How to merge two datatables with PrimaryKeys?

I have two data tables and want merge columns and create as third table.

//Given
dt1 {A(PK),B,C,D}
dt2 {a(PK),b,c,d}
//Want
dtResult {A,B,C,D,a,b,c,d}

Do not want to lose ExtendedProperties so not Selecting DataColumn.ColumnName and adding as columns.

Code:

DataTable dt1 = new DataTable();
dt1.TableName = "dt1";
dt1.PrimaryKey = new DataColumn[] { dt1.Columns.Add("A") };
dt1.Columns.Add("B");
dt1.Columns.Add("C");
dt1.Columns.Add("D");

DataTable dt2 = new DataTable();
dt2.TableName = "dt2";
dt2.PrimaryKey = new DataColumn[] { dt2.Columns.Add("a") };
dt2.Columns.Add("b");
dt2.Columns.Add("c");
dt2.Columns.Add("d");

var dtResult = dt1.Clone();
dtResult.TableName = "dtResult";

Working fine till here

Now want to merge...

Multiple merge tries type 1

dtResult.Merge(dt2.Clone()); 
dtResult.Merge(dt2.Clone(), false);
dtResult.Merge(dt2.Clone(), true);
dtResult.Merge(dt2.Clone(), false, MissingSchemaAction.Add);
dtResult.Merge(dt2.Clone(), false, MissingSchemaAction.AddWithKey); 
dtResult.Merge(dt2.Clone(), true, MissingSchemaAction.Add);
dtResult.Merge(dt2.Clone(), true, MissingSchemaAction.AddWithKey);

In all above tries, Columns are merged into dtResult but throws NullReferenceException. StackTrace:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=System.Data
  StackTrace:
   at System.Data.Merger.MergeSchema(DataTable table)
   at System.Data.Merger.MergeTableData(DataTable src)
   at System.Data.Merger.MergeTable(DataTable src)
   at System.Data.DataTable.Merge(DataTable table, Boolean preserveChanges, MissingSchemaAction missingSchemaAction)
   at Demo.Program.Test5() in C:\App\Demo\Program.cs:line 54
   at Demo.Program.Main(String[] args) in C:\App\Program.cs:line 36

Multiple merge tries type 2

dtResult.Merge(dt2.Clone(), false, MissingSchemaAction.Error);
dtResult.Merge(dt2.Clone(), true, MissingSchemaAction.Error);

In all above tries, Columns are NOT merged into dtResult and System.Data.DataException StackTrace:

System.Data.DataException
  HResult=0x80131920
  Message=Target table  missing definition for column a.
  Source=System.Data
  StackTrace:
   at System.Data.Merger.MergeSchema(DataTable table)
   at System.Data.Merger.MergeTableData(DataTable src)
   at System.Data.Merger.MergeTable(DataTable src)
   at System.Data.DataTable.Merge(DataTable table, Boolean preserveChanges, MissingSchemaAction missingSchemaAction)
   at Demo.Program.Test5() in C:\App\Demo\Program.cs:line 54
   at Demo.Program.Main(String[] args) in C:\App\Demo\Program.cs:line 36

Multiple merge tries type 3

dtResult.Merge(dt2.Clone(), false, MissingSchemaAction.Ignore);
dtResult.Merge(dt2.Clone(), true, MissingSchemaAction.Ignore);

In all above tries, Columns are NOT merged into dtResult and NO exception

Apparently, the NullReferenceException occurs because the two tables have primary keys. We need to do something to avoid that conflict.

There you go:

var dtResult = dt1.Clone();
dtResult.TableName = "dtResult";

var dt2Cloned = dt2.Clone();
dt2Cloned.PrimaryKey = null;
dtResult.Merge(dt2Cloned);

var primaryKeys = dtResult.PrimaryKey.ToList();
foreach (var col in dt2.PrimaryKey)
{
    primaryKeys.Add(dtResult.Columns[col.ColumnName]);
}
dtResult.PrimaryKey = primaryKeys.ToArray();

This can probably be optimized a little but it gets you the result you need so it's a good start.