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

I want to accept a string array开发者_如何学JAVA 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")?

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>.

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.

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);