Question

I have the following code:

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

Where the function GetDbColumns() returns an List<string>.
So columNames is of the type List<List<string>>.
Is it possible to create a List<string>, so each element of the list of GetDbColumns is added to the result of the LinQ query?

Answer 1

You can use the “select many” construction:

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();

Or here is an alternative way of using SelectMany:

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();

And finally, this is another way to put it (but it includes the somewhat ugly code 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();