[英]Using Where( Expression<Func<T, bool>> ) in IGrouping
考慮以下Linq to Entities查詢:
return (from lead in db.Leads
join postcodeEnProvincie in postcodeEnProvincies
on lead.Postcode equals postcodeEnProvincie.Postcode
where (lead.CreationDate >= range.StartDate) && (lead.CreationDate <= range.EndDate)
group lead by postcodeEnProvincie.Provincie into g
select new Web.Models.GroupedLeads() {
GroupName = g.Key,
HotLeads = g.Count(l => l.Type == Data.LeadType.Hot),
Leads = g.Count(),
PriorityLeads = g.Count(l => l.Type == Data.LeadType.Priority),
Sales = g.Count(l => l.Sold),
ProductA = g.Count(l => l.Producten.Any(a => ((a.Name.Equals("productA", StringComparison.CurrentCultureIgnoreCase)) || (a.Parent.Name.Equals("productA", StringComparison.CurrentCultureIgnoreCase))))),
ProductB = g.Count(l => l.Producten.Any(a => ((a.Name.Equals("productB", StringComparison.CurrentCultureIgnoreCase)) || (a.Parent.Name.Equals("productB", StringComparison.CurrentCultureIgnoreCase))))),
ProductC = g.Count(l => l.Producten.Any(a => ((a.Name.Equals("productC", StringComparison.CurrentCultureIgnoreCase)) || (a.Parent.Name.Equals("productC", StringComparison.CurrentCultureIgnoreCase))))),
ProductC = g.Count(l => l.Producten.Any(a => ((a.Name.Equals("productD", StringComparison.CurrentCultureIgnoreCase)) || (a.Parent.Name.Equals("productD", StringComparison.CurrentCultureIgnoreCase)))))
}).ToList();
如果你像我一樣,你的腳趾會重復產品選擇邏輯。 這種模式也在另一個地方重復。 我首先嘗試用IEnumerable上的擴展方法替換它,這當然不起作用:Linq to Entities需要一個Expression來解析和翻譯。
所以我創建了這個方法:
public static System.Linq.Expressions.Expression<Func<Data.Lead, bool>> ContainingProductEx(string productName)
{
var ignoreCase = StringComparison.CurrentCultureIgnoreCase;
return (Data.Lead lead) =>
lead.Producten.Any(
(product =>
product.Name.Equals(productName, ignoreCase) ||
product.Parent.Name.Equals(productName, ignoreCase)
));
}
以下選擇現在完全正常:
var test = db.Leads.Where(Extensions.ContainingProductEx("productA")).ToList();
但是,這不會編譯,因為IGrouping不包含Where接受Expression的覆蓋:
return (from lead in db.Leads
join postcodeEnProvincie in postcodeEnProvincies
on lead.Postcode equals postcodeEnProvincie.Postcode
where (lead.CreationDate >= range.StartDate) && (lead.CreationDate <= range.EndDate)
group lead by postcodeEnProvincie.Provincie into g
select new Web.Models.GroupedLeads()
{
GroupName = g.Key,
HotLeads = g
.Where(l => l.Type == Data.LeadType.Hot)
.Count(),
Leads = g.Count(),
PriorityLeads = g
.Where(l => l.Type == Data.LeadType.Priority)
.Count(),
Sales = g
.Where(l => l.Sold)
.Count(),
ProductA = g
.Where(Extensions.ContainingProductEx("productA"))
.Count(),
ProductB = g
.Where(Extensions.ContainingProductEx("productB"))
.Count(),
ProductC = g
.Where(Extensions.ContainingProductEx("productC"))
.Count(),
ProductD = g
.Where(Extensions.ContainingProductEx("productD"))
.Count()
}).ToList();
將g轉換為IQueryable編譯,但隨后產生“內部.NET Framework數據提供程序錯誤1025”。
有沒有辦法將這個邏輯包裝在自己的方法中?
這是一個可以使用LINQKit解決的問題。 它允許從其他表達式中調用表達式,並在其調用者中內聯調用的表達式。 遺憾的是,它只支持一些非常具體的情況,所以我們需要調整你的表達式生成方法。
不是將產品名稱傳遞給表達式生成方法,而是將它作為返回表達式的參數:
public static Expression<Func<Data.Lead, string, bool>> ContainingProductEx()
{
var ignoreCase = StringComparison.CurrentCultureIgnoreCase;
return (lead, productName) =>
lead.Producten.Any(
(product =>
product.Name.Equals(productName, ignoreCase) ||
product.Parent.Name.Equals(productName, ignoreCase)
));
}
接下來,我們需要在聲明查詢之前調用該方法:
var predicate = Extensions.ContainingProductEx();
您的查詢現在可以寫為:
from lead in db.Leads.AsExpandable()
//...
ProductA = g
.Where(lead => predicate.Invoke(lead, "productA"))
.Count(),
ProductB = g
.Where(lead => predicate.Invoke(lead, "productB"))
.Count(),
ProductC = g
.Where(lead => predicate.Invoke(lead, "productC"))
.Count(),
ProductD = g
.Where(lead => predicate.Invoke(lead, "productD"))
.Count()
而不是擔心在您的查詢中創建一個您可以引用的函數指針/表達式(可能不可能),為什么不創建一個單獨的私有方法,它接受一個IEnumerable<Lead>
,一個字符串,並返回一個int並引用它查詢中的方法組? 我認為您的困惑源於嘗試在集合上創建擴展方法,而不是創建集合中的方法和您正在尋找的值。
就像是:
ProductA = GetLeadsForProduct(g, "productA")
private int GetLeadsForProduct(IEnumerable<Lead> leads, string productType)
{
return leads.Count(l => l.Producten.Any(a => ((a.Name.Equals(productType, StringComparison.CurrentCultureIgnoreCase)) || (a.Parent.Name.Equals(productType, StringComparison.CurrentCultureIgnoreCase)))))
}
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