Question
I'm trying to create a filter method for Entity framework List and understand better the Expression<Func<...
I have a Test Function like this.
public IQueryable<T> Filter<T>(IEnumerable<T> src, Expression<Func<T, bool>> pred)
{
return src.AsQueryable().Where(pred);
}
and if I do this:
context.Table.Filter(e => e.ID < 500);
or this:
context.Table.Filter(e => e.SubTable.Where(et => et.ID < 500).Count() > 0 && e.ID < 500);
it all works well.
But if I do this:
context.Table.Filter(e => e.SubTable.Filter(et => et.ID < 500).Count() > 0 && e.ID < 500);
or this:
context.Table.Where(e => e.SubTable.Filter(et => et.ID < 500).Count() > 0 && e.ID < 500);
I receive one error. LINQ to Entities does not recognize the method ...Filter...
Why it works in one case and not in the adder? What should I change in the Filter for it to work with related tables. I prefer to stay away from other external library's as what I want is to learn how it works and be able to use it in any scenario in future.
In the first two cases the filter runs in the database correctly.
Accepted Answer
Jon and Tim already explained why it doesn't work.
Assuming that the filter code inside Filter is not trivial, you could change Filter so that it returns an expression EF can translate.
Let's assume you have this code:
context.Table.Where(x => x.Name.Length > 500);
You can now create a method the returns this expression:
Expression<Func<YourEntity, bool>> FilterByNameLength(int length)
{
return x => x.Name.Length > length;
}
Usage would be like this:
context.Table.Where(FilterByNameLength(500));
The expression you build inside FilterByNameLength can be arbitrarily complex as long as you could pass it directly to Where.
Popular Answer
It's useful to understand the difference between Expression<Func<>> and Func<>.
An Expression e => e.ID < 500 stores the info about that expression: that there's a T e, that you're accessing the property ID, calling the < operator with the int value 500. When EF looks at that, it might turn it into something like [SomeTable].[ID] < 500.
A Func e => e.ID < 500 is a method equivalent to:
static bool MyMethod(T e) { return e.ID < 500; }
It is compiled as IL code that does this; it's not designed to be 'reconstituted' into a SQL query or anything else, only run.
When EF takes your Expression, it must understand every piece of it, because it uses that to build a SQL query. It is programmed to know what the existing Where method means. It does not know what your Filter method means, even though it's a trivial method, so it just gives up.
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