How do I compose existing Linq Expressions

Question

I want to compose the results of two Linq Expressions. They exist in the form

Expression<Func<T, bool>>

So the two that I want to compose are essentially delegates on a parameter (of type T) that both return a boolean. The result I would like composed would be the logical evaluation of the booleans. I would probably implement it as an extension method so my syntax would be something like:

Expression<Func<User, bool>> expression1 = t => t.Name == "steve";
Expression<Func<User, bool>> expression2 = t => t.Age == 28;
Expression<Func<User, bool>> composedExpression = expression1.And(expression2);

And later on in my code I want to evaluate the composed expression

var user = new User();
bool evaluated = composedExpression.Compile().Invoke(user);

I have poked around with a few different ideas but I fear that it is more complex than I had hoped. How is this done?

Answer 1

Here is an example:

var user1 = new User {Name = "steve", Age = 28};
var user2 = new User {Name = "foobar", Age = 28};

Expression<Func<User, bool>> expression1 = t => t.Name == "steve";
Expression<Func<User, bool>> expression2 = t => t.Age == 28;

var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

var result = Expression.Lambda<Func<User, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);

Console.WriteLine(result.Compile().Invoke(user1)); // true
Console.WriteLine(result.Compile().Invoke(user2)); // false

You can reuse this code via extension methods:

class User
{
  public string Name { get; set; }
  public int Age { get; set; }
}

public static class PredicateExtensions
{
  public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1,Expression<Func<T, bool>> expression2)
  {
    InvocationExpression invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

    return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
  }
}

class Program
{
  static void Main(string[] args)
  {
    var user1 = new User {Name = "steve", Age = 28};
    var user2 = new User {Name = "foobar", Age = 28};

    Expression<Func<User, bool>> expression1 = t => t.Name == "steve";
    Expression<Func<User, bool>> expression2 = t => t.Age == 28;

    var result = expression1.And(expression2);

    Console.WriteLine(result.Compile().Invoke(user1));
    Console.WriteLine(result.Compile().Invoke(user2));
  }
}