mvc c#合并两个以上的查询结果 [英] mvc c# combine more than two query results

问题描述

我有几个具有相同结果结构的通用查询，我试图将它们合并到一个变量中，以便可以将它们传递给视图.

I have several generic queries that has the same structure of results and I'm trying to combine them in one variable so that I can pass them to the view.

var query1 = db1.table.Where(a => a.ID == filter1).Select(x=> new ViewModel { //columns}).LastOrDefault();
var query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel  { //columns}).LastOrDefault();
var query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel  { //columns}).LastOrDefault();

var combi = query1.Concat(query2);

我无法结合使用这两个查询结果，我尝试使用Concat或Union，但它给出了以下错误.

I cannot combine the two query results, I tried using Concat or Union but it gives the error below.

'ViewModel'不包含'Concat'的定义，也找不到扩展方法'Concat'接受类型为'ViewModel'的第一个参数(您是否缺少using指令或程序集引用?)

'ViewModel' does not contain a definition for 'Concat' and no extension method 'Concat' accepting a first argument of type 'ViewModel' could be found (are you missing a using directive or an assembly reference?)

谢谢！

推荐答案

如果将 var 关键字替换为实际类型，则会发现您拥有以下内容:

If you were to replace the var keyword with the actual types you'd see that you have this:

ViewModel query1 = db1.table.Where(a => a.ID == filter1).Select(x => new ViewModel { /* columns */ }).LastOrDefault();
ViewModel query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel { /* columns */ }).LastOrDefault();
ViewModel query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel { /* columns */ }).LastOrDefault();

为了执行 .Concat(...)，它应该是 IQueryable< ViewModel> 或 IEnumerable< ViewModel> .

In order to do a .Concat(...) it should be IQueryable<ViewModel> or IEnumerable<ViewModel>.

这是导致您感到悲伤的 .LastOrDefault()调用.

It's the .LastOrDefault() call that is causing you grief.

尝试一下:

IQueryable<ViewModel> query1 = db1.table.Where(a => a.ID == filter1).Select(x => new ViewModel { /* columns */ });
IQueryable<ViewModel> query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel { /* columns */ });
IQueryable<ViewModel> query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel { /* columns */ });

var combi = query1.Concat(query2).Concat(query3).LastOrDefault();

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