How Can I Dynamically Select Anonymous Type Properties in LINQ Using Expression Trees?

Use expression trees to dynamically select LINQ anonymous type attributes

It is possible to use expression trees to build complex LINQ queries to dynamically select anonymous types. While queries that select a single attribute are easy to generate, selecting multiple attributes in an anonymous type requires a different approach.

To do this, anonymous types can be dynamically defined at runtime using reflective emission and helper classes. Here's an example:

SelectDynamic extension method:

<code>public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
{
    ... // 实现细节
}</code>

LinqRuntimeTypeBuilder auxiliary class:

<code>public static class LinqRuntimeTypeBuilder
{
    ... // 实现细节
}</code>

This approach allows the creation of complex dynamic selections without Intellisense support, which is useful for late-bound data controls.

The following example generates a query that selects the Name and Population properties from the Countries entity where City equals "London":

<code>var v = Countries.Where(c => c.City == "London")
    .SelectDynamic(new[] { "Name", "Population" });</code>

As a result, the variable v will contain a dynamic type with Name and Population properties.

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