如何强制LINQ总和（）返回0，而源集合为空 [英] How to force LINQ Sum() to return 0 while source collection is empty

问题描述

基本上，当我做下面的查询，如果没有线索相匹配下面的查询会抛出异常。在这种情况下，我想preFER有总和相等0而不是抛出的异常。
这会不会有可能在查询本身 - 我的意思，而不是存储查询和检查 query.Any（）

 双重收益= db.Leads.Where（L =＆GT; l.Date.Day == date.Day
                ＆功放;＆安培; l.Date.Month == date.Month
                ＆功放;＆安培; l.Date.Year == date.Year
                ＆功放;＆安培; l.Property.Type == ProtectedPropertyType.Password
                ＆功放;＆安培; l.Property.PropertyId ==物业ID）.SUM（L =＆GT; l.Amount）;
 

解决方案

试着改变你的查询这样的：

  db.Leads.Where（L =＆GT; l.Date.Day == date.Day
            ＆功放;＆安培; l.Date.Month == date.Month
            ＆功放;＆安培; l.Date.Year == date.Year
            ＆功放;＆安培; l.Property.Type == ProtectedPropertyType.Password
            ＆功放;＆安培; l.Property.PropertyId ==物业ID）
         。选择（L =＆GT; l.Amount）
         .DefaultIfEmpty（0）
         。和（）;
 

这样，你的查询只会选择金额字段。如果集合是空的，它会返回一个元素与值 0 ，然后总和将被应用。

Basically when I do the following query, if no leads were matched the following query throws an exception. In that case I'd prefer to have the sum equalize 0 rather than an exception being thrown. Would this be possible in the query itself - I mean rather than storing the query and checking query.Any() ?

double earnings = db.Leads.Where(l => l.Date.Day == date.Day
                && l.Date.Month == date.Month
                && l.Date.Year == date.Year
                && l.Property.Type == ProtectedPropertyType.Password
                && l.Property.PropertyId == PropertyId).Sum(l => l.Amount);

解决方案

Try changing your query to this:

db.Leads.Where(l => l.Date.Day == date.Day
            && l.Date.Month == date.Month
            && l.Date.Year == date.Year
            && l.Property.Type == ProtectedPropertyType.Password
            && l.Property.PropertyId == PropertyId)
         .Select(l => l.Amount)
         .DefaultIfEmpty(0)
         .Sum();

This way, your query will only select the Amount field. If the collection is empty, it will return one element with the value of 0 and then the sum will be applied.

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