Should I Use Two "Where" Clauses or "&&" in My Linq Query

Should I use two where clauses or && in my LINQ query?

I personally would always go with the && vs. two where clauses whenever it doesn't make the statement unintelligible.

In your case, it probably won't be noticeble at all, but having 2 where clauses definitely will have a performance impact if you have a large collection, and if you use all of the results from this query. For example, if you call .Count() on the results, or iterate through the entire list, the first where clause will run, creating a new IEnumerable<T> that will be completely enumerated again, with a second delegate.

Chaining the 2 clauses together causes the query to form a single delegate that gets run as the collection is enumerated. This results in one enumeration through the collection and one call to the delegate each time a result is returned.

If you split them, things change. As your first where clause enumerates through the original collection, the second where clause enumerates its results. This causes, potentially (worst case), 2 full enumerations through your collection and 2 delegates called per member, which could mean this statement (theoretically) could take 2x the runtime speed.

If you do decide to use 2 where clauses, placing the more restrictive clause first will help quite a bit, since the second where clause is only run on the elements that pass the first one.

Now, in your case, this won't matter. On a large collection, it could. As a general rule of thumb, I go for:

Readability and maintainability
Performance

In this case, I think both options are equally maintainable, so I'd go for the more performant option.

When using LINQ, what is the difference between && and multiple where clauses?

The first one will be translated into:

objectList.Where(o => o.value1 < 100).Where(o=> o.value2 > 10)

while the second one will be translated in:

objectList.Where(o => o.value1 < 100 && o.value2 > 10)

So, in the first one, you will have a first filtered sequence that is filtered again (first sequence contains all the objects with value < 100, the second one containing all the objects with value > 10 from the first sequence), in while the second one you will do the same comparisons in the same labda expression. This is valid fro Linq to objects, for other providers it depends how the expression is translated.

What's the difference between multiple where clauses and && operator in LINQ-to-SQL?

Ok here are my findings after going through Reflector output for a while. LINQ-to-Objects combine the consecutive where predicates when using WhereArrayIterator or WhereListIterator causing it to ALMOST behave like && operator, but not as exactly:

When you use x.a==1 && x.b==1 the where clause translates into a Func<TSource, bool> looking like this:

bool daspredicate(TSource x)
{
    return x.a==1 && x.b==1
}

However, when you use consecutive Where clauses there is a slight performance penalty, at least from non-JITted IL-aspect. Here is how code looks like after combining:

bool predicate1(TSource x)
{
     return x.a==1;
}
bool predicate2(TSource x)
{
     return x.b==1;
}
bool daspredicate(TSource x)
{
    return predicate1(x) && predicate2(x);
}

As you can see this involves additional function call overhead. This can be quite expensive unless JIT inlines the functions. I'm sure it does a good job at it but we now know JIT's job becomes much easier if we combine our Where statements ourselves, unless necessary.

On the SQL-side of things though, the queries are the same. Even before execution, debugger evaluates the query object into the same SQL statement. I couldn't go too far in Linq namespace because things seemed much more complex, but since queries are the same, there should be no penalty unlike LINQ-to-objects example above.

EDIT: I've seen instances where multiple where statements resulting in nested sub-queries on SQL server. I think it's better to stick to single where statements whenever you can to be on the safe side.

LINQ: One where clause versus multiple chained where clauses

I set up a test project using the model and context described in this article and logged the SQL for two different queries, following the pattern in your question. The queries I made were:

db.Blogs
    .Where(b => b.BlogId == 0)
    .Where(b => b.Name == "Foo");

and

db.Blogs
    .Where(b => b.BlogId == 0 && b.Name == "Foo");

The generated SQL for both queries is identical:

SELECT 
    [Extent1].[BlogId] AS [BlogId], 
    [Extent1].[Name] AS [Name]
    FROM [dbo].[Blogs] AS [Extent1]
    WHERE (0 = [Extent1].[BlogId]) AND (N'Foo' = [Extent1].[Name])

So it seems (at least for simple cases like this), there is no significant performance difference one way or the other. I guess you could argue it takes the expression visitor a bit longer to examine your tree if you use the multiple Where approach, but this is negligible in the long run.

