c# - 添加列表到列表

Question

我有以下代码:

var columnNames = (from autoExport in dataContext.AutoExports
               where autoExport.AutoExportTemplate != null
                  && ContainsColumn(autoExport.AutoExportTemplate, realName)
               select GetDbColumnNames(autoExport.AutoExportTemplate, realName)).ToList();

其中函数GetDbColumns()返回 List<string> .所以 columNames 的类型是 List<List<string>> .是否可以创建 List<string> , 所以 GetDbColumns 列表的每个元素都添加到 LinQ 查询的结果中？

Answer 1

您可以使用“多选”结构:

var columnNames = (
    from autoExport in dataContext.AutoExports
    where autoExport.AutoExportTemplate != null
          && ContainsColumn(autoExport.AutoExportTemplate, realName)
    from column in GetDbColumnNames(autoExport.AutoExportTemplate, realName)
    select column).ToList();

或者这是使用 SelectMany 的替代方法:

var columnNames = (
    from autoExport in dataContext.AutoExports
    where autoExport.AutoExportTemplate != null
          && ContainsColumn(autoExport.AutoExportTemplate, realName)
    select autoExport
).SelectMany(x => x.GetDbColumnNames(autoExport.AutoExportTemplate, realName))
.ToList();

最后，这是另一种表达方式(但它包含了一些丑陋的代码 x => x):

var columnNames = (
    from autoExport in dataContext.AutoExports
    where autoExport.AutoExportTemplate != null
          && ContainsColumn(autoExport.AutoExportTemplate, realName)
    select autoExport.GetDbColumnNames(autoExport.AutoExportTemplate, realName)
).SelectMany(x => x).ToList();