返回匿名2字符串类型的通用列表的返回类型是什么？

Question

I have this function:

public static List<string> JoinDataTablesMultipleColumns(DataTable dt1, DataTable dt2, string col1, string col2, string col3) {
 var multiJoin = from table1 in dt1.AsEnumerable()
 join table2 in dt2.AsEnumerable()
 on new {
   Col1 = table1.Field < string > (col1),
   Col2 = table1.Field < string > (col2),
   Col3 = table1.Field < string > (col3),
 }
 equals new {
   Col1 = table2.Field < string > (col1),
   Col2 = table2.Field < string > (col2),
   Col3 = table2.Field < string > (col3),
 }
 select new {
   SeriesID = table1.Field < string > ("SeriesID"),
   CatID = table2.Field < string > ("CatID")
 };

 //if ( multiJoin != null )
 //   return multiJoin.ToList();
 //else
 return null;
}

It works perfectly except that I have the wrong return type. The only way I can compile and run it is by returning null. When I stop the debugger before returning and examine multiJoin.ToList() in the watch window , it's exactly what I want:

But the return type is wrong. I keep trying List<string, string> , but that won't compile. Probably a simple thing but I am stuck. Any ideas?

Answer 1

As you note, you're getting a list of anonymous type. Anonymous types are not designed to cross out of the methods that create them . There are ways to make that work, but they are confusing, difficult and arcane.

Do not attempt to do this with anonymous types. In C# 7, you can use a tuple type and return List<(string, string)> , and then do

select (table1.Field<string>("SeriesID"), table2.Field<string>("CatID"))

In other versions of C#, make a nominal pair type and create an instance of it in your select clause.

Answer 2

Instead of attempting returning an anonymous type (you can't declare an anonymous return type for your method), create a named type to hold your data:

public class Item
{
    public string SeriesID { get; set;}
    //...
}

and use it instead of the anonymous type that you are currently selecting at the end of your query.

...select new Item {
   SeriesID = table1.Field < string > ("SeriesID"),
   CatID = table2.Field < string > ("CatID")
 };