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c#linqparsingreflectiondelegates
mikeschuld
mikeschuld

Reputation: 963

Transforming a filter string into a C# delegate

I have a set of classes which I am using for a Data Access Layer for some clients. As part of the data access I am allowing a set of filters to be sent in this format:

"{Member[.Member....]}{Operator}{Value}"

I would like to turn these strings into delegates for use in a LINQ query like this:

.Where([delegate returned by a factory])

Here is a more concrete example:

IEnumerable<Parent> parents = GetSomeParents();
string filter = "Child.Id=5";
var expression = FilterFactory<Parent>.GetExpression(filter);

parents = parents.Where(expression);

expression would contain the delegate: parent => parent.Child.Id == 5

Is there a way using reflection to construct the FilterFactory in a Generic way to handle any member path I send in? Paths with indexing aren't required, but would be nice.

Upvotes: 1

Views: 788

Answers (1)

justin.m.chase
justin.m.chase

Reputation: 13685

Yes absolutely! This is a really fun thing to do too.

One way you can do this is to use the LINQ Dynamic Query Library and get the expression compiler they have in there. I also have something very similar in my project MetaSharp.

But you could also do it yourself if the syntax or features don't quite match what you're wanting. The general idea is that you need to parse the string and build up an Expression tree that represents what you are parsing. In .NET the expression tree objects can be found in System.Linq.Expressions. Once you have your tree you can call Compile() on it and it will be dynamically compiled into a delegate right then. Try reading about the State Machine and Visitor patterns to figure out the best way to parse an arbitrary expresssion like you have above.

PS I would not recommend using regular expressions!

Upvotes: 4

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