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How to turn this func into expression?

I play with expression trees and try to better understand how they work. I wrote an example of the code I'm working with, and hopefully someone can help me.

So, I have this somewhat dirty query:

/// <summary>
/// Retrieves the total number of messages for the user.
/// </summary>
/// <param name="username">The name of the user.</param>
/// <param name="sent">True if retrieving the number of messages sent.</param>
/// <returns>The total number of messages.</returns>
public int GetMessageCountBy_Username(string username, bool sent)
{
    var query = _dataContext.Messages
        .Where(x => (sent ? x.Sender.ToLower() : x.Recipient.ToLower()) == username.ToLower())
        .Count();
    return query;
}

_dataContextis the context of the entity structure data. This query works beautifully, but is not easy to read. I decided to include the inline IF expression in the Funcfollowing way:

public int GetMessageCountBy_Username(string username, bool sent)
{
    Func<Message, string> userSelector = x => sent ? x.Sender : x.Recipient;
    var query = _dataContext.Messages
        .Where(x => userSelector(x).ToLower() == username.ToLower())
        .Count();
    return query;
}

This seems to work fine, but there is a problem. Since the query is against IQueryable<T>, this LINQ expression is translated into SQL for execution in the data source. This is great, but because of this, he does not know what to do with the call userSelector(x)and throws an exception. He cannot translate this delegate into expression.

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:

Expression<Func<Message, string>> userSelectorExpression = x => sent ? x.Sender : x.Recipient;
Func<Message, string> userSelector = userSelectorExpression.Compile();

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LINQ to Entities LINQ node type 'Invoke' .

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LINQ to Entities 'System.String userSelector (Message, Boolean)', .

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    /// <summary>
    /// Retrieves the total number of messages for the user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="sent">True if retrieving the number of messages sent.</param>
    /// <returns>The total number of messages.</returns>
    public int GetMessageCountBy_Username(string username, bool sent)
    {
        var query = _dataContext.Messages
            .Count(UserSelector(username, sent));
        return query;
    }

, :

    /// <summary>
    /// Retrieves a list of messages from the data context for a user.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="page">The page number.</param>
    /// <param name="itemsPerPage">The number of items to display per page.</param>
    /// <returns>An enumerable list of messages.</returns>
    public IEnumerable<Message> GetMessagesBy_Username(string username, int page, int itemsPerPage, bool sent)
    {
        var query = _dataContext.Messages
            .Where(UserSelector(username, sent))
            .OrderByDescending(x => x.SentDate)
            .Skip(itemsPerPage * (page - 1))
            .Take(itemsPerPage);
        return query;
    }

, UserSelector(string, bool). :

    /// <summary>
    /// Builds an expression to be reused in a LINQ query.
    /// </summary>
    /// <param name="username">The name of the user.</param>
    /// <param name="sent">True if retrieving sent messages.</param>
    /// <returns>An expression to be used in a LINQ query.</returns>
    private Expression<Func<Message, bool>> UserSelector(string username, bool sent)
    {
        return x => ((sent ? x.FromUser : x.ToUser).Username.ToLower() == username.ToLower()) && (sent ? !x.SenderDeleted : !x.RecipientDeleted);
    }

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    public int GetMessageCountBy_Username(string username, bool sent)
    {
        Expression<Func<Message, bool>> userSelector = x => ((sent ? x.FromUser : x.ToUser).Username.ToLower() == username.ToLower()) && (sent ? !x.SenderDeleted : !x.RecipientDeleted);

        var query = _dataContext.Messages
            .Count(userSelector);
        return query;
    }

. , , . , LINQ, lambdas :)

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0

:

return sent
    ? _dataContext.Messages.Count(x => x.Sender.ToLower() == username.ToLower())
    : _dataContext.Messages.Count(x => x.Recipient.ToLower() == username.ToLower());
+2

Perhaps this may be useful to help you abstract the conditions (predicates): http://www.albahari.com/nutshell/predicatebuilder.aspx

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