How To Change DataType of a DataColumn in a DataTa

Question

I have:

DataTable Table = new DataTable;
SqlConnection = new System.Data.SqlClient.SqlConnection("Data Source=" + ServerName + ";Initial Catalog=" + DatabaseName + ";Integrated Security=SSPI; Connect Timeout=120");

SqlDataAdapter adapter = new SqlDataAdapter("Select * from " + TableName, Connection);
adapter.FillSchema(Table, SchemaType.Source);
adapter.Fill(Table);

DataColumn column = DataTable.Columns[0];

What I want to do is:

Assume currently column.DataType.Name is "Double". I want it to become "Int32".

How do I achieve this?

Answer 1

Consider also altering the return type:

select cast(columnName as int) columnName from table

Answer 2

Once a DataTable has been filled, you can't change the type of a column.

Your best option in this scenario is to add an Int32 column to the DataTable before filling it:

dataTable = new DataTable("Contact");
dataColumn = new DataColumn("Id");
dataColumn.DataType = typeof(Int32);
dataTable.Columns.Add(dataColumn);

Then you can clone the data from your original table to the new table:

DataTable dataTableClone = dataTable.Clone();

Here's a post with more details.

Answer 3

I created an extension function which allows changing the column type of a DataTable. Instead of cloning the entire table and importing all the data it just clones the column, parses the value and then deletes the original.

    /// <summary>
    /// Changes the datatype of a column. More specifically it creates a new one and transfers the data to it
    /// </summary>
    /// <param name="column">The source column</param>
    /// <param name="type">The target type</param>
    /// <param name="parser">A lambda function for converting the value</param>
    public static void ChangeType(this DataColumn column, Type type, Func<object, object> parser)
    {
        //no table? just switch the type
        if (column.Table == null)
        {
            column.DataType = type;
            return;
        }

        //clone our table
        DataTable clonedtable = column.Table.Clone();

        //get our cloned column
        DataColumn clonedcolumn = clonedtable.Columns[column.ColumnName];

        //remove from our cloned table
        clonedtable.Columns.Remove(clonedcolumn);

        //change the data type
        clonedcolumn.DataType = type;

        //change our name
        clonedcolumn.ColumnName = Guid.NewGuid().ToString();

        //add our cloned column
        column.Table.Columns.Add(clonedcolumn);

        //interpret our rows
        foreach (DataRow drRow in column.Table.Rows)
        {
            drRow[clonedcolumn] = parser(drRow[column]);
        }

        //remove our original column
        column.Table.Columns.Remove(column);

        //change our name
        clonedcolumn.ColumnName = column.ColumnName;
    }
}

You can use it like so:

List<DataColumn> lsColumns = dtData.Columns
    .Cast<DataColumn>()
    .Where(i => i.DataType == typeof(decimal))
    .ToList()

//loop through each of our decimal columns
foreach(DataColumn column in lsColumns)
{
    //change to double
    column.ChangeType(typeof(double),(value) =>
    {
        double output = 0;
        double.TryParse(value.ToString(), out output);
        return output;  
    });
}

The above code changes all the decimal columns to doubles.

Answer 4

Dim tblReady1 As DataTable = tblReady.Clone()

'' convert all the columns type to String 
For Each col As DataColumn In tblReady1.Columns
  col.DataType = GetType(String)
Next

tblReady1.Load(tblReady.CreateDataReader)

Answer 5

I've taken a bit of a different approach. I needed to parse a datetime from an excel import that was in the OA date format. This methodology is simple enough to build from... in essence,

1. Add column of type you want
2. Rip through the rows converting the value

3. Delete the original column and rename to the new to match the old

   private void ChangeColumnType(System.Data.DataTable dt, string p, Type type){
           dt.Columns.Add(p + "_new", type);
           foreach (System.Data.DataRow dr in dt.Rows)
           {   // Will need switch Case for others if Date is not the only one.
               dr[p + "_new"] =DateTime.FromOADate(double.Parse(dr[p].ToString())); // dr[p].ToString();
           }
           dt.Columns.Remove(p);
           dt.Columns[p + "_new"].ColumnName = p;
       }
   

Answer 6

While it is true that you cannot change the type of the column after the DataTable is filled, you can change it after you call FillSchema, but before you call Fill. For example, say the 3rd column is the one you want to convert from double to Int32, you could use:

adapter.FillSchema(table, SchemaType.Source);
table.Columns[2].DataType = typeof (Int32);
adapter.Fill(table);