Question

How do you write a dynamic Linq query for the following simple search criteria?
1) StudentNumber
2) LastName
3) LastName and FirstName

//if (!String.IsNullOrEmpty(StudentNumber))
var results  = (from s in Students              
                where s.StudentNumber == 1001
                select s
               );

//else if (!String.IsNullOrEmpty(LastName) & (String.IsNullOrEmpty(FirstName))

var results  = (from s in Students              
                where s.LastName == "Tucker"
                select s
               );

//else if (!String.IsNullOrEmpty(LastName) & (!String.IsNullOrEmpty(FirstName))            
var results  = (from s in Students              
                where s.LastName == "Tucker" && s.FirstName == "Ron"
                select s
               );   

Answer 1

You need to declare your results variable outside of any individual query. This will allow you append different filters based upon your varying criteria, and append as many filters as you need. An example:

var results = Students.AsEnumerable(); // use .AsQueryable() for EF or Linq-to-SQL

if (!string.IsNullorEmpty(StudentNumber)) 
{
    results = results.Where(s => s.StudentNumber.Equals(StudentNumber));
}
else if (!string.IsNullOrEmpty(LastName))
{
    results = results.Where(s => s.LastName.Equals(LastName));

    if (!string.IsNullOrEmpty(FirstName))
    {
         results = results.Where(s => s.FirstName.Equals(FirstName));
         // filter is in addition to predicate against LastName
    }
}

// results can be used here

If dealing with Linq-to-Entities or -Sql, type the initial query with Students.AsQueryable(); so that the filtering happens at the database rather than inside the application.

---

Is there a way I can construct the WHERE clause first and use it in a
Linq query without if…else

If you want to build the entire where before the first step of the query, it’s the same logic. You are conditionally building the predicate, so you will have some sort of if/else involved. However, to build the entire predicate first, you could build against a Func<Student, bool> for Linq to Objects.

Func<Student, bool> predicate;
if (!string.IsNullOrEmpty(StudentNumber))
{
    predicate = s => s.StudentNumber.Equals(StudentNumber);
}
else if (!string.IsNullOrEmpty(LastName))
{
    predicate = s => s.LastName.Equals(LastName);

    if (!string.IsNullOrEmpty(FirstName))
    {
        Func<Student, bool> p = predicate;
        predicate = s => p(s) && s.FirstName.Equals(FirstName);
    }
}
else
{
    predicate = s => true;
}

var query = Students.Where(predicate);

You’ll notice it’s the exact same if/else structure. You could collapse that down into a complicated conditional expression

Func<Student, bool> predicate;
predicate = s =>
    !string.IsNullOrEmpty(StudentNumber)
    ? s.StudentNumber.Equals(StudentNumber)
    : !string.IsNullOrEmpty(LastName)
        ? !string.IsNullOrEmpty(FirstName)
            ? s.LastName.Equals(LastName) && s.FirstName.Equals(FirstName)
            : s.LastName.Equals(LastName)
        : true;

var query = Students.Where(predicate);

But I find that pretty well difficult to follow, certainly as compared to the longer if/else. This predicate is also bigger than the one we build via the if/else, because this one contains all the logic, it’s not just the logic we conditionally added.