使用 Linq 动态添加条件连接

我有一个基本的搜索控件，它根据下拉列表提供的预定义搜索/过滤条件列出 CRM 中的公司。每个下拉菜单的默认选择是“全部”，否则用户将选择特定的项目。我希望能够根据选择动态构建 Linq 查询。在 5 个选择器中，它们提供了我可以与 Company 表进行匹配的值，但其中两个选择器（如果选择其中一个或两个）将需要一个或多个联接，否则不应对基本结果集再次采取任何操作。我希望这是有道理的。

我不知道如何有效地做到这一点。这是我的代码：

private void Search()
{
    EnergyPubsCRMDataContext dc = new EnergyPubsCRMDataContext();

    var results = (from c in dc.Companies
                   select c);


    //only create the join if the selected index > 0
    if (ddlIndustry.SelectedIndex > 0)
    {
        //A company can be in 1 or more industries, thus here I want to join
        //with the CompanyIndustry table and have a WHERE clause to match on the ddlIndustry.SelectedValue
    }

    //only create the join if the selected index > 0
    if (ddlServices.SelectedIndex > 0)
    {
        //A company can offer 1 or more services. Here I want to join to the CompanyService table
        //on the CompanyID and have a WHERE clause to match the ddlServices.SelectedValue
    }        

    //These work OK to shape the overal query further (they don't need joins)
    if (ddlCountry.SelectedIndex > 0)
        results = results.Where(c => c.CountryID == Convert.ToInt32(ddlCountry.SelectedValue));

    if (ddlStateRegion.SelectedIndex > 0)
        results = results.Where(c => c.StateRegionID == Convert.ToInt32(ddlStateRegion.SelectedValue));

    if (ddlAccountManagers.SelectedIndex > 0)
    {
        Guid g = new Guid(ddlAccountManagers.SelectedValue);
        results = results.Where(c => c.UserId == g);
    }

    results = results.OrderBy(c => c.CompanyName);

    //Bind to Grid....        
}

if (ddlIndustry.SelectedIndex > 0)
{
    //A company can be in 1 or more industries, thus here I want to join
    //with the CompanyIndustry table and have a WHERE clause to match on the ddlIndustry.SelectedValue
    results = results.Where(c => c.CompanyIndustry.IndustryID == ddlIndustry.SelectedValue);
}

假设您的数据库/DBML 中有正确的外键。

这将隐式生成连接。