CommandBehavior.SchemaOnly in C#

The CommandBehavior.SchemaOnly option in C# is used with ADO.NET DataReader to retrieve only the schema information of the result set returned by a SQL query, without actually fetching any data. This is useful when you want to examine the structure of the result set, such as column names and data types, without incurring the overhead of fetching the actual data. It can be particularly helpful for tasks like dynamic query generation or metadata inspection.

Let's illustrate how to use CommandBehavior.SchemaOnly with a complete C# source code example and its expected output:

using System;
using System.Data;
using System.Data.SqlClient;

class Program
{
    static void Main()
    {
        string connectionString = "Server=your_server_name;Database=your_database_name;User Id=your_username;Password=your_password;";

        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            connection.Open();

            string sqlQuery = "SELECT * FROM Customers";
            using (SqlCommand command = new SqlCommand(sqlQuery, connection))
            {
                // Use CommandBehavior.SchemaOnly to retrieve schema information only
                using (SqlDataReader reader = command.ExecuteReader(CommandBehavior.SchemaOnly))
                {
                    DataTable schemaTable = reader.GetSchemaTable();

                    // Display schema information
                    Console.WriteLine("Column Name\tData Type");
                    foreach (DataRow row in schemaTable.Rows)
                    {
                        string columnName = row["ColumnName"].ToString();
                        string dataType = row["DataType"].ToString();
                        Console.WriteLine($"{columnName}\t\t{dataType}");
                    }
                }
            }
        }
    }
}

Explanation of the code:

1. We set up the database connection string, specifying the server name, database name, username, and password.
2. Inside a using block, we create a SqlConnection object and open the database connection.
3. We define a SQL query to select all columns from a "Customers" table.
4. We create a SqlCommand object with the SQL query and the database connection.
5. Within another using block, we execute the command using ExecuteReader(CommandBehavior.SchemaOnly). This ensures that only schema information is retrieved.
6. We use the GetSchemaTable() method of the DataReader to get a DataTable containing the schema information.
7. We iterate through the rows of the schema table, extracting the column name and data type, and display them in the console.

Expected Output

Column Name     Data Type
CustomerID      System.Int32
FirstName       System.String
LastName        System.String
Email           System.String
PhoneNumber     System.String

In this example, CommandBehavior.SchemaOnly is used to retrieve and display the schema information (column names and data types) for the "Customers" table without fetching any actual data. This can be helpful for tasks that require understanding the structure of a result set before working with its data.