Runtime: SqlBulkCopy.WriteToServer fails with null reference exception when using a named column mapping and IDataReader not implemented via DbDataReader

[drunkcod]

How to reproduce:
1) implment IDataReader without inherting from DbDataReader
2) Create a SqlBulkCopy object having a column mapping where from and to are column names.
3) WriteToServer will fail due to a null reference exception.

```c#
//how to repro. (Using DataBoss.Data 0.0.42 from NuGet on netstandard 2.0)
using DataBoss.Data;
using System;
using System.Data.SqlClient;
using System.Diagnostics;

namespace ConsoleApp3
{
class Program
{
static void Main(string[] args)
{
var db = new SqlConnection("Server=.;Integrated Security=SSPI");

    db.Open();
    db.ExecuteNonQuery("create table #Temp(id int not null)");

    var toInsert = SequenceDataReader.Create(new[] { 1, 2, 3 }, x => x.Map("id", id => id));

    var bulkCopy = new SqlBulkCopy(db)
    {
        DestinationTableName = "#Temp",
    };

    for (var i = 0; i != toInsert.FieldCount; ++i)
    {
        var n = toInsert.GetName(i);
        bulkCopy.ColumnMappings.Add(n, n);
    }
    bulkCopy.WriteToServer(toInsert);
  }

}
}
```

Known workaround: Create the column mapping by ordinal (change bulkCopy.ColumnMappings.Add(n, n); to 2.Add(i, n)".

Likely culprit as far as I can discern is mishandling the _DbDataReaderSource here:

[EDIT] Add C# syntax highlight + space formatting by @karelz
[EDIT] Clarified repro code, added link to my guess on source. @drunkcod

[karelz]

@drunkcod where is the code snippet from? Can you please just link it instead of screenshot? (I couldn't find it in CoreFX)

[drunkcod]

Clarified that the snippet is for reproduction (and what packages to includede) and linked to the screnshotted line (sorry didn't know one could do that before)
https://github.com/dotnet/corefx/blob/master/src/System.Data.SqlClient/src/System/Data/SqlClient/SqlBulkCopy.cs#L848

[satano]

Hello

This is (very probably) duplicate of dotnet/runtime#23843, and the fix was already mreged dotnet/corefx#24655.

@drunkcod As a workaround for now, you have to inherit from DbDataReader.

[satano]

Hmmmm... On the second look, it seems, that it is the same issue, but another place, so another fix is needed. I will try to reproduce it and preprare PR in couple of days.

[drunkcod]

@satano - duplicate looks legit!
My "likely culprit" is from reading the code hunting for a non null-checked access so if it doesn't repro on current dev-bits you can likely assume it's my code reading that's at fault and the issue will disappear whenever the those bits are trickled out to nuget.

Since it works if I use "ordinal to name" instead of "name to name" column mappings and my use-case will work wonderfully well with that I'll simply go with that. One could argue that's a more elegant way anyhow.

Thanks for the swift responses and sorry for the missing link and bad search-fu in the issue tracker.

[karelz]

Duplicate of dotnet/runtime#23843
Thanks for detecting the dupe @satano!