How does C# lambda work?

I'm trying to implement method Find that searches the database.

I forgot to mention that I'm using Postgresql, so I can't use built in LINQ to SQL.

I want it to be like that:

var user = User.Find(a => a.LastName == "Brown");

Like it's done in List class. But when I go to List's source code (thanks, Reflector), I see this:

public T Find(Predicate<T> match)
{
    if (match == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    for (int i = 0; i < this._size; i++)
    {
        if (match(this._items[i]))
        {
            return this._items[i];
        }
    }
    return default(T);
}

How can I implement this thing? I need to get those parameters to make the search.

Solution

Okay, I understood now that I need to do LINQ to SQL to do all this good expressions stuff, otherwise I'd have to spend a lot of time reimplementeing the wheel.

Since I can't use LINQ to SQL, I implemented this easy method:

public static User Find(User match, string orderBy = "")
    {
        string query = "";
        if (!String.IsNullOrEmpty(match.FirstName)) query += "first_name='" + match.FirstName + "'";
        if (!String.IsNullOrEmpty(match.LastName开发者_StackOverflow中文版)) query += "last_name='" + match.LastName+ "'";
        return Find(query + (!String.IsNullOrEmpty(orderBy) ? orderBy : ""));
    }

This is how to use it:

var user = User.Find(new User { FirstName = "Bob", LastName = "Brown" });

---

Your method should accept Expression<Func<User>>.

This will give you expression tree instead of delegate which you can analyze and serialize to SQL or convert to any other API call your database have.

If you want everything to be generic, you may wish to go on with implementing IQueryable interface. Useful information can be found here: LINQ Tips: Implementing IQueryable Provider

Although for a simple scenario I would suggest not to complicate everything and stick with using Expression Trees and returning plain IEnumerable<T> or even List<T>.

For your case first version of code could look like this:

public IEnumerable<T> Get(Expression<Func<T, bool>> condition)
{
    if (condition.Body.NodeType == ExpressionType.Equal)
    {
        var equalityExpression = ((BinaryExpression)condition.Body);

        var column = ((MemberExpression)equalityExpression.Left).Member.Name;

        var value = ((ConstantExpression)equalityExpression.Right).Value;

        var table = typeof(T).Name;

        var sql = string.Format("select * from {0} where {1} = '{2}'", table, column, value);

        return ExecuteSelect(sql);
    }

    return Enumerable.Empty<T>();
}

And it's complexity grows fast when you want to handle new and new scenarios so make sure you have reliable unit tests for each scenario.

C# Samples for Visual Studio 2008 contain ExpressionTreeVisualizer that will help you to dig into Expression Trees more easily to understand how to extract information you need from it.

And of course, if you can stick with using existing implementation of LINQ, I would suggest to do it. There are Linq to SQL for SQL Server databases, Linq to Entities for many different databases, Linq to NHibernate for NHbernate projects.

Many other LINQ providers can be found here: Link to Everything: A List of LINQ Providers. Amount of work to implement LINQ provider is not trivial so it's a good idea to reuse tested and supported solution.

---

Exactly the same way. Just replace this._items with your users collection.

Also replace the type parameter T with the type User.

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A lambda expression in source code can be converted to either a compiled executable delegate or an expression tree upon compilation. Usually we associate lambda's with delegates but in your case since you say you want access to the parameters (in this case I assume you mean LastName and "Brown" then you want an expression tree.

Once you have an expression tree, you can parse it to see exactly what it is an translate it to whatever you actually need to do.

Here are a few questions about expression trees.

Expression trees for dummies?

Bit Curious to understand Expression Tree in .NET

Sounds like you're definitely reinventing a very complicated wheel though. I'm sure it'll be a useful learning experience, but you should look into LINQ to Entities or LINQ to SQL for real-world programming.

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Maybe I just haven't understood the question, but there's already a method for doing what you want: Enumerable.Where.

If you need to find a single element then use SingleOrDefault or FirstOrDefault instead.

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You could do it something like this:

public static IEnumerable<User> Find(Predicate<User> match)
{
    //I'm not sure of the name
    using (var cn = new NpgsqlConnection("..your connection string..") )
    using (var cmd = new NpgsqlCommand("SELECT * FROM Users", cn))
    using (var rdr = cmd.ExecuteReader())
    {
        while (rdr.Read())
        {
           var user = BuildUserObjectFromIDataRecord(rdr);
           if (match(user)) yield return user;
        }
    }
}

And then you can call it like this

var users = User.Find(a => a.LastName == "Brown");

Note that this returns any number of users, you still have to implement the BuildUserObjectFromIDataRecord() function, and that it will always want to iterate over the entire users table. But it gives you the exact semantics you want.

---

Okay, I understood now that I need to do LINQ to SQL to do all this good expressions stuff, otherwise I'd have to spend a lot of time reimplementeing the wheel.

Since I can't use LINQ to SQL, I implemented this easy method:

public static User Find(User match, string orderBy = "")
    {
        string query = "";
        if (!String.IsNullOrEmpty(match.FirstName)) query += "first_name='" + match.FirstName + "'";
        if (!String.IsNullOrEmpty(match.LastName)) query += "last_name='" + match.LastName+ "'";
        return Find(query + (!String.IsNullOrEmpty(orderBy) ? orderBy : ""));
    }

This is how to use it:

var user = User.Find(new User { FirstName = "Bob", LastName = "Brown" });

---

One way would be to create an anonymous delegate, like so:

Predicate<User> Finder = delegate(User user)
{
    return user.LastName == "Brown";
}

var User = User.Find(Finder);