CREATE FUNCTION failed because T-SQL and CLR types for return value do not match

Question

I have created a .Net assembly that returns a set of records. When I create a function in SQL Server, I get the message

CREATE FUNCTION for ...failed because T-SQL and CLR types for return value do not match.

I am a bit lost, does anyone have a clue what is going wrong? This is my assembly:

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.SqlServer.Server;
using System.Net;
using System.IO;
using System.Data.SqlTypes;
using System.Net.Json;
namespace SQLHttpRequest
{
    public partial class HTTPFunctions
    {

       public class mailBox
       {
           public bool Expanded { get; set; }
           public string ID { get; set; }
           public string Name { get; set; }
           public string ParentID { get; set; }
           public int ChildFolderCount { get; set; }
           public int UnreadItemCount { get; set; }
           public int TotalItemCount { get; set; }
       }

       public static void FillRow(object mailBoxObject
           , out SqlBoolean Expanded
           , out SqlString ID
           , out SqlString Name
           , out SqlString ParentID
           , out SqlInt32 ChildFolderCount
           , out SqlInt32 UnreadItemCount
           , out SqlInt32 TotalItemCount
            )
        {
           var mb = (mailBox)mailBoxObject;
           Expanded = mb.Expanded;
           ID = mb.ID;
           Name = mb.Name;
           ParentID = mb.ParentID;
           ChildFolderCount = mb.ChildFolderCount;
           UnreadItemCount = mb.UnreadItemCount;
           TotalItemCount = mb.TotalItemCount;
      }

    [Microsoft.SqlServer.Server.SqlFunction(DataAccess = DataAccessKind.Read
            , TableDefinition = "Expanded BIT, ID NVARCHAR(100),Name NVARCHAR(max),ParentID NVARCHAR(max),ChildFolderCount INT, UnreadItemCount INT, TotalItemCount INT"
            , FillRowMethodName = "FillRow")]
    public static List<mailBox> GetMailFolders(string WellKnownFolderName, string FolderID, string UserID, string password)
       {
           List<mailBox> mbs = ReadFolder(WellKnownFolderName, UserID, password);
           // Method ReadFolder Fills the mbs List
           return mbs;
        }
   }
}

The assembly receives some parameters and returns a List of mail folders. The assembly is succesfully created in SQL Server:

 CREATE ASSEMBLY SQLHttpRequest
 FROM N'C:\...\SQLHttpRequest.dll'
 WITH PERMISSION_SET=UNSAFE;

When I try to create the function

    CREATE FUNCTION dbo.usp_GetMailFolders(
         @WellKnownFolderName  nvarchar(max),
         @FolderID    nvarchar(max),
         @UserID        nvarchar(max),
         @password      nvarchar(max)
    )
    RETURNS TABLE
    (
        Expanded BIT,
        ID NVARCHAR(100),
        Name NVARCHAR(100),
        ParentID NVARCHAR(100),
        ChildFolderCount int,
        UnreadItemCount int,
        TotalItemCount int
    )
    AS
    EXTERNAL NAME SQLHttpRequest.[SQLHttpRequest.HTTPFunctions].GetMailFolders;

I receive the follwing error:

CREATE FUNCTION for "usp_GetMailFolders" failed because T-SQL and CLR types for return value do not match.

Can anyone spot what I am missing here?

Thanks in advance for your response!

Answer 1

Try to change signature of function GetMailFolders to IEnumerable.

public static IEnumerable GetMailFolders(string WellKnownFolderName, string FolderID, string UserID, string password)

I looked again, and it seems to me to try to put nvarchar instead of varchar in the code to create the tool.