How does PredicateBuilder work

by

Let’s say you have:

Expression<Func<Person, bool>> isAdult = p1 => p1.Age >= 18;

// I've given the parameter a different name to allow you to differentiate.
Expression<Func<Person, bool>> isMale = p2 => p2.Gender == "Male";

And then combine them with PredicateBuilder

var isAdultMale = isAdult.And(isMale);

What PredicateBuilder produces is an expression that looks like this:

// Invoke has no direct equivalent in C# lambda expressions.
p1 => p1.Age >= 18 && Invoke(p2 => p2.Gender == "Male", p1)

As you can see:

  1. The resulting lambda reuses the parameters of the first expression.
  2. Has a body that invokes the second expression by passing the parameters of the first expression as a replacement for the second expression’s parameters. The resulting InvocationExpression is sort of like the expression-equivalent of a method-call (calling a routine by passing in arguments for parameters).
  3. Ands the first expression’s body and this InvocationExpression together to produce the body of the resulting lambda.

The idea is that the LINQ provider should be able to understand the semantics of this operation and take a sensible course of action (e.g. generate SQL like WHERE age >= 18 AND gender="Male").

Often though, providers have problems with InvocationExpressions, because of the obvious complications of processing a ‘nested expression-call inside an expression.’

To get around this, LINQKit also provides the Expand helper. This essentially ‘inlines’ the invocation call smartly by replacing the call with the body of the nested expression, substituting uses of the nested expression’s parameters appropriately (in this case, replacing p2 with p1). This should produce something like:

p1 => p1.Age >= 18 && p1.Gender == "Male"

Note that this how you would have manually combined those predicates if you’d done it yourself in a lambda. But with LINQKit around, you can get these predicates from independent sources and easily combine them:

  1. Without writing “by hand” expression code.
  2. Optionally, in a way that is transparent to consumers of the resulting lambda.

More Related Contents:

  1. How do I apply OrderBy on an IQueryable using a string column name within a generic extension method?
  2. How do I create an expression tree calling IEnumerable.Any(…)?
  3. How to write this nested foreach loop query in LINQ?
  4. How to use LINQ to select object with minimum or maximum property value
  5. Linq: GroupBy, Sum and Count
  6. Update all objects in a collection using LINQ
  7. LINQ Aggregate algorithm explained
  8. Mutating the expression tree of a predicate to target another type
  9. LINQ to Entities only supports casting EDM primitive or enumeration types with IEntity interface
  10. Why does Enumerable.All return true for an empty sequence? [duplicate]
  11. Am I misunderstanding LINQ to SQL .AsEnumerable()?
  12. Extension methods syntax vs query syntax
  13. Conversion from List to array T[]
  14. Internal .NET Framework Data Provider error 1025
  15. How set value a property selector Expression
  16. Find all child controls of specific type using Enumerable.OfType() or LINQ
  17. What is the performance of the Last() extension method for List?
  18. Should I use two “where” clauses or “&&” in my LINQ query?
  19. Max or Default?
  20. Performance of Find() vs. FirstOrDefault() [duplicate]
  21. How do I do a left outer join with Dynamic Linq?
  22. Remove item from list based on condition
  23. LINQ performance FAQ
  24. Last and LastOrDefault not supported
  25. A dictionary where value is an anonymous type in C#
  26. How do I dynamically create an Expression predicate?
  27. Expressing recursion in LINQ
  28. How to wrap Entity Framework to intercept the LINQ expression just before execution?
  29. C# Searching for files and folders except in certain folders
  30. Linq where column == (null reference) not the same as column == null

Leave a Comment