8.1 Creating DataColumns

When creating a new DataColumn, the overloaded constructor allows up to four optional arguments to be specified to initialize the ColumnName, DataType, Expression, and ColumnMapping properties. The following example creates a column and specifies both its name and data type:

// create the column and set the name and data type using properties 
DataColumn col = new DataColumn();
col.ColumnName = "MyTextColumn";
col.DataType = typeof(System.String);

// the code below is identical to the above three lines
DataColumn col = new DataColumn("MyTextColumn", typeof(System.String));

// set the maximum length in characters of the new column
col.MaxLength = 50;

The following example demonstrates another overloaded constructor and subsequently sets some properties of the column:

// create another column
DataColumn col = new DataColumn("Id", typeof(System.Int32));
col.AllowDBNull = false;   // the column doesn't allow null values
col.DefaultValue = -1;     // set the default value
col.Caption = "ID #";      // column title used by certain bound controls
col.Unique = true;         // the column value is each row must be unique
col.ReadOnly = true;       // the column value cannot be changed once the
                           // the row is added to a table

If the column name isn't supplied, the names Column1, Column2, Column3, and so on, are assigned when the column is added to the table. An exception is raised if the name of the column being added to the table is the same as the name of an existing column in the table.