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Combining DataTables with Different Schema

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21 Nov 2017CPOL
There is a problem with accessing Excel files via OleDb which contain more than 255 columns: they are truncated and need to be fetched using ranges. But what do you do with DataTables after that?


This came up because there is a problem in QA with OleDb access to Excel files which means that only 255 columns are fetched at a time. That's not difficult to work around - use Ranges to fetch the separate columns into different tables. But ... what then? It's a pain to handle two (or more) separate tables in order to process a single row, and the DataTable.Merge method combines two tables with the same schema to provide more rows, not more columns.

This tip provides a "combine columns" method to "merge" tables producing the same number of rows, but more columns. Do note that it will fail if the column names overlap - It wouldn't be difficult to add code to get round that, but it would slow the process so I'll leave it as an exercise for the reader.

The Code that Does the Work

/// <summary>
/// Merge a list of tables into a single table with more columns.
/// </summary>
/// <remarks>
/// This isn't the most efficient method, I'm sure - but the code
/// would get seriously incomprehensible to do it all via one
/// single Linq method call. For the sake of readability and
/// maintainability I'll live with the inefficiency.
/// </remarks>
/// <param name="baseTable"></param>
/// <param name="additionalTables"></param>
/// <returns></returns>
public static DataTable MergeTables(DataTable baseTable, params DataTable[] additionalTables)
    // Build combined table columns
    DataTable merged = baseTable;
    foreach (DataTable dt in additionalTables)
        merged = AddTable(merged, dt);
    return merged;
/// <summary>
/// Merge two tables into a single table with more columns.
/// </summary>
/// <param name="baseTable"></param>
/// <param name="additionalTable"></param>
/// <returns></returns>
public static DataTable AddTable(DataTable baseTable, DataTable additionalTable)
    // Build combined table columns
    DataTable merged = baseTable.Clone();                  // Include all columns from base table in result.
    foreach (DataColumn col in additionalTable.Columns)
        string newColumnName = col.ColumnName;
        merged.Columns.Add(newColumnName, col.DataType);
    // Add all rows from both tables
    var bt = baseTable.AsEnumerable();
    var at = additionalTable.AsEnumerable();
    var mergedRows = bt.Zip(at, (r1, r2) => r1.ItemArray.Concat(r2.ItemArray).ToArray());
    foreach (object[] rowFields in mergedRows)
    return merged;

Only one line is complicated:

var mergedRows = bt.Zip(at, (r1, r2) => r1.ItemArray.Concat(r2.ItemArray).ToArray());

Zip just executes a Lambda on each "pair" of items: a DataRow from the base table, and a DataRow from the additional table.

The Lambda just concatenates the two item collections into a single collection and converts it into an array of objects. The array just makes it easier to describe the collection content in the following loop: using a var there confuses the compiler a bit. (Now you see why I do this the inefficient way!)

Using the Code

It's pretty obvious how to use it - you need a few tables:

ID Name Email
1 Mike
2 Susan


City Country
London England
Cardiff Wales


Favourite Colour Favourite Fruit
Green Apple
Yellow Banana

As in:

DataTable dt1 = new DataTable();
dt1.Columns.Add("ID", typeof(int));
dt1.Columns.Add("Name", typeof(string));
dt1.Columns.Add("Email", typeof(string));
dt1.Rows.Add(1, "Mike", "");
dt1.Rows.Add(2, "Susan", "");
var dt2 = new DataTable();
dt2.Columns.Add("City", typeof(string));
dt2.Columns.Add("Country", typeof(string));
dt2.Rows.Add("London", "England");
dt2.Rows.Add("Cardiff", "Wales");
var dt3 = new DataTable();
dt3.Columns.Add("Favourite Colour", typeof(string));
dt3.Columns.Add("Favourite Fruit", typeof(string));
dt3.Rows.Add("Green", "Apple");
dt3.Rows.Add("Yellow", "Banana");

Then just combine the tables:

DataTable dtMerged = MergeTables(dt1, dt2, dt3);
ID Name Email City Country Favourite Colour Favourite Fruit
1 Mike London England Green Apple
2 Susan Cardiff Wales Yellow Banana


  • 2017-11-21 First version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Wales Wales
Born at an early age, he grew older. At the same time, his hair grew longer, and was tied up behind his head.
Has problems spelling the word "the".
Invented the portable cat-flap.
Currently, has not died yet. Or has he?

Comments and Discussions

QuestionGetting error in Zip method while compiling the code. Please help me to resolve this issue. Pin
Member 144796713-Jun-19 20:15
MemberMember 144796713-Jun-19 20:15 

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Posted 21 Nov 2017

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