standards-c-sharp

Usage of IQueryable<T>

In WebApi

Return IQueryable<T> from a WebApi endpoint when you want to allow artitrary server-side filtering and paging. For example, to allow the client to arbitrarily query the products database:

public IQueryable<Product> Get() 
{
    return allProducts.AsQueryable();
}

The client can then construct arbitrary queries such as:

• http://localhost/odata/Products?$orderby=Category,Price desc
• http://localhost/Products?$filter=substringof('zz',Name)

In a class library

Return IQueryable<T> from a class library when all of the following are true:

• You want a to expose a dataset that can be queried arbitrarily.
• The dataset is not already materialized in memory (just return IEnumerable<T>).
• You can take advantage of the provided expression tree to optimize retrieval of the data.

An example is a LINQ to SQL provider. The C# expression tree provided by the consumer is translated into actual SQL statements that can be executed by the target database. Note: this is a dark art and not for the faint-hearted.

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