Entity Framework 4: convert a string condition to a lambda expression?

Question

I want to accept a string array of where conditions from the client like field == value. It would be really nice to create a specification object that could accept the string in the constructor and output a lambda expression to represent the Where clause. For example, I could do the following:

var myCondition = new Specification<Product>( myStringArrayOfConditions); 
var myProducts = DB.Products.Where( myCondition);

How could you turn "name == Jujyfruits" into
DB.Products.Where(p => p.name == "JujyFruits")?

Why can't you pass a lambda into the method, so instead of passing the string .Where("name == Jujyfruits") you pass in .Where(x => x.name == "Jujyfruits")? I don't really know what you're trying to do. — Phill
– Phill, Commented May 2, 2011 at 5:38
If the "specification" (where clause) comes from the client, it will be in the form of a string. I would need to convert it in to something EF could understand, which is normally a lambda expression. — Zachary Scott
– Zachary Scott, Commented May 3, 2011 at 1:02

Heinzi · Accepted Answer · 2011-05-03 06:42:51Z

15

You can use

Reflection to get the Property Product.name from the string name and
the LINQ Expression class to manually create a lambda expression.

Note that the following code example will only work for Equals (==) operations. However, it is easy to generalize to other operations as well (split on whitespace, parse the operator and choose the appropriate Expression instead of Expression.Equal).

    var condition = "name == Jujyfruits";

    // Parse the condition
    var c = condition.Split(new string[] { "==" }, StringSplitOptions.None);
    var propertyName = c[0].Trim();
    var value = c[1].Trim();

    // Create the lambda
    var arg = Expression.Parameter(typeof(Product), "p");
    var property = typeof(Product).GetProperty(propertyName);
    var comparison = Expression.Equal(
        Expression.MakeMemberAccess(arg, property),
        Expression.Constant(value));
    var lambda = Expression.Lambda<Func<Product, bool>>(comparison, arg).Compile();

    // Test
    var prod1 = new Product() { name = "Test" };
    var prod2 = new Product() { name = "Jujyfruits" };
    Console.WriteLine(lambda(prod1));  // outputs False
    Console.WriteLine(lambda(prod2));  // outputs True

About the constructor thing: Since Func<T, TResult> is sealed, you cannot derive from it. However, you could create an implicit conversion operator that translates Specification<T> into Func<T, bool>.

edited May 3, 2011 at 6:42

answered May 2, 2011 at 5:26

Heinzi

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10 Comments

Zachary Scott Over a year ago

So you could do something like Products.Where( p => lambda(p))?

Heinzi Over a year ago

@Dr. Zim: Actually, Products.Where(lambda) should be sufficient.

AnteSim Over a year ago

If you're going to go down this route, consider caching the delegate (var lambda). There is a reasonable performance hit from compiling expression trees and you definitely wouldn't want to do it prior to each search query.

Heinzi Over a year ago

@AnteSim: That depends on how you define "reasonable". Compiling the lambda in my example above takes ~5ms on my machine. I guess the database query part of the search takes significantly longer...

AnteSim Over a year ago

@Heinzi: What happens when he adapts this code to filter the search based on multiple properties? In isolation, your code is fine. But you just need to be aware that this is a point of optimisation should the need arise.

|

Jim Ross · Accepted Answer · 2011-05-02 17:52:38Z

3

We've recently discovered the Dynamic LINQ library from the VS2008 sample projects. Works perfectly to turn string based "Where" clauses into expressions.

This link will get you there.

answered May 2, 2011 at 17:52

Jim Ross

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1 Comment

Zachary Scott Over a year ago

It definitely uses a string in the where clause. It's a good answer. However, it produces a side effect where it changes the data type.

Paul Suart · Accepted Answer · 2011-05-02 05:02:07Z

0

You need to turn your search term into a predicate. Try something like the following:

string searchString = "JujyFruits";
Func<Product, bool> search = new Func<Product,bool>(p => p.name == searchString);

return DB.Products.Where(search);

answered May 2, 2011 at 5:02

Paul Suart

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3 Comments

Zachary Scott Over a year ago

Would there be a way around the big switch statement testing the name of every property of Product? Could you get a list of the "primitive" properties of Product, test against the field magic string, and construct a lambda expression where p.name is expressed as a magic string?

Paul Suart Over a year ago

Hmm, I'm not sure this is trivial. See this question: stackoverflow.com/q/714799/68432

Zachary Scott Over a year ago

linqspecs.codeplex.com is very much like your answer, which reinforces that your method is probably the generally accepted approach of making specifications first class citizens.

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