How to create LINQ Expression Tree to select an anonymous type

Question

I would like to generate the following select statement dynamically using expression trees:

var v = from c in Countries
        where c.City == \"London\"
           


        

Answer 1

The accepted answer is very useful, but I needed something a little closer to a real anonymous type.

A real anonymous type has read-only properties, a constructor for filling in all of the values, an implementation of Equals/GetHashCode for comparing the values of each property, and an implementation ToString that includes the name/value of each property. (See https://msdn.microsoft.com/en-us/library/bb397696.aspx for a full description of anonymous types.)

Based on that definition of anonymous classes, I put a class that generates dynamic anonymous types on github at https://github.com/dotlattice/LatticeUtils/blob/master/LatticeUtils/AnonymousTypeUtils.cs. The project also contains some unit tests to make sure the fake anonymous types behave like real ones.

Here's a very basic example of how to use it:

AnonymousTypeUtils.CreateObject(new Dictionary
{
    { "a", 1 },
    { "b", 2 }
});

Also, another note: I found that when using a dynamic anonymous type with Entity Framework, the constructor must be called with the "members" parameter set. For example:

Expression.New(
    constructor: anonymousType.GetConstructors().Single(), 
    arguments: propertyExpressions,
    members: anonymousType.GetProperties().Cast().ToArray()
); 

If you used one of the versions of Expression.New that does not include the "members" parameter, Entity Framework would not recognize it as the constructor of an anonymous type. So I assume that means a real anonymous type's constructor expression would include that "members" information.