如何在LINQ中实现动态的"where"子句? [英] How do I implement a dynamic 'where' clause in LINQ?

问题描述

我想要一个动态的where条件.

I want to have a dynamic where condition.

在以下示例中:

var opportunites =  from opp in oppDC.Opportunities
                    join org in oppDC.Organizations 
                        on opp.OrganizationID equals org.OrgnizationID
                    where opp.Title.StartsWith(title)
                    select new
                    {
                        opp.OpportunityID,
                        opp.Title,
                        opp.PostedBy,
                        opp.Address1,
                        opp.CreatedDate,
                        org.OrganizationName
                    };

有时候我有Title，有时我没有.我也想在where子句中动态添加日期.

Some times I have Title and sometimes I don't. And also I want to add date in where clause dynamically.

例如，类似以下SQL:

For example, like this SQL:

string whereClause;
string SQL = whereClause == string.Empty ? 
     "Select * from someTable" : "Select * from someTable" + whereclause

推荐答案

您可以这样重写它:

 var opportunites =  from opp in oppDC.Opportunities
                            join org in oppDC.Organizations on opp.OrganizationID equals org.OrgnizationID
                            select new
                            {
                                opp.OpportunityID,
                                opp.Title,
                                opp.PostedBy,
                                opp.Address1,
                                opp.CreatedDate,
                                org.OrganizationName
                            };

if(condition)
{
   opportunites  = opportunites.Where(opp => opp.Title.StartsWith(title));
}

编辑:是的，要在评论中回答您的问题，可以继续添加到原始Queryable中.记住，这都是延迟执行的，因此，到此为止，它都在构建IQueryable，因此您可以根据需要继续将它们链接在一起:

To answer your question in the comments, yes, you can keep appending to the original Queryable. Remember, this is all lazily executed, so at this point all it's doing it building up the IQueryable so you can keep chaining them together as needed:

if(!String.IsNullOrEmpty(title))
{
   opportunites  = opportunites.Where(.....);
}

if(!String.IsNullOrEmpty(name))
{
   opportunites  = opportunites.Where(.....);
}

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