无法将Distinct函数与C＃和LINQ中的NTEXT字段一起使用

Question

I have a field called Description in a table that is of the type NTEXT. My issue is that I need to use the method .Distinct() but am not able to as "The ntext data type cannot be selected as DISTINCT because it is not comparable."

There is another question very similar to this found here but the accepted solution does not work for me. When I try it I get a new error saying "Argument data type ntext is invalid for argument 1 of len function."

Below is the Linq statement I had originally and the one below it is what I tried to do after reading the answer to the other similar question .

return (from vl in db.ValueLog
        join vc in db.ValueCodes on vl.ValueCode equals vc.ValueCode
        select new ValueLogItem
        {
            ValueId = vl.ValueId ,
            Description = vl.Description.Substring(0),
            Quantity = vl.Quantity,
            Code = vc.ValueCode
        }).Distinct().ToList();

Does anyone have any ideas on how to get around this restriction besides just converting the field? Thanks!

I am well aware that NTEXT, TEXT, and IMAGE have all been deprecated since 2005, but I am not able to change the field's type at the moment as I do not have access to do that.

I am also well aware that NTEXT, TEXT, and IMAGE can't be made DISTINCT. I am trying to figure out a work around in LINQ. Doing it in SQL is extremely simple.

Answer 1

I am writing down the ways you can try to compare

First Try GroupBy instead of Distinct. GroupBY works in similar way

Second Convert the Description into Byte array and compare the arrays. For that, you can use

static bool ByteArrayCompare(byte[] a1, byte[] a2) 
{
    return StructuralComparisons.StructuralEqualityComparer.Equals(a1, a2);
}

or

return a1.SequenceEqual(b1);

or

public bool Equality(byte[] a1, byte[] b1)
{
   int i;
   if (a1.Length == b1.Length)
   {
      i = 0;
      while (i < a1.Length && (a1[i]==b1[i])) //Earlier it was a1[i]!=b1[i]
      {
          i++;
      }
      if (i == a1.Length)
      {
          return true;
      }
   }

   return false;
}

Answer 2

The basic issue is that TEXT, NTEXT, and IMAGE data fields are not allowed in a SELECT using DISTINCT. One of SQL Server's errors specifically states:

Server: Msg 421, Level 16, State 1, Line 1
The text/ntext/image data type cannot be selected 
as DISTINCT because it is not comparable.

If you can't change the data type, you won't be able to use DISTINCT on that field. So, CAST/CONVERT the text field to VARCHAR(MAX) on the fly if you can't change the underlying data type.