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DevForce Resource Center » Development guide » Query » Build queries dynamically » Create dynamic "Where" clauses
Last modified on August 15, 2012 17:20
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The need for the ability to create a dynamic where clause occurs fairly frequently in applications that need to filter data based on a users input. In these cases you don't really know ahead of time what properties a user is going to want to query or what the conditions of the query are likely to be. In fact, you may not even know the type that you will be querying for until runtime. So you will need to be able to compose the query based on the inputs determined at runtime.
Flavors of dynamic where clauses
Ideally, when building a query we want to specify as little dynamically as we have to. The less dynamic a query is the better the compile time checking and intellisense will be for the query. Everything depends upon what we know and when we know it. Do we know the type of the query at compile time but need to wait until runtime to determine the properties to be queried and the conditions regarding these properties? Or do we not even know the type of the query until runtime?
Whatever the case, the IdeaBlade.Linq.PredicateBuilder and the IdeaBlade.Linq.PredicateDescription classes are the tools used to deal with compile time uncertainty. As we review the PredicateBuilder API, you will see both typed and untyped overloads for most methods in order to support both compile and runtime scenarios.
What’s a predicate?
A LINQ where clause is intimately tied to the concept of a "predicate". So what is a predicate and how does it relate to a LINQ where clause.
A predicate is a function that evaluates an expression and returns true or false. The code fragment...
| C# | p.ProductName.Contains("Sir") |
| VB | p.ProductName.Contains("Sir") |
...is a predicate that examines a product and returns true if the product’s ProductName contains the Sir string.
The CLR type of the predicate in our example is:
| C# | Func<Product, bool> |
| VB | Func(Of Product, Boolean) |
Which we can generalize to:
| C# | Func<T, bool> |
| VB | Func(Of T, Boolean) |
This is almost what we need in order to build a LINQ where clause. The LINQ Where extension method is defined as follows:
| C# | public static IQueryable<T> Where<TSource>( this IQueryable<T> source1, Expression<Func<T,bool>> predicate) |
| VB | public static IQueryable(Of T) Where(Of TSource) _ (Me IQueryable(Of T) source1, Expression(Of Func(Of T,Boolean)) predicate) |
Note that the "predicate" parameter above is
| C# | Expression<Func<T, bool>> |
| VB | Expression(Of Func(Of T, Boolean)) |
When we refer to the idea that a "predicate" is needed in order to build a LINQ where clause, what we really mean is that we need to build a "predicate expression", or a Expression that is resolvable to a simple predicate.
Create a Where clause with two predicates
The snippet below comprises two statements, each of which uses PredicateBuilder.Make to create a PredicateDescription representing a single predicate (filter criteria).
| C# | PredicateDescription p1 = PredicateBuilder.Make(typeof(Product), "UnitPrice", FilterOperator.IsGreaterThanOrEqualTo, 24); PredicateDescription p2 = PredicateBuilder.Make(typeof(Product), "Discontinued", FilterOperator.IsEqualTo, true); |
| VB | Dim p1 As PredicateDescription = PredicateBuilder.Make(GetType(Product), "UnitPrice", _ FilterOperator.IsGreaterThanOrEqualTo, 24) Dim p2 As PredicateDescription = PredicateBuilder.Make(GetType(Product), "Discontinued", _ FilterOperator.IsEqualTo, True) |
A new query can then be composed and executed by an EntityManager as usual:
| C# | var query = anEntityManager.Products.Where(p1.And(p2)) // The above query is the same as: //var queryb = anEntityManager.Products.Where(p => p.UnitPrice > 24 && p.Discontinued); |
| VB | Dim query = anEntityManager.Products.Where(p1.And(p2)) ' The above query is the same as: 'var queryb = anEntityManager.Products.Where(p => p.UnitPrice > 24 && p.Discontinued); |
We could have combined the individual PredicateDescriptions into another PredicateDescription variable:
| C# | CompositePredicateDescription p3 = p1.And(p2); var query = anEntityManager.Products.Where(p3); |
| VB | Dim p3 As CompositePredicateDescription = p1.And(p2) Dim query = anEntityManager.Products.Where(p5) |
Completely dynamic query
The Where clause in the previous examples were still applied to a strongly typed (non-dynamic) IQueryable<Product> implementation. To be more precise, the query root was of type IEntityQuery<Product>.
What if we didn't know we were querying for Product at compile type? We'd know the type at runtime but we didn't know it as we wrote the code. We could use the EntityQuery.CreateQuery factory method and pass it the runtime type. CreateQuery returns a nominally-untyped EntityQuery object.
| C# | var queryType = typeof(Product); // imagine this was passed in at runtime. var baseQuery = EntityQuery.CreateQuery(queryType , anEntityManager); var query = baseQuery.Where(p1.And(p2)); |
| VB | Dim queryType = GetType(Product) ' imagine this was passed in at runtime. Dim baseQuery = EntityQuery.CreateQuery(queryType , anEntityManager) Dim query = baseQuery.Where(p1.And(p2)) |
The EntityQuery result of CreateQuery implements ITypedEntityQuery. The Where clause in this example made use of the ITypedEntityQuery extension method instead of the IQueryable<T> extension method:
| C# | public static ITypedEntityQuery Where(this ITypedEntityQuery source, IPredicateDescription predicateDescription); |
| VB | Public Shared ITypedEntityQuery Where(Me ITypedEntityQuery source, _ IPredicateDescription predicateDescription) |
The query that will be executed is exactly the same regardless of which extension method is used. Both queries are Product queries. The critical difference is that the compiler can't determine the type of the query in the second case. That's why it resolved to a query typed as ITypedEntityQuery, which implements IQueryable, instead of IEntityQuery<T> which implements IQueryable<T>.
There are two ramifications:
- Completely dynamic queries, those typed as ITypedEntityQuery, must be executed with one of the EntityManager.Execute methods instead of ToList().
- The compiler declares that the type of the dynamic query execution result is IEnumerable instead of IEnumerable<T>.
The following example illustrates:
| C# | // IQueryable<T> implementation var query1 = anEntityManager.Products.Where(p3); IEnumerable<Product> products1 = query1.ToList(); // IQueryable implementation var baseQuery = EntityQuery.CreateQuery(typeof(Product), anEntityManager); var query2 = baseQuery.Where(p1.And(p2)); // can't call query2.ToList() because query2 is not an IQueryable<T> IEnumerable results = anEntityManager.ExecuteQuery(query2); // next line is required in order to IEnumerable<Product> products2 = results.Cast<Product>(); |
| VB | ' IQueryable<T> implementation Dim query1 = anEntityManager.Products.Where(p3) Dim products1 As IEnumerable(Of Product) = query1.ToList() ' IQueryable implementation Dim baseQuery = EntityQuery.CreateQuery(GetType(Product), anEntityManager) Dim query2 = baseQuery.Where(p1.And(p2)) ' can't call query2.ToList() because query2 is not an IQueryable<T> Dim results As IEnumerable = anEntityManager.ExecuteQuery(query2) ' next line is required in order to Dim products2 As IEnumerable(Of Product) = results.Cast(Of Product)() |