When using LINQ, what is the difference between && and multiple where clauses?

The first one will be translated into:

objectList.Where(o => o.value1 < 100).Where(o=> o.value2 > 10)

while the second one will be translated in:

objectList.Where(o => o.value1 < 100 && o.value2 > 10)

C# Linq: Combine multiple .Where() with an OR clause

If you want to build your query programmatically and have it execute on your SQL server instead of fetching all records and querying in memory, you need to use the set of static methods on the Expression class and build your query using those. In your example:

public class Query // this will contain your 20 fields you want to check against
{
    public int? Field1; public int? Field2; public int? Field3; public int Field4;
}

public class QueriedObject // this is the object representing the database table you're querying
{
    public int QueriedField;
}

public class Program
{
    public static void Main()
    {
        var queryable = new List<QueriedObject>().AsQueryable();
        var query = new Query { Field2 = 1, Field3 = 4, Field4 = 2 };

        // this represents the argument to your lambda expression
        var parameter = Expression.Parameter(typeof(QueriedObject), "qo");

        // this is the "qo.QueriedField" part of the resulting expression - we'll use it several times later
        var memberAccess = Expression.Field(parameter, "QueriedField");

        // start with a 1 == 1 comparison for easier building - 
        // you can just add further &&s to it without checking if it's the first in the chain
        var expr = Expression.Equal(Expression.Constant(1), Expression.Constant(1));

        // doesn't trigger, so you still have 1 == 1
        if (query.Field1.HasValue)
        {
            expr = Expression.AndAlso(expr, Expression.Equal(memberAccess, Expression.Constant(query.Field1.Value)));
        }
        // 1 == 1 && qo.QueriedField == 1
        if (query.Field2.HasValue)
        {
            expr = Expression.AndAlso(expr, Expression.Equal(memberAccess, Expression.Constant(query.Field2.Value)));
        }
        // 1 == 1 && qo.QueriedField == 1 && qo.QueriedField == 4
        if (query.Field3.HasValue)
        {
            expr = Expression.AndAlso(expr, Expression.Equal(memberAccess, Expression.Constant(query.Field3.Value)));
        }

        // (1 == 1 && qo.QueriedField == 1 && qo.QueriedField == 4) || qo.QueriedField == 2
        expr = Expression.OrElse(expr, Expression.Equal(memberAccess, Expression.Constant(query.Field4)));

        // now, we combine the lambda body with the parameter to create a lambda expression, which can be cast to Expression<Func<X, bool>>
        var lambda = (Expression<Func<QueriedObject, bool>>) Expression.Lambda(expr, parameter);

        // you can now do this, and the Where will be translated to an SQL query just as if you've written the expression manually
        var result = queryable.Where(lambda);       
    }
}

Multiple WHERE Clauses with LINQ extension methods

Two ways:

results = results.Where(o => (o.OrderStatus == OrderStatus.Open) &&
                             (o.CustomerID == customerID));

or:

results = results.Where(o => (o.OrderStatus == OrderStatus.Open))
                 .Where(o => (o.CustomerID == customerID));

I usually prefer the latter. But it's worth profiling the SQL server to check the query execution and see which one performs better for your data (if there's any difference at all).

A note about chaining the .Where() methods: You can chain together all the LINQ methods you want. Methods like .Where() don't actually execute against the database (yet). They defer execution until the actual results are calculated (such as with a .Count() or a .ToList()). So, as you chain together multiple methods (more calls to .Where(), maybe an .OrderBy() or something to that effect, etc.) they build up what's called an expression tree. This entire tree is what gets executed against the data source when the time comes to evaluate it.

LINQ: differences between single Where with multiple conditions and consecutive Wheres with single condition

Semantically, there is no difference in the case of Where (contrast OrderBy, which needs more care). At the implementation level, it is simply multiple predicates with simple expression trees instead of a single predicate with a complex expression tree; but most engines will cope fine with either.

For what you are doing, multiple Where is ideal.

Should I Use Two "Where" Clauses or "&&" in My Linq Query