Compare 2 tables columns names, add it if doesn't exist

Question

I have a question about comparing 2 tables. If in tables 1 doesn't contain column's name from tables 2, add the column with values. So i did it with my code, but don't know why it gives me error that the column already belongs to tables1. What did i do wrong here? Is there any better way to do it?

Example, table1:

Name   LastName
a       aa
b       bb

table2:

Name    Product
s       dd
a       ss

result:

Name   LastName    Product
a       aa         dd
b       bb         ss      

My code:

 for (int i = 0; i < excelTb2.Columns.Count; i++)
                {
                    for (int j = 0; j < Temp.Columns.Count; j++ )
                    {
                        if (Temp.Columns[j].ColumnName.ToString() != excelTb2.Columns[i].ColumnName.ToString())
                        {
                            excelTb2.Columns.Add(Temp.Columns[j].ColumnName.ToString());

                            for (int ok = 0; ok < 2; ok++)
                            {
                                excelTb2.Rows[ok][Temp.Columns[j].ColumnName] = Temp.Rows[ok][j];
                            }

                        }
                    }
                }

Answer 1

Columns is a collection. You could check if the column name is already present using Contains

for (int j = 0; j < Temp.Columns.Count; j++ )
{
    if(!excelTb2.Columns.Contains(Temp.Columns[j].ColumnName))
    {
        excelTb2.Columns.Add(Temp.Columns[j].ColumnName.ToString());
        ...
    }
}

This will remove the need of the nested loop that is the main cause of the error you get

Answer 2

You could use this Merge method which merges the schema of two DataTables and joins data on row-index(if both tables contain a column, the data of table1 will be taken as requested):

public static DataTable MergeOnRowIndex(DataTable table1, DataTable table2)
{
    var data = table1.AsEnumerable()
        .Select((r, i) => new
        {
            Row1 = r,
            Row2 = i >= table2.Rows.Count ? null : table2.Rows[i]
        });
    DataTable table3 = new DataTable();
    foreach (DataColumn col in table1.Columns)
    {
        table3.Columns.Add(col.ColumnName, col.DataType);
    }
    foreach(DataColumn col in table2.Columns)
        if(!table3.Columns.Contains(col.ColumnName))
            table3.Columns.Add(col.ColumnName, col.DataType);

    if(data.Any())
    {
        foreach(var x in data)
        {
            var newRow = table3.Rows.Add();
            for (int i = 0; i < table1.Columns.Count; i++)
            {
                newRow[i] = x.Row1.ItemArray[i];
            }
            if (x.Row2 != null)
            {
                for (int i = table1.Columns.Count; i < table3.Columns.Count; i++)
                {
                    DataColumn currentColumn = table3.Columns[i];
                    newRow[currentColumn.ColumnName] = x.Row2[currentColumn.ColumnName];
                }
            }
        }
    }
    return table3;
}

Here i've used the method to get your desired result on your sample data:

var table = new DataTable();
table.Columns.Add("Name");
table.Columns.Add("LastName");
var otherTable = new DataTable();
otherTable.Columns.Add("Name");
otherTable.Columns.Add("Product");
table.Rows.Add("a","aa");
table.Rows.Add("b","bb");
otherTable.Rows.Add("s","dd");
otherTable.Rows.Add("a","ss");

DataTable result = MergeOnRowIndex(table, otherTable);