Question

I’ve been trying to learn more about using Lamba expression trees and so I created a simple example. Here is the code, this works in LINQPad if pasted in as a C# program.

void Main()
{
    IEnumerable<User> list = GetUsers().Where(NameContains("a"));
    list.Dump("Users");
}

// Methods
public IEnumerable<User> GetUsers()
{
    yield return new User{Name = "andrew"};
    yield return new User{Name = "rob"};
    yield return new User{Name = "chris"};
    yield return new User{Name = "ryan"};
}

public Expression<Func<User, bool>> NameContains(string namePart)
{
    return u => u.Name.Contains(namePart);
}

// Classes
public class User
{
    public string Name { get; set; }
}

This results in the following error:

The type arguments for method ‘System.Linq.Enumerable.Where(System.Collections.Generic.IEnumerable, System.Func)’ cannot be inferred from the usage. Try specifying the type arguments explicitly.

However if I just substitute the first line in main with this:

IEnumerable<User> list = GetUsers().Where(u => u.Name.Contains("a"));

It works fine. Can tell me what I’m doing wrong, please?

Answer 1

The Enumerable.Where method takes a Func<T, bool>, not an Expression<Func<T, bool>>. Perhaps you’re confusing with Queryable.Where, which does take an expression as a parameter… In your case you don’t need an expression, you just need a delegate that can be executed against each item in the sequence. The purpose of expressions is (mostly) to be analysed and translated to something else (SQL for instance), to perform the query against an external data source