.First().Quantity == DBNull.Value ? p.Quantity : p.Quantity - p.LoadingsDetails.Sum(n => n.Quantity)
    }).AsQueryable();

Return Errors :

Operator '==' cannot be applied to operands of type 'decimal' and 'DBNull'