如何将无参数构造函数转换为MemberInitExpression

医生

我可以MemberInitExpression从实际上返回无参数构造函数的函数中创建无参数构造函数的或任何表达式吗？

public IQueryable<GroupView> GetViewSelect(IQueryable<ItemGroup> myQ)
{
    return myQ.SelectMany(GetExpression1(), GetExpression2());
}        

internal static Expression<Func<ItemGroup, IEnumerable<ItemDetail>>> 
    GetExpression1()
{
    // m is the ItemGroup
    // m.item is Item
    // m.item.itemDetail is a collection of ItemDetail
    return m => m.item.itemDetail; 
}

internal static Expression<Func<ItemGroup, ItemDetail, GroupView>> 
    GetExpression2()
{
    // m is the ItemGroup
    // n is the ItemDetail
    // and it's error at runtime coz LINQ doesnt know the GetGroupView.. :(
    return (m, n) => GetGroupView(m, n);
}

internal static GroupView GetGroupView(ItemGroup m, ItemDetail n)
{
    // I think the same error will occurs
    // coz LINQ doesnt know the GetItemView and GetItemDetailView
    return new GroupView
    {
        Id = m.id,
        Name = m.name,
        ItemDetailsTotalCount = 
            m.item.ItemDetails.Sum(nn => nn.item.itemDetails.Count),

        Item = GetItemView(m.item),
        ItemDetail = GetItemDetailView(n)
    };
}    

internal static ItemView GetItemView(Item m)
{
    return new ItemView
    {
        Id = m.id,
        Name = m.name,
        DetailCount = m.ItemDetail.Count
    };
}

internal static ItemDetailView GetItemDetailView(ItemDetail n)
{
    return new ItemDetailView
    {
        Id = n.id,
        Name = n.name,            
        Supplier = GetSupplierView(n.supplier)
    };
}

internal static SupplierView GetSupplierView(Supplier n)
{
    return new SupplierView
    {
        Id = n.id,
        Name = n.name,
        Email = n.email ?? "no email",
        Phone = n.phone ?? "no phone"
    };
}

当然，上述所有方法都不起作用，但是我只是想避免一遍又一遍地重新键入相同的无参数构造函数，以便每次我想要获得不同的视图时都获得视图构造函数。

例如，我想这样称呼它

public IQueryable<ItemView> GetViewSelect(IQueryable<Item> myQ)
{
    return myQ.Select(GetExpression3());
}        

public IQueryable<ItemView> GetViewSelect(IQueryable<ItemDetail> myQ)
{
    return myQ.Select(GetExpression3());
}        

internal static Expression<Func<Item, ItemView>> GetExpression3()
{
    return m => GetItemView(m); 
}

internal static Expression<Func<ItemDetail, ItemView>> GetExpression4()
{
    return m => GetItemView(m.item);
}

而不是每次我像下面这样调用它时都编写相同的无参数构造函数。

internal static Expression<Func<Item, ItemView>> GetExpression3()
{
    // writing same parameterless constructor again
    return m => new ItemView
    {
        Id = m.id,
        Name = m.name,
        DetailCount = m.ItemDetail.Count
    }; 
}

internal static Expression<Func<ItemDetail, ItemView>> GetExpression4()
{
    // the same thing again
    return m => new ItemView
    {
        Id = m.item.id,
        Name = m.item.name,
        DetailCount = m.item.ItemDetail.Count
    };
}

internal static Expression<Func<ItemGroup, ItemDetail, GroupView>> 
    GetExpression2()
{
    return (m, n) => new GroupView
    {
        Id = m.id,
        Name = m.name,
        ItemDetailsTotalCount = 
            m.item.ItemDetails.Sum(nn => nn.item.itemDetails.Count),

        Item = new ItemView
        {
            Id = m.item.id,
            Name = m.item.name,
            DetailCount = m.item.ItemDetail.Count
        },
        ItemDetail = new ItemDetailView
        {
            Id = n.id,
            Name = n.name,            
            Supplier = new SupplierView
            {
                Id = n.id,
                Name = n.name,
                Email = n.email ?? "no email",
                Phone = n.phone ?? "no phone"
            }
        }
    };
}

所以我实际上是在寻找一种转换的方法

new ItemView
{
    Id = m.id,
    Name = m.name,
    DetailCount = m.ItemDetail.Count
}

到无MemberInitExpression参数的构造函数中，所以也许我可以像下面那样使用它，而无需进行任何成员分配。

internal Expression<Func<Item, ItemView>> GetExpression4e
    (ParameterExpression m)
{
    // looking for something like this.. 
    MemberInitExpression ItemViewInitExpression = 
        Something("GetItemView", new Object[] {m});

    return Expression.Lambda<Func<Item, ItemView>>
        (ItemViewInitExpression, m);
}

Xanatos

internal static Expression<Func<Item, ItemView>> GetExpression3()
{
    // writing same parameterless constructor again
    return m => new ItemView
    {
        Id = m.id,
        Name = m.name,
        DetailCount = m.ItemDetail.Count
    }; 
}

此方法已经产生了MemberInitExpression。如果你这样做

var type = GetExpression3().Body.GetType();

你会得到 System.Linq.Expressions.MemberInitExpression

如果只需要传递一些参数，则传递一些参数：

internal static Expression<Func<Item, ItemView>> GetExpression3(int id, string name, int count)    
{
    // writing same parameterless constructor again
    return m => new ItemView
    {
        Id = id,
        Name = name,
        DetailCount = count
    }; 
}

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