How To Return Nvarchar(max) From CLR?

How to return nvarchar(max) from CLR?
----------------------
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
[Microsoft.SqlServer.Server.SqlFunction(DataAccess = DataAccessKind.Read)]
public static SqlString Select_Description(int Obj_ID)
{

using (SqlConnection conn = new SqlConnection("context connection=true"))
{
try
{
string sql = @"Select description from tbl_detail";

SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
return (SqlString)cmd.ExecuteScalar();

}
catch (Exception e)
{
throw e;
}

finally
{
conn.Close();
}
}
}

};
----------------------

Code above work fine with any [description]< 4000 characters. Get an error if description field have values> 4000 characters.

Try to solve ==> Looking for the net they said change SqlString ==> SqlChars

Here my modify code
------------------------
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
[Microsoft.SqlServer.Server.SqlFunction(DataAccess = DataAccessKind.Read)]
public static SqlString Select_Description(int Obj_ID)
{

using (SqlConnection conn = new SqlConnection("context connection=true"))
{
try
{
string sql = @"Select description from tbl_detail";

SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
return (SqlChars)cmd.ExecuteScalar(); //<== is this correct syntax?

}
catch (Exception e)
{
throw e;
}

finally
{
conn.Close();
}
}
}

};
------------------------

the error come when compile

Error 1 Cannot implicitly convert type 'System.Data.SqlTypes.SqlChars' to 'System.Data.SqlTypes.SqlString'. An explicit conversion exists (are you missing a cast?)

Please teach me how to solve this problem. Thank you.
I'm newbie on CLR and .NET