How Can I Efficiently Convert a DataReader into a List in .NET?

Process data efficiently: Convert DataReader to List

When processing data in a .NET environment, you may want to convert a DataReader (a data stream that only reads forward) into a more manageable format, such as a List. This transformation allows you to access and process structured data more efficiently.

Solution: Extension methods

One way to convert is to use extension methods. An example is as follows:

<code class="language-csharp">public static IEnumerable<T> Select<T>(this IDataReader reader,
                                       Func<IDataReader, T> projection)
{
    while (reader.Read())
    {
        yield return projection(reader);
    }
}</code>

This extension method allows you to use projection functions to select data from a DataReader and convert it to an IEnumerable.

Examples of usage

To convert data to List you can use the following code:

<code class="language-csharp">using (IDataReader reader = ...)
{
    List<Customer> customers = reader.Select(r => new Customer {
        CustomerId = r["id"] is DBNull ? null : r["id"].ToString(),
        CustomerName = r["name"] is DBNull ? null : r["name"].ToString()
    }).ToList();
}</code>

This example converts the rows in the DataReader into a list of Customer objects.

Alternative: specialized methods for entity types

Alternatively, you can create a dedicated static method in the Customer entity:

<code class="language-csharp">public static Customer FromDataReader(IDataReader reader) { ... }</code>

This method is responsible for creating a Customer object based on the data in the DataReader.

Using this method you can simplify the conversion process:

<code class="language-csharp">using (IDataReader reader = ...)
{
    List<Customer> customers = reader.Select<Customer>(Customer.FromDataReader)
                                     .ToList();
}</code>

Through these techniques, you can efficiently convert the data in DataReader into List, thereby leveraging the power of LINQ for further data manipulation and processing.

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