创建一个调用表达式树的表达式树

Question

I have a property on a class

Expression<Func<Product, int>> predicate;

that will be assigned to different expressions throughout the application.

In a method called GetProducts I would like to retrieve Products from the DB using Entity Framework, using this predicate. I will then have a variable called myInt that I would like to use as the int parameter, that will then be assigned a value.

So I tried

dbContext.Products.Where(p => predicate(p, myInt))

but I got Non-invocable member 'predicate' cannot be used like a method. error.

It looks like I need to do some expression tree manipulation, to create a new expression tree, with myInt baked in it. How can I do this?

Thank you for your help.

Answer 1

OK, got it.

ParameterExpression prm = Expression.Parameter(typeof(Product));
InvocationExpression inv = Expression.Invoke(predicate, prm, Expression.Constant(myInt));
var lambda = (Expression<Func<Product,bool>>) Expression.Lambda(inv, prm);
dbContext.Products.Where(lambda);

The key is Expression.Invoke that can invoke the predicate, using supplied parameters.

EDIT - After trying it, this only works with linq2sql - with Entity Framework it can't translate InvocationExpression to SQL. Instead, I used LinqKit , as follows:

dbContext.Products.AsExpandable().Where(p => predicate.Invoke(p, myInt))

Answer 2

You defined predicate as a Type but you are using it as a method.

You can define a predicate in the following way:

private bool predicate(Product product, int myInt)
{
    //put your logic here
    return true;
}

You can also use lambda expressions:

product => product.SomeValue > 5

Edit:

Expression<Func<Product, int>> predicate = (product,val) => product.SomeValue > val;
var filtered = dbContext.Products.Where(predicate);

• Avoid using types when naming a parameters (ie don't name an integer MyInt )