Add additional WHERE clause to existing Linq query

Question

I have a method which accepts a linq query:

 public IEnumerable<User> GetAll(System.Linq.Expressions.Expression<Func<UserDTO, bool>> query)
 {
     return GetAllDTO(query);
 }

What I would like to be able to do is append an additional WHERE clause to this existing query so it looks something like this:

 public IEnumerable<User> GetAll(System.Linq.Expressions.Expression<Func<UserDTO, bool>> query)
 {
    return GetAllDTO(query).Where(x => x.Organisation == "something")
 }

But this will load ALL the records and that match the query and THEN apply the where clause. I want to add the where clause to the original query so that only the records matching both are returned.

Don't modify the result of GetAllDTO(...). Modify the query before you call the method. — vesan
– vesan, Commented Oct 28, 2014 at 0:57

Mike Cowan · Accepted Answer · 2014-10-28 03:54:07Z

This example modifies the query before executing it:

private IEnumerable<int> GetAll(Expression<Func<int, bool>> currentQuery)
{
     Expression left = currentQuery.Body;
     BinaryExpression right = Expression.GreaterThan(
         currentQuery.Parameters[0], Expression.Constant(0));
     BinaryExpression combined = Expression.AndAlso(left, right);
     Expression<Func<int, bool>> final = Expression.Lambda<Func<int, bool>>(
         combined, currentQuery.Parameters[0]);
     return GetAllInt(final);
}

If currentQuery starts as x => x != 5, the function above will return x => (x != 5) && (x > 0).

Here's the remaining example code:

private static readonly List<int> TheList = 
    new List<int> { 0, 1, 0, 2, 0, 3, 0, 4, 0, 5 };

public static void Main(string[] args)
{
    Expression<Func<int, bool>> initialQuery = x => x != 5;
    IEnumerable<int> result = GetAll(initialQuery);
    foreach (int i in result)
    {
        Console.WriteLine(i);
    }

    Console.ReadLine();
}

And the GetAllInt method:

private static IEnumerable<int> GetAllInt(Expression<Func<int, bool>> query)
{
    return TheList.Where(query.Compile());
}

This prints out:

This may not fit your situation exactly but should at least give you a starting point.

Simon · Accepted Answer · 2014-10-28 18:39:28Z

1

In the end I managed it like this:

public IEnumerable<User> GetAll(System.Linq.Expressions.Expression<Func<UserDTO, bool>> query)
{
  var prefix = query.Compile();
  query = c => prefix(c) && c.Organisation == organisationID;
}

answered Oct 28, 2014 at 18:39

Simon

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