Graeme Hill's Dev Blog

Entity Framework dynamic queries and parameterization

Star date: 2014.139

Update (August 09/2014) There is a new paragraph at the bottom of the article explaining a workaround to this problem.

Here's an interesting quirk in Entity Framework 6's SQL generation. If we write some C# like this:

var name = "foo";
var query = myData.Query<Person>().Where(p => p.Name == name);

Then the generated SQL will essentially looks like this (I simplified it a bit):

SELECT [name], [email] FROM People WHERE [name]=@p__linq__0

The parameter @p__linq__0 will be set to "foo". However if I instead write the C# part like this:

var query = myData.Query<Person>().Where(p => p.Name == "foo");

Then the generated SQL looks like:

SELECT [name], [email] FROM People WHERE [name]='foo'

Instead of parameterizing the name in the where clause it just directly embedded a string literal. This isn't a security concern since I'm sure the inputs are sanitized but it is a performance concern. There is often great benefit in parameterized queries because SQL Server knows that two queries which differ only in their parameter values can use the same query plan and it allows you to take advantage of a lot of optimizations. Often the first query will be much slower than all subsequent queries which use the same query plan, so if every value of the name parameter needs its own query plan then they are all going to get first execution overhead.

The first question is how does it even know the difference?

The answer is that the function Querable.Where() does not take a parameter of Func<T, bool>. Instead its parameter is type Expression<Func<T, bool>> which means that you are literally giving it a syntax tree that it can inspect and optionally compile to a Func<T, bool>. The Entity Framework provider for SQL Server will inspect that expression and convert it to SQL instead of compiling it as a C# function because it doesn't want to execute the function as part of the C# program running in local memory, it wants to execute it on the database server as part of the SQL query.

The next question is what's the difference between using a variable and directly using a string literal?

The difference is in the syntax tree that gets generated. The AST (abstract syntax tree) will store a variable or a constant depending on the syntax you use. This makes sense and is actually a good feature, but the problem is that it's quite unusual in C# code for it to make a functional difference whether you declare a variable in a separate line or just directly use the literal. There is a high degree of programmer surprise here.

And finally, what difference does it make to the Entity Framework SQL generator?

Well, if your AST says to use a constant then the generated SQL uses a constant. If your AST says to use a variable then the generated SQL uses a variable which results in a parameterized query. Seems pretty logical at first glance but I would still classify this as a bug. Just because you phrased something as a constant in C# doesn't necessarily mean that you want it to also be a constant in SQL. There is unlikely to be a downside to parameterizing the query, but it is very likely that there will be a downside to NOT parameterizing it (especially if it's a complex query).

There's a very easy workaround for static queries that are known at compile time, simply put the value in a variable and you don't have to worry. It gets a little trickier with dynamically generated queries though. Take a look at an example like this:

// this code is the same as: filter = (p) => p.Name == "name";
var expectedName = Expression.Constant("name");
var param = Expression.Parameter(typeof(Person));
var namePropery = Expression.Property(param, typeof(Person).GetProperty("Name"));
var equal = Expression.Equal(nameProperty, expectedName);
var filter = Expression.Lambda(equal, param);

Creating queries like this is pretty common when the query needs to be dynamically generated based on user input like if you have an API with a syntax that allows the caller to create an arbitrary filter (eg: OData). If you want a dynamically generated query to NOT use a constant then you are a little bit screwed. Here's a thread on StackOverflow that illustrates the problem:

http://stackoverflow.com/questions/7220626/local-variable-and-expression-trees

As Jon Skeet explains, there is some compiler magic involved in generating an expression that can access a local variable even though the expression may be evaluated in some other scope. This is part of the newish closure functionality in .NET and is very cool, but sadly doesn't help us when the expression is dynamically generated unless you want to do some custom IL generation at runtime or invent a workaround like using a pool of global variables or something hacky like that (I tested and it does actually work with global variables, but that doesn't count as a solution).

When it comes down to it, I don't actually want to change the expression at all. I think the expression is correct and it's actually the SQL generation in Entity Framework that is wrong.

Update (August 09/2014) There is a workaround to this issue and it turns out that it's pretty simple. You just have to wrap your constant in another class and then reference it by either a PropertyExpression or a FieldExpression. It's also pretty easy to do this in a generic way:

public static class ExpressionHelper
{
    public static MemberExpression WrappedConstant<TValue>(TValue value)
    {
        var wrapper = new WrappedObj<TValue>(value);
        return Expression.Property(
            Expression.Constant(wrapper),
            typeof(WrappedObj<TValue>).GetProperty("Value"));
    }

    private class WrappedObj<TValue>
    {
        public TValue Value { get; set; }
        public WrappedObj(TValue value)
        {
            this.Value = value;
        }
    }
}

Now a dynamic expression can be created like this:

// this code is the same as: filter = (p) => p.Name == "name";
var expectedName = ExpressionHelper.WrappedConstant("name");
var param = Expression.Parameter(typeof(Person));
var namePropery = Expression.Property(param, typeof(Person).GetProperty("Name"));
var equal = Expression.Equal(nameProperty, expectedName);
var filter = Expression.Lambda(equal, param);

Now when you get Entity Framework 6 to generate sql with this filter you'll end up with this:

SELECT [name], [email] FROM People WHERE [name]=@p__linq__0

Yay!

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