DataColumn Class (ADO.NET)

The DataColumn class represents a column in a DataTable. It defines the schema of the column, including its name, data type, constraints, and default value.

Overview

A DataTable is a collection of DataColumn objects and DataRow objects. Each DataColumn defines the properties of a single column within the table. This includes:

• ColumnName: A unique string identifier for the column.
• DataType: The type of data the column can store (e.g., string, int, DateTime).
• AllowDBNull: A boolean value indicating whether the column can contain null values.
• DefaultValue: The default value for new rows in this column.
• MaxLength: The maximum length for string-based data types.
• Unique: A boolean value indicating whether all values in the column must be unique.
• Expression: A string defining an expression to calculate the column's value or filter rows.

Creating a DataColumn

You can create a DataColumn programmatically or add it to a DataTable through its Columns collection.

Programmatic Creation

You can instantiate a DataColumn using its constructors. The most common constructor takes the column name as an argument.

// Create a DataColumn for employee ID
DataColumn idColumn = new DataColumn("EmployeeID", typeof(int));
idColumn.Unique = true;
idColumn.AllowDBNull = false;
idColumn.AutoIncrement = true;
idColumn.AutoIncrementSeed = 1;
idColumn.AutoIncrementStep = 1;

// Create a DataColumn for employee name
DataColumn nameColumn = new DataColumn("FirstName", typeof(string));
nameColumn.AllowDBNull = false;
nameColumn.MaxLength = 50;

// Create a DataColumn for hire date
DataColumn hireDateColumn = new DataColumn("HireDate", typeof(DateTime));
hireDateColumn.DefaultValue = DateTime.Now;
                

Adding to a DataTable

Once created, a DataColumn can be added to the Columns collection of a DataTable.

DataTable employeesTable = new DataTable("Employees");

// Add the columns created above
employeesTable.Columns.Add(idColumn);
employeesTable.Columns.Add(nameColumn);
employeesTable.Columns.Add(hireDateColumn);
                

DataColumn Properties in Detail

DataType

This property is crucial as it dictates the kind of data that can be stored in the column and affects how data is compared and processed. Common types include:

• typeof(int)
• typeof(string)
• typeof(DateTime)
• typeof(decimal)
• typeof(bool)
• typeof(byte[])

AllowDBNull

Setting this to false enforces that every row must have a value for this column. If you try to add a row with a null value to a column where AllowDBNull is false, a ConstraintException will be thrown.

DefaultValue

When a new row is added to the DataTable, if a value is not explicitly provided for a column, the DefaultValue of that column will be used.

Unique

If set to true, this property ensures that no two rows in the DataTable can have the same value in this column. A ConstraintException is thrown if a duplicate value is attempted.

Expression

The Expression property is a powerful feature that allows you to perform calculations or filtering directly within the DataTable. It supports a syntax similar to SQL's WHERE and SELECT clauses.

DataColumn fullNameColumn = new DataColumn("FullName", typeof(string), "FirstName + ' ' + LastName");
employeesTable.Columns.Add(fullNameColumn);
                    

DataRow[] filteredRows = employeesTable.Select("EmployeeID > 100");
                    

Constraints

In addition to the properties like Unique and AllowDBNull, you can define more complex constraints on DataColumn objects using the Constraints collection of the DataTable. Common constraints include:

• UniqueConstraint: Ensures all values in a column (or set of columns) are unique.
• ForeignKeyConstraint: Establishes relationships between tables.
• UniqueConstraint: Ensures all values in a column (or set of columns) are unique.

See Also