CODEWITHSUNDEEP
c#entity-frameworklinq-to-entitiesexpression-trees
Skadoosh
Skadoosh

Reputation: 2523

Filter using Expression Trees

I need to query database and filter based on parameters passed into the function. I am passing two date parameters (used as a date range), a name, and a status parameters. All the parameters can have 'and' or 'or' conditions. Basically, I would like to build an linq expression based on which parameters are populated and pass it to Entity Framework to return a result set.

How can I do this with minimum 'if' statements? If you could be kind enough to provide an explanation with your example code, that would be awesome. I am trying to learn expression trees so an explanation would help.

At this point I don't have much code. That is why I have posted here. I can list the method signature. What exactly are you looking for?

public enum EmployeeStatus
{
    FullTime,
    PartTime,
    Contract
}

public IEnumerable<Employee> FilterEmployees(DateTime? startDate, 
    DateTime? endDate, string employeeName, EmployeeStatus employeeStatus)
{   }

Upvotes: 0

Views: 706

Answers (2)

Aaron Newton
Aaron Newton

Reputation: 2316

All the parameters can have 'and' or 'or' conditions. - you could consider using the PredicateBuilder. See http://www.albahari.com/nutshell/predicatebuilder.aspx. Why? Because this allows you to write a single query, but add an AND/OR predicate only if it is needed. You may or may not require this feature, but it is a good feature to be aware of. There is no database overhead until the query is actually called - it provides a means to conditionally build an IQueryable where you may not want to match against fields under certain conditions. E.g. I used this the other day to ignore a product code field - with min-length 10 - on searches where the input string was less than 10 characters.

This will allow you to add AND/OR statements using an if-condition like so:

public IQueryable<Employee> FilterEmployees(IQueryable<Employee> query, DateTime startDate, DateTime endDate, string employeeName, EmployeeStatus employeeStatus)
{
    var predicate = PredicateBuilder.True<Employee>();

    //All names starting with 'A'
    predicate = predicate.And(x => x.Name.StartsWith("A"));

    //Add a condition only if the employee is PartTime
    if (employeeStatus == EmployeeStatus.PartTime)
    {
        //Add condition for when they start
        predicate = predicate.And(x => x.StartDate >= startDate);
    }
    else
    {
        //Say we don't care about the start date for the other employee statuses,
        //but we want to add condition for when non-part-time employees are due to leave
        predicate = predicate.And(x => x.EndDate <= endDate);
        //or their name ends in 'z'
        predicate = predicate.Or(x => x.Name.EndsWith("z"));
    }

    IQueryable<Employee> employees = query.FindBy(predicate); //you should probably use a repository here to return your query

    return employees

}

Note - this is intended as psuedo-code to demonstrate and may have errors - see the above link for the proper implementation.

Upvotes: 2

user1793714
user1793714

Reputation: 329

public IQueryable<Employee> FilterEmployees(IQueryable<Employee> query, DateTime? startDate, DateTime? endDate, string employeeName, EmployeeStatus employeeStatus)
{
    if (startDate != null)
        query = query.Where(x => x.StartDate >= startDate);

    // etc...

    return query;
}

Upvotes: 4

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