如果您合并两个查询，我们是否需要首先在每个查询上运行ToList（）？

我有一些LINQ to SQL疑问，想要得到他们的结果union。我需要先运行ToList()它们吗？或者我可以将它们保留为IQueryable原来的样子，然后将它们合并，在合并结束时只需打个电话ToList()？而且这根本没有任何性能改进吗？还是本质上是同一回事？

var query1 = ( from a in this.Context.Blah select a); // .ToList()??
var query2 = ( from b in this.Context.Yada select b); // .ToList()??
var query3 = (query1.Union(query2)).ToList();

Hamid Pourja.. 5

方案1：

var query1 = ( from a in this.Context.Blah select a); // you have a query
var query2 = ( from b in this.Context.Yada select b); // you have another query
var query3 = (query1.Union(query2)); // you build a new query
var result = query3.ToList(); // you execute a query and get result

方案2：

//this builds a query and execute it and get result from database
var query1 = ( from a in this.Context.Blah select a).ToList(); // you execute a query

//this builds a query and execute it and get result from database
var query2 = ( from b in this.Context.Yada select b).ToList(); // you execute another query

//this builds a query and execute it but it does not hit database because you are working
//with IEnumerable here not IQueryable
var query3 = (query1.Union(query2)); // you build a new query
var result = query3.ToList(); // you execute another query and get result

方案2执行3个查询，并有2次往返数据库，而第一个则只需一次往返就可以获取您想要的内容。

建立查询和执行查询之间有很多差异。调用方法，如ToList，ToArray，First，Single和......将执行，因为调用类似的方法你已经建立了查询Where，Union，Except和...只是建立你的查询。

方案1：

var query1 = ( from a in this.Context.Blah select a); // you have a query
var query2 = ( from b in this.Context.Yada select b); // you have another query
var query3 = (query1.Union(query2)); // you build a new query
var result = query3.ToList(); // you execute a query and get result

方案2：

//this builds a query and execute it and get result from database
var query1 = ( from a in this.Context.Blah select a).ToList(); // you execute a query

//this builds a query and execute it and get result from database
var query2 = ( from b in this.Context.Yada select b).ToList(); // you execute another query

//this builds a query and execute it but it does not hit database because you are working
//with IEnumerable here not IQueryable
var query3 = (query1.Union(query2)); // you build a new query
var result = query3.ToList(); // you execute another query and get result

方案2执行3个查询，并有2次往返数据库，而第一个则只需一次往返就可以获取您想要的内容。

建立查询和执行查询之间有很多差异。调用方法，如ToList，ToArray，First，Single和......将执行，因为调用类似的方法你已经建立了查询Where，Union，Except和...只是建立你的查询